Designing something that can be assembled and used, may be sufficient for one-off shop equipment and proof-of-principle. In a cost competitive production environment, it is not enough. We need product that that works well, that is reliable, and that is easy and cheap to manufacture. This requires significant effort by design and engineering, and an understanding of engineering principles, such as Design For Manufacture and Assembly (DFMA).
DFMA is a strategy that reduces cost and improves quality.
There are a number of websites and articles that summarize rules for Design For Manufacture and Assembly.[1]
Simplify the design and reduce the number of parts.
Standardize and use common parts and materials.
Design for ease of fabrication.1
Design within process capabilities and avoid unneeded surface finish requirements.
Mistake-proof product design and assembly (poka-yoke)
Design for parts orientation and handling.2
Minimize flexible parts and interconnections.
Design for ease of assembly.
Design for efficient joining and fastening.
Design modular products.
Design for automated production. 3
Design printed circuit boards for assembly.
Congratulations, you are the mechanical designer for your company’s new Handy Dandy Super Duper Nutating Widget! The widget you are to design, requires custom parts to be fabricated. It requires catalogue parts and materials to be ordered. All parts must be stored in inventory until they are needed. The inventory must be searchable, and tracked by accounting. The parts must be kitted. The widget must be assembled and tested. It must be moved safely around the plant. It must be packaged and shipped. The widget must get sold. The widget must be use-able by the customer. The widget may require service, possibly by the customer. The widget must eventually be disposed of.[3] You must communicate and work with co-workers from other technical specialties, managers, and manufacturing. You are part of a team. Mechanical design must account for all of this. All of your design decisions must be made in context of a design strategy.
DFMA requires a top-down design process, in which the designer looks for solutions that meet all the requirements. The design review team should include the other designers on the project, and manufacturing, and sales, and any other stakeholders. You should model and document several designs, to the point that you can evaluate performance and cost. Your very first idea ought to work, but it probably is not the best idea you can come up with.
DFMA manuals tell us to reduce the number of parts. Fewer parts means fewer assembly steps, fewer fasteners, and reduced inventory.
A typical widget contains parts that accomplish its basic functionality, and then a bunch of other pieces that connect everything together, and provide protection, and possibly handling and/or mounting to other equipment.
There are a number of causes of excessive parts…
Designers start from a base plate or some other existing structure, and they design a mount bracket for each and every part. You should be trying very hard to mount everything directly to some primary base or chassis.
Designers are pressured to re-use existing parts and designs. This can save tooling and inventory costs. It can also result in kludges. All sorts of cost is generated by the design and fabrication of brackets to connect components to holes that were not intended for those components.
Parts are designed in sequence instead of all-together. A part designed early on is finalized, and fabricated, and cannot be modified to solve the DFMA and other design issues that come up later. As noted below, concurrent design is good practice.
Each mechanical part should be made to do as many things as possible. Any part called “bracket” probably can be eliminated from your design.
Good DFMA practice is for all parts to be mounted onto a stable base. The stable base can be your widget’s primary structure. It can be your primary structure mounted to an assembly fixture.4 It is very desirable that there be no need to flip the stable base over. It is nice if the base does not have to be rotated on the work bench. Parts attached to the structure should be retained by gravity or by configuration, leaving both hands to manage fasteners and tools.
If your assembly is a stable base as noted above, you can use one hand to hold the part, while the other holds a fastener and driver. The part must be light enough to be lifted with one hand. There must be some satisfactory hand-hold on the part. If fasteners are oriented vertically, they are retained in their holes. Hex socket, Torx, and Robertson5 screws can hang horizontally off their drivers, as shown on Figure 1, allowing one handed installation of the fasteners. Long fasteners will be retained horizontally in deep holes. If a part is a two-handed lift, it could still be retained one-handed by a pin or some integral feature.
Figure 1 shows a device fairly conveniently assembled with screws, using two hands.
The base part either is clamped to a work bench, or it is heavy enough to stay in place while other parts are installed.
The hex socket head cap screw hangs horizontally on the key. A Robertson or Torx screw would work too. A slotted or Phillips screw would be very much less easy to manage.
If the installed part had some sort of retention feature like a pin,6 or flange, the assembler will have two hands to manage the screws.
This part is small enough to not need hand holds, however, an indent on either side would make it more graspable, and would probably have no effect on fabrication cost.
The assembly in Figure 1 works fine with the manual driver. If the worker were using a power driver, both hands would be required, and the installed part would have to retain itself in place.
If your part to be installed can be easily picked up one-handed by a fifth percentile woman,7 you should have no serious handling problems. Any worker can pick the part up, place it on the assembly, retaining it in place if necessary, and they have a hand free to operate a tool.
If your part is heavier than this, the handling of it must be thought through. There are lots of problems, and solutions.
If your part can be easily lifted by two hands, it should be retained by gravity, or by one or more pins. One hand retaining it in place leaves the other hand to manage fasteners and tools, as per Figure 1. If the part retains itself, both hands are available. This matters. If the part requires two hands to retain in place, assembly requires two workers.
If your part is heavy, provide hand-holds. If your part is too heavy for safe, convenient lifting, consider breaking it up into smaller, lift-able parts. The additional assembly steps may be cheaper than the additional worker, or the extra lifting equipment.
At some point, your part must be lifted by a chain hoist, or some powered lift.8 Design in lift points like lifting holes or eye bolts. Make sure these can take the weight of whatever is being lifted. If there is any obstruction between the part and its mount point, make sure the lifting system can make the necessary movements.
Managing heavy parts is not just a productivity issue. Heavy equipment lifted high above people’s heads is a safety hazard, as is repetitive strain. This all is in addition to property damage.
Try to standardize on a small number of fasteners and other small, standard hardware. A worker with thirty two different fasteners kitted, must select the correct one for each attachment. They take more time and/or they make more mistakes. Reducing the quantity of fastener types, simplifies warehousing and kitting.
If power tools are being used to install fasteners, we9 do not want to change the bit or reset the torque.
Each and every hex socket cap screw requires a different sized key.10 This must be searched for. The set of hex keys must be kept complete. If you specify Phillips, Torx, or Robertson sockets, the assembler will spend less time searching for tools, or using the wrong tool. The ratchet drive kit in Figure 2 belongs to the author. There ought to be more slotted and Robertson drives in it. Otherwise, we see four Phillips drives and seven Torx drives, one of which is on the extender. There is a total of seventeen hex drives, in both English and metric sizes.
Cables are the responsibility of electronics. They connect intimately with the mechanics, so mechanical and electrical designers must work as a team.
Cabling strategy number one is to not have cables. DFMA manuals recommend minimizing flexible objects. Get all your circuits onto one printed circuit board (PCB). There are PCB mounted connectors that mount through panels, thus eliminating the need for additional fasteners, and cables connecting between the PCB and panel. If you have a second PCB, mount it to the first PCB directly through board-to-board connectors.
In Figure 4 the big PCB sits on the base, and is attached to the front panel through its connectors. Retaining nuts are not absolutely necessary. The second, smaller PCB is mounted to the big one using a board-to-board connector. Its LEDs extend through the cover. The cover can be designed to snap in place. It can also help retain the top PCB. There is no need for threaded fasteners on this box.
Cables are expensive to build and test. They must be routed through your system, and probably tied down somehow. Cable ties, like fasteners, must be documented, ordered, purchased, stored, kitted and installed.
Connectors should be inserted through the inside face of a panel, as shown to the right on Figure 5. The two connectors and their wires are a sub-assembly that can be built and tested outside the box. Manufacturing has the option of subcontracting the cable assembly to an outside vendor who specializes in this. The cable is built in parallel with the chassis, so it is not part of your assembly schedule critical path. Your service people have the option of replacing the cable, rather than repairing the wires in situ.
If the connector is installed through the front, as shown on the left in Figure 5, the wiring will have to be done in place. This is less flexible for manufacturing.
Unfortunately, front mounted connectors look better. If looks matter, you may have to live with the inconvenience of a front mounted connector. Sometimes, you can treat the panel as part of the cable!
Consider designating an ugly panel in your otherwise stylish product. Install your ugly external components on the ugly panel. Methodically follow all the DFMA rules on the ugly panel, including the rear mounting of connectors. If your rear, ugly panel connector is small enough to fit through the front hole and the front connector threaded ring, you have a removable cable assembly, with all the advantages noted above.
A handy point is not shown on Figure 5. Connectors with screw-on nuts, as shown, usually are designed to be inserted into “D” shaped holes. This constrains the connector from rotating. All the assembler needs to do is wrench the nut. This is good DFMA practice.
Look for pinch points when hardware is installed on top of cables. These can be difficult to assemble, and they can be dangerous.[15].
Design your parts so that there is obviously only one way to install them. This prevents mistakes. It minimizes time examining drawings, and phone calls to the design office.
Make things obviously asymmetric as in Figure 6A.
Add orienting features. This can be a flat on a diameter as in Figure 6B. On a pair of castings, it probably costs nothing to add orienting arrows.
If you have a rectangular hole pattern or a pitch circle, move one hole out of the pattern, also shown in Figure 6B.
Note how we design for assembly, before we design for fabrication. When you are building your widget, obviously, you do fabrication first. When you are designing, you solve all the requirements, you work out the assembly process, then you design the parts and work out fabrication.
Rule #1: Talk to your fabricators. They are the experts on how your fabrication generates cost.
Any parts you design must be fabricated. In theory, you prepare a drawing of the part, and manufacturing figures out how to make it. In reality, the designer selects a fabrication process,11 uses its advantages, and copes with its limitations. As a designer, you must understand fabrication processes. These are described in quite a bit of detail in Product Design for Manufacture and Assembly[2].
The following list is nowhere near complete. It provides a general understanding, and it shows off the strategic thinking you must do to select processes.
Tips on Designing Cost Effective Machined Parts, by Joe Osborn[5] is an excellent reference on dealing with machine shops.
Machining ranges from accurate to extremely accurate. Machining wastes material, especially with large, cut-from-billet parts. Part by part, machining is expensive. There is minimal tooling required to do it, so machining is an economical way to produce single parts. If accurate features are required, consider designing your widget around one accurate part. All the other pieces in your design can be fabricated from cheaper processes.
Generally, if you use any process other than machining, you need to work around loose tolerances. In any sort of production, this is worthwhile.
Corners of machined pockets must be rounded. A larger corner radius allows for a larger, faster cutter. A small radius means a small cutter that cannot withstand the side loads of heavy cutting. It may take several passes to cut a deep pocket. Through holes should be rounded too, although there are ways to fabricate sharp, through corners. Study your part and count the number of setups on the mill or lathe. Each setup is expensive. Orient your machining radii to not force extra setups. Almost all machined parts today are fabricated with CNC, so tool changes are done automatically, quickly and cheaply. Machining easily produces orthogonal parts. Non-orthogonal angles require tricky setups, and/or very careful design and documentation.
The material cost and machining time of a housing machined from billet varies with the cube of its size. Small housings can be very cheap. Large housings probably should be fabricated from pieces, from sheet metal, or cast.
Beware! Machining allows bad drawings, since the tolerances required to make your part work are well within the process’ capability, even if the drafter did not call them up. If you have been machining all your parts and you have started using other processes, your drafting may have to improve.12
In Figure 7, the part shown can be fabricated in one set-up on a lathe. The front and side features can be fabricated by the cutting tools, shown. The part can be cut off by the tool shown. Any other features on the inaccessible face require the part to be re-chucked in an extra process, at extra cost. Material is often fed through the rear of a lathe chuck. The part is fabricated and then cut off. The material is then automatically fed, and the next part fabricated. According to Joe Osborn[5], round parts often are CNC machined by vertical mills. The rear face still is not accessible without the extra set-up. When you design round parts, try to ensure that all your features can be machined from one side.
The assemblies in Figure 8 show a number of design issues. Both assemblies hold the same printed circuit board, contained in a machined box. The box on the left requires at least four set-ups on a vertical mill. The first set-up machines out the insides. The second and third set-ups do the holes on the sides. The fourth set-up drills and countersinks holes on the bottom. The box on the right requires one set-up only. The tapped holes and access to the connectors all are machined from the top.13 On the right, the pan head screws through the cover, do not require countersinks. The covers are flat, and they can be fabricated more cheaply by punching from sheet metal. The countersinks on the left would be a rapid, automatic tool change on a CNC mill, but they are an extra process in a sheet metal shop. The bottom screws and the hexagonal standoffs on the left hand view are parts that must be must be documented, ordered, purchased, stored, kitted and installed. The extra features on the right are done as part of the CNC machining process, so the cost is minimal.
Thin, flat panels are cut using punches, water-jets or lasers. Often, the flat panel is bent into some 3D shape as with Figure 9. Holes with sharp corners are easily punched. Pockets must be machined, in an extra process, at extra cost. Countersunk or centre bored holes are an extra process, at extra cost. The panel can be cut out accurately. Sheet metal bending is much less accurate than punching or machining. You must talk to your fabricator and ensure you understand their tolerances. If the bends are not specified and fabricated properly, they crack.14 Make sure you specify proper bend radii. Make sure the fabricator follows your drawings. Sheet metal parts can be assembled in the shop by riveting or welding to create more elaborate 3D shapes.
Operation | Tolerance |
Punching | repeatable to .004”. |
Punch feature to punch feature | .005” |
Formed bends | 1/2∘ |
Hole to edge | ±.010” |
“Standard” sheet metal tolerances | ±.060” |
Punched holes | ±.003” |
1Hole to bend | ±.015” |
Bend to bend | ±.020” |
Table 1 shows tolerances from a precision sheet metal shop.[13]
If a large number of smaller sheet metal parts are required, the fabricator may build a progressive die. The material is provided as a roll of sheet, it is fed into the die, and punched and bent, at a very high rate. If you need lots of parts, make a point of understanding progressive dies.
Sheet metal allows the fabrication of large, hollow, covered structures, with minimal waste of material. A carefully designed sheet metal box can be extremely strong and rigid. Attachment points should be located close to gussets and walls.
You can specify accurate hole patterns, but you cannot locate them accurately from bends. Read up on composite positional tolerances in your GD&T text.[7] Make sure the composite tolerance works in your design. It is possible to machine an accurate hole pattern, after bending, as an extra process, at extra cost.
If you are making lots of components, investigate the packing of your flat punch-outs on the metal sheet. If you design your components to pack tightly, you reduce your scrap rate. 15
You can weld sheet metal. You can burn, water-jet or laser cut metal plates and weld them. You can weld structural sections like tubes, angles, I beams and wide flange beams into a space frame.
Welding is not an accurate process. You need to talk to your welder and ensure you specify fabricate-able tolerances on your drawings. If you need accurate tolerances, you need to machine your weldment, which is an extra process, at extra cost.
Make sure the welding equipment fits into the space where you want your weld.
Many commercial metals either are heat treated or work hardened. When you weld them, they become annealed. Stress relief and heat treating each are an extra process, at extra cost. These processes are not necessarily feasible. If the material is tricky to weld, it must be fabricated by a better trained, more expensive worker. Know your metallurgy.
If your part is a safety critical structure, you may have to worry about workmanship, quality standards, and leaving a paper trail.
Welding allows all sorts of elaborate non-orthogonal structures at low cost. If these are carefully designed, they can be extremely strong and rigid.
Casting requires expensive tooling. This must be amortized over your production run. Design changes require modifications to tooling, or new tooling. Casting patterns can be produced by rapid prototyping, making one-off prototypes feasible.
Compared to permanent moulding and die-casting, sand and investment castings use cheaper tools, making them suitable for shorter production runs. The latter two processes can cast high melting temperature materials such as steel. Casting dies usually are made of steel, restricting the cast material to low temperature metals such as aluminium and zinc, plastic, or investment casting wax. Try to make the dies two-piece, and make sure your part can be ejected from them.
Once you have paid for tooling, castings are cheap. Castings can be very complex with little impact on cost, at least once the tooling is paid for. A cast or moulded housing can be styled with all sorts of weird, cool looking non-orthogonal shapes.
Castings can be machined to provide accurate features and small holes, but this is an extra process, at extra cost. Try to make your design work with the as-cast features. For example, probably, you cannot cast clearance holes for M4 screws. Maybe you can cast holes for M8 screws!
Castings and mouldings can be made from anything that can be melted into a liquid, including thermoplastics. Thermo-setting plastics and fibre reinforced plastics also are fabricated with moulds, and the comments above, apply.
When you do castings, you should do aggressive DFA,16 reducing the part count, taking advantage of the casting process, and looking for an optimal layout for assembly.
Rapid prototyping, RP, is a process in which a part is constructed in some sort of 3D raster form, from a computer 3D model. The material normally is some form of plastic. The resulting part(s) can be used for design visualization, as functional parts, or as casting patterns. There now are RP machines that produce metal parts.
The popular technology seems to be Fused Deposition Modelling, or FDM. A nozzle is mounted on a precision XYZ translation state, and molten thermo-plastic is squirted out into the final 3D form. The author has encountered FDM machines of varying quality. Any FDM part has a basket weave finish. Maybe you can sand down external faces that must be smooth! The higher quality machines have smaller, more precise nozzles. Other RP processes can do very smooth surfaces and full colour parts, but the plastic may not be functional for you.
The part must be retained in place on a platter as it is created. This will limit your design capabilities on the cheaper machines. A more expensive machine the author has encountered has an additional nozzle that creates a support structure out of plastic with a very low melting temperature. This is dissolved away once the rapid prototyping is complete, and the support plastic disposed of. I believe it went down a drain somewhere, which may not be acceptable to you.
In spite of being called “Rapid Prototyping”, the processes are very slow, and not suitable for mass production.
The fancy machine the author encountered used a platter assembly which was consumed in the RP process. Will these be available when the manufacturer stops supporting the machine? You want to know how standard the coils of plastic raw material are, otherwise, these may disappear along with the manufacturer.17 It would be nice if somebody made a device that recycled your plastic into raw FDM material.
Consider not buying the RP machine. There are Rapid Prototyping vendors out there, who can do multiple processes and who keep their technology up to date. They can run whichever process exactly suits your requirement. Is it worth buying an expensive machine that you use only once or twice a month.
If you are desiging a casting or moulding, consider getting your foundry to do the rapid prototyping. You don’t need the rapid prototyping machine. You do need feedback on how manufacturable your part is.
When fabrication or anything else, is subcontracted, each part appears on the BOM as a single item, which must be documented, ordered, purchased, stored, kitted and installed. The single BOM entry could be hiding quite a bit of process. At a management level, this probably is a good thing. As designer, you need to be aware of all the production steps.
A suggested DFMA analysis is to capture the BOM as a spreadsheet. For each part, mark out the number of steps needed to get it all the way to installed on the final product. Expand the subcontracts. Whether or not you do the work yourself, you pay for it. As designer, you need to understand the number of process steps you are forcing.
Many products require service and maintenance. Your widget may require extensive testing as part of the production process. Service may be done under warranty. Either you pay for this directly, or the service is part of your customer’s total cost of ownership. This may be done by your own highly trained personnel, or it may be done by untrained customers.
Service components should be located in accessible positions.
Fasteners, adjusters, and anything that requires observation, should be oriented towards where they will be accessed.
Probably, covers should be convenient to remove.18 Try to minimize the number of screws.19 Consider using quick release and/or captive fasteners.
There should not be any component other than a cover between your service people and the components to be serviced.
You should be able to turn the system on and operate with covers on or off, and you should be able to install or remove covers while it works.
The covers should not affect critical system alignments.
At the end of the project, a designer submits documentation. Documentation is not the only skill a designer must master, but it is a link in a chain. Documentation is necessary for manufacturing, and to support continued engineering. It also is a valuable tool during the design process. You need to inform the rest of the project team what you are doing. If you use the official documents for this, you have done the official documents, and you don’t need to re-do them. Your team must conduct effective design reviews. If assembly drawings, even very preliminary ones, are provided, the assembly process can be reviewed.
With 3D CAD, you can generate fabrication drawings early in the design process with all the critical tolerances. You can review the tolerances and do tolerance stacks. These do affect your design.
Manufacturing can cope with bad or non-existent documentation. You can telephone back and forth and hold meetings. You can show your fabricator how the assembly works, allowing them to work out functional dimensions and tolerances. Ultimately, manufacturing can generate its own documentation. You, the designer, will not be in control of it. Is that a good thing?
When you subcontract work, you need a well defined work statement. You have a purchase order. Your purchase order calls up your drawing or document. Your purchase order is at least approximately a contract, and your document is a clause in that contract. Prepare your documents accordingly.
There is no free lunch. All of the comments above about things that must be documented, ordered, purchased, stored, kitted and installed, apply whether you do the work in-house, or subcontract. In the long run, you pay for time wasted on the phone clearing up confusion. If 10% of your stuff needs re-work, this will be built into your price. If you are difficult to deal with, the shop can always refuse your business.
There is more about subcontracting under the Notes.
In anything more than one-off fabrication and assembly, you have a department that orders material and parts, that maintains a warehouse, and that kits parts for whoever is doing the assembly.
Any item moving in and out of a warehouse is a financial transaction. Warehouses are controlled by accounting, not engineering. The people who do the actual work are low-level clerks with very limited authority. They follow procedures. It can be amazing to see how much stuff they are keeping track of. Your design and documentation must work with these procedures.
The following is a limited production scenario designers need to understand.
Parts are stored in the warehouse by part number or stock code.
When a widget is to be built, manufacturing reads the BOM, and searches through the warehouse for the parts.
Any parts not found, must be ordered.
Anything with a vendor and part number will be ordered from that vendor.
Anything with your company part number on it will have a document keyed to that part number. The document must be examined for ordering instructions.
A fabricated part will be ordered from the appropriate vendor, which purchasing must identify somehow.
An assembly will have its own BOM, which will be processed.
A specification control may have ordering instructions on it.
All the parts will be kitted for assembly.
If you have documented sub-assemblies, manufacturing can assemble these ahead of time and put them back on the warehouse shelves. This is convenient when the sub-assembly is used on multiple products. It is convenient if the sub-assembly requires a different specialized worker than the main assembly.
Mass production processes are very much more elaborate.
Any modification to your widget must be performed and documented methodically, if you are not to create chaos in manufacturing. You need to follow design change rules.
For any given part or assembly with a drawing and part number, do not change form, fit or function.
If you must change form, fit or function, you must create a new part number.
The revision number on your drawing should not be contained in your part number.20
Anything called up by part number on a BOM must function in whatever assembly the BOM belongs to. Obviously, if you change the length or you delete holes, the part is no longer functional in existing assemblies. If you add holes, your new part probably is functional in existing applications. On your new assembly that requires the holes, the old parts are not functional. If you have these in stock, these are what will be kitted.
If manufacturing trusts you to follow the above design change rules, they can stock everything by part number. If they cannot trust you, everything must be stocked by part number and revision number.
You are part of a team. Most engineering and design projects are multi-disciplinary. You need to talk to co-workers. You need their co-operation.
You and your co-workers need to share information. You can set up blog sites and wikis. Microsoft Project stores and manages documents. The author likes to set up websites in his project folders.21 It would be nice if the design team all sat in the same area, where they could see each other, and get in the habit of talking.
Manufacturing is part of your design team. They need to sit in on the design reviews. You need to understand their requirements.
Design For Manufacture and Assembly means that engineering does something more than just make the thing work, somehow. Management must commit to the engineering time needed to make the thing fabricate-able and assemble-able.
Figure 10, comes from Product Design for Manufacture and Assembly.[2] Their reference is Munro and Associates http://leandesign.com, with no mention of a publication.22 This is generic data that must vary wildly from company to company and product to product. Regardless, it shows that the design department’s direct cost is very much less than its effect on the overall product cost, especially if there is significant production involved. Extra time and effort taken by designers to assure efficient manufacturing will reduce cost and improve quality.
Mechanical design time should take some fixed percentage of the total production cost of the widget. It does not matter if you make a hundred thousand assemblies at ten bucks each, or one piece at a million dollars. The chart in Figure 10 shows 5% of total cost.
A ten dollar assembly may seem simple, but every cent saved will be multiplied by 100,000. Design and evaluate multiple versions of the widget. For each version, thoroughly study and plan the process. This should not wait for the completion of the design.
The mechanical design department must develop design rules, standardizing hardware, and anticipating requirements that occur over and over again in your industry.
If design is done in sequence, then task A is completed. Task B starts some time later. It is not possible for task A to recognize and solve task B problems, or capitalize on opportunities created by task B.
Concurrent design means that all the design tasks are performed simultaneously. This is messy, and it requires a lot of communication and teamwork. That communication and teamwork creates all sorts of opportunities to be clever and implement DFMA.
Your drawings are how you communicate with the outside world. When you subcontract work, you need clear specifications, and you don’t want shops wasting time trying to interpret your stuff. For anything done in-house, you need to set an example of how work ought to be done, and you still don’t want the shop wasting time trying to interpret your stuff.
Joe Osborn spends over a third of his article Tips on Designing Cost Effective Machined Parts[5] on Drawings and Prints. This should not be surprising. In a jobbing shop, the time spent reading your drawings is part of the fabrication cost. Time is wasted when they cannot find information quickly. Time and material are wasted when they make mistakes. The shop may be willing to do free re-work for goodwill’s sake, but they will account for it when they quote future jobs.
Are you still using 2D CAD? Change your background screen from black to white. Do it now. We’ll wait! This is the only possible explanation for Figure 11, an issue noted by Osborn.
Drawings now are sent out electronically. With 3D CAD like SolidWorks, a common process is to send the drawing out as PDF, accompanied by a STEP file of the 3D model. Full sized prints are nice, but they do not fit through email, and you may not have a 36” plotter.
If you are sending out files in PDF format, assume your vendor has a letter sized23 black and white printer. Your colours may generate an unreadable half tone. Generate PDF files in black and white mode. There are lots of printers out there that do B or A3 size. Maybe your fabricator has one! The author has not seen a machine shop with a full sized plotter.
See Drawing Sizes and Lettering, in the Notes, below.
Be careful sending out DXF files. These provide reliable geometry. Most CAD packages use their own special font to implement GD&T symbols. These may or may not work on your fabricator’s CAD.
In The Quest for Imperfection[8] Charles Murray states that Japanese car manufacturers do not inspect their components as carefully as their American and European counterparts, yet they achieve higher quality and reliability at assembly. This was presented as a counter-intuitive example of oriental inscrutability. This author suspects that this is an example of good drafting practice. The Japanese send out drawings with achievable tolerances. The vendors charge less. Everybody inspects less. The assemblies are designed around the realistic, looser tolerances.
Your specifications, fabrication drawings and defined models (MBD?) must be inspectable. You must be able to inspect each part and confirm that either it conforms to your requirements, or it does not.
Drawings are inspectable when…
…all features are controlled by dimensions and tolerances.
…all tolerances are achievable. If the shop is making a “best effort”, you have no control over the final part.
…drafting, and dimensioning and tolerancing standards are followed. Standards such as ASME Y14.5[7] provide an unambiguous definition of the drafting terms and symbols.
…critical features are accessible for inspection. At design time, you should be able to describe an inspection procedure. Perhaps you need to design an inspection fixture!
If you persistently call up unfabricatable tolerances, your vendors will learn to not take your numbers seriously. They may be encouraged in this by your purchasing department. Vendors who run high level quality programs, may have to refuse your business.
Data security – For some reason, your CAD software is no longer available. From complete drawings stored in a format like PDF, you can re-model and re-draw your parts in your new CAD, correctly to scale. This is a good training exercise for your drafters and designers.
The bill of material is part of your design and documentation. Obviously, the BOM will be used to create requisitions and purchase orders. Not so obviously, information moves from engineering, to the purchasing/accounting department. It would be nice if information did not have to re-typed at each transfer of responsibility. If your BOM is set up in the same format as your requisition form, you can copy and paste. If your BOM is in the same format as your MRP/ERP database, you can copy and paste, or electronically transfer the data. This saves time and it eliminates errors.
Type once, only.
You do not want non-technical clerks re-interpreting your BOM entries. You and your accounting department must agree on a data format. Accounting must trust you to fill in BOM entries correctly. Talk to them. Even if they don’t want to copy and paste or electronically transfer, create the opportunity.
Databases tend to have fixed length fields. They tend to have separate fields for vendor and/or part number and/or description. BOM entries must conform to this. The author would like it if BOMs and databases had a single description field sixty characters long, but he has not seen it. If your own company part number is called up, someone must pull out a document, read it and follow the instructions. If the manufacturer, part number and description fits in the database field, everything is simple. Sometimes, the manufacturer’s information is too long, or their ordering process is too complicated to fit in your BOM entry. You can write up ordering instructions on your specification control, and call the part up by your own number.
ITEM | DESCRIPTION |
1 | CAP SCR HEX SOCK .190-32X0.63 SST |
2 | HSCS SS 10-32X5/8 |
3 | MCMASTER CARR 92196A271 HSHCS |
4 | HX SCK CAP SCR ST STL 10-32UNC X 5/8 |
5 | SPAE NAUR 370-053 HX SK CP SCR 10-32X5/8 |
6 | HEX SOCHET CAP SCRW STSTL 10-32X5/8 |
7 | HXSCK KPSCR 1032UNFX.62 SS |
8 | CAP SCREW ST STL 10-32X5/8 |
All of the BOM entries on Table 2 describe the same thing, a hex socket head cap screw, stainless steel, 10-32UNF X 5/8. The typos are deliberate. Some people have eccentric ways of writing things. Some people are trying to fit things in small database fields. Some people do not like typing. Some people can’t spell. A clerk must order parts, writing a requisition that may have to be interpreted by another clerk working for the vendor. Yet another clerk will set up MRP/ERP and assign stock codes and warehouse storage, possibly to each and every part above. The person doing the actual assembly must be able to identify the parts on the work bench in front of them. Probably, they don’t have a McMaster Carr catalogue at hand. Something called a “cap screw” could be a hex socket head cap screw, or it could be something with a hexagonal head.
If BOMs are prepared manually, create copy-and-paste lists for standard parts like screws. This assures consistent BOM entries, and it reduces hunt and peck typing. In 3D CAD, use the automatic BOM feature, and set up libraries of standard components with correct BOM entries.
Obviously, 3D CAD provides 3D visualization of the design. Lots of people are not good at reading drawings.
A big benefit of 3D parametric CAD, as opposed to 2D CAD or drafting boards, is that drawings update automatically as the models are changed. Assembly and fabrication drawings can be generated early, and used to communicate with co-workers. An exploded assembly drawing shown off in early design reviews, shows manufacturing how you intend to assemble your widget. Your assembly procedure gets reviewed early in the project, when it is easy to make changes. On fabrication drawings generated early in the design process, you can apply tolerances to the critical features. Tolerance stacks tell you whether or not your design can be fabricated and assembled. You can change your design to eliminate the accurate features.
3D CAD generates Bills Of Material (BOMs). Again, these can be generated early, providing a view of parts to be ordered. This is absolutely superior to bills of material written out on the backs of envelopes, used to generate purchase orders, then discarded. It is also absolutely superior to bills of material generated several months after you delivered your product, or several months after manufacturing has worked up it own BOMs.
Insert fasteners and other standard hardware into your CAD model. These must be called up on the BOM. Attaching the fasteners reveals all sorts of design and assembly problems. Sometimes, they are inaccessible. Sometimes, there is no room to install them. When you see how many different fasteners you have, you can change your design to standardize them a bit, or a lot!
Do not cut corners when modelling and drafting. You can send 3D models out to the shop and skip the drawings, but Joe Osborn[5] recommends drawings. Model Based Definition (MBD) is not an efficient process in limited production jobbing shops.
In higher production shops, fabricators read your drawings, and prepare their own fabrication drawings. Your drawings should not specify fabrication procedure.24 Your drawings should specify what you will accept from the vendor. Let the vendor figure out how to do stuff. An alternate MBD procedure is to mark only the critical dimensions and tolerances on drawings. The fabricator receives the drawing and the 3D model, which is enough information for them to prepare their own drawings. It is understood that the specified dimensions will be inspected, and that everything else is less critical.
The author’s experience mostly is machining and sheet metal. In 3D, he prefers to model at nominal size, 25 and apply tolerances. If your vendor is not reading your drawings and they are working off your model or DXF copies of your drawings, you may have to model at median size. Either way, you need systematic CAD practice. The shops need to understand and trust how your CAD models work.
This is all stuff not directly pertaining to DFMA, but interesting and relevant.
One summer in the 1970s, the author worked in a factory shipping office. The forms from a trucking company stated that the drivers were not required to lift more than seventy pounds (32kg). Back in the day, truckers were assumed to be manly men!
In today’s work environments, you have to account for tiny women. On a loading dock, you may get away with requiring some minimum strength. If you are moving and testing high technology or scientific apparatus, somebody may not have factored heavy lifting into their choice of doctoral thesis.
Handling Function | Male and female | Male only |
Lift an object from the floor and place it on a surface equal to or greater than 152cm (5.0ft) above the floor. | 14kg (31lb) | 21.9kg (48lb) |
Lift an object from the floor and place it on a surface not greater than 152cm (5.0ft) above the floor. | 16.8kg (37lb) | 25.4kg (56lb) |
Lift an object from the floor and place it on a surface not greater than 91cm (3.0ft) above the floor. | 20kg (44lb) | 39.5kg (87lb) |
Carry an object 10m (33ft) or less. | 19kg (42lb) | 37.2kg (82lb) |
MIL-STD-1472G[4] contains specifications on what people should be able to lift and push and pull around. Since this is a US military publication, you should assume that their people are younger and physically fitter than your people. Table 3 comes from MIL-STD-1472G, Table XXXVIII.
The nearest thing MIL-STD-1472 has to single handed lifting is their Table XLI Static Muscle Strength. A 5th percentile woman can exert a mean force of 103N or 23lb, doing a standing one-handed pull.26 103N is the weight of a 10.5kg mass. If her arm is extended (cantilevered), the forces must be much less than this.
Almost certainly, you cannot constrain your lifting to males only. Table XXXVIII is not for repetitive lifting. For six lifts per minute, the standard recommends reducing these forces by 50%. Reduce it further if the worker must exert these forces repetitively, day after day, week after week.
Most of the author’s experience has been with laser optics and remote sensing. These are complex, expensive devices requiring all sorts of test procedures to get them working. They require maintenance and repair, later. Some of this is done under warranty. Optics require alignment, which must survive continued assembly, and use in the field. Your industry probably is different, but similar design rules will be feasible, and a good idea. Regard the following as an example, not as a set of hard and fast rules.
Covers are surprisingly difficult to design. Do not leave this for last.
Covers have multiple functions, providing protection from dust, liquid, EMI/RFI, and people’s fingers. The cover may shield people from things like fans and class IV lasers. The cover may have to look good. These requirements must all be managed simultaneously.
Covers have to be attached somehow, so they are an integral part of your structural design. They are part of your plan for assembly and access.
You ought to be able to power your system up and run it, and be able to take the covers on and off without disturbing any of this. Do not attach components with wires and hoses, to removable covers. Hinging a door allows you to attach cables and hoses, and not strain the connections.
If things, such as optics, must be kept aligned, the structure must be very much more rigid than the covers. Alignment must not be affected by the installation and removal of the covers.
Making it look cool can impact fabrication and assembly.
Style costs more. For any given product package, there is a way to do it that is cheap, functional and ugly. Pretty things are expensive. Good styling may (have to) compromise easy assembly and access. Get management to commit to a budget.27
Keep it simple. You are engineers and mechanical designers, not artists. Don’t put curves on things for the sake of putting curves on things.
Bad styling is worse than unstyled functional. You tried. You screwed up.
Assemblies with edges that must line up accurately, are difficult and expensive to produce by processes other than machining. If your styling strategy is to have significant gaps or overhangs, inaccuracies will be harder to see, and manufacturing of an attractive, professional product will be easier and cheaper.
Matching colours is an expensive challenge if components are coloured by different processes. Paint, moulded and coloured plastics, coloured anodizing, and paint from some other shop, will produce matching colours only with a lot of effort. Contrasting colours can be made attractive, with a lot less expensive hassle.
Like style, lightweight costs more. Are you sure you are adding value to your product?
Costs of lightweight design…
Design time
You can reduce weight by reducing structural safety factors. These sometimes are called factors of ignorance. You need to do rigorous structural calculations so that you are less ignorant.
Materials
Lightweight materials are more expensive, and difficult to fabricate.
Fatigue
If your structural components are subject to fatigue, their lifespan is finite. There must be a maintenance procedure in which your product is inspected and/or its use methodically logged. The component must be replaced, or your entire product decommissioned. This all is part of total cost of ownership.
Risk management
Does your product have a catastrophic failure mode? Is it really worth the risk? Errors and ommission insurance, and lawsuits are costs too.28
How well do you understand the structural loads on your product? How good are you at structural analysis, really?29 If using some exotic material, how well do you understand it, and its fabrication and limitations?
Many structures such as optical mounts and tooling fixtures, need to be rigid. If they are rigid enough, they are very strong. Make sure you are solving the correct problem.
If you are diecasting or injection moulding, a significant reduction in weight makes for a reduction in material costs. This may be worthwhile over a long production run.
If a customer is primarily concerned about the product being sturdy and reliable, light weight may not impress them. This is especially true if structural failure is perceived as catastrophic.
Lots of factory equipment is designed on the assumption that somebody will drive a forklift into it.
Consider the following when you design title blocks and drawing standards.
American | Metric | Font Size | Remark |
A size | A4 size | 1/8in (3mm) | Letter sized sheet |
B size | A3 size | 1/8in (3mm) | A sized prints are easily read |
C size | A2 size | 1/8in (3mm) | A sized prints are readable. |
D size | A1 size | 5/32in (4mm) | A sized prints are marginally readable. B sized prints and PDFs are readable. Full sized prints hanging on a wall are readable from a distance of 6ft (2m). Full sized prints probably don’t fit on desks and workbenches. |
E size | A0 size | 5/32in (4mm) | A sized prints are not readable. B sized prints and PDFs are marginally readable. Full sized prints hanging on a wall are readable from a distance of 6ft (2m). Full sized prints are unlikely to fit on desks or workbenches. |
Table 4 shows something the author saw somewhere in the 21st Edition of the Machinery’s Handbook, but he cannot find it now. The “Remarks” are the author’s. Font sizes matter.
Full sized plotters are becoming rare. Why create drawings you cannot print 1:1?
If the author were setting up a new CAD and documentation system there would be only two title blocks, A sized portrait, and B sized landscape. These fit in a three ring binder. Your plant or your vendors may someday equip workers with computer stations or laptops to access drawings.
The font size .08in or 2mm is readable on full sized copies, and fairly readable on B size printed scaled down on A sized sheet. It is easily read on a 1080p monitor (1920 X 1080). It is marginally readable on the 1366 X 768 display of a cheap laptop.
The font size .10in or 2.5mm on a B size drawing is readable on an A sized print, and on the cheap laptop.
This is an expansion of some ideas, presented above under Subcontracting.
When you send drawings out for subcontract, you need them to be as clear as possible. On a one-off project, you can talk to the vendor, and provide extra information. In production, the clerical staff in the purchasing office must reproduce the conversations, and identify and produce any extra drawings required by the fabricator.
“We have to sell parts!” – Joe Osborn[5]
There was a discussion on Eng-Tips[11] on critical dimensions on drawings. Some manufacturing engineers described their need to understand the function of the parts they were fabricating and inspecting, so that they could identify the critical features. Let’s assume we send out the assembly drawing.
Purchasing needs to know that the assembly drawing must be sent out. This is not obvious from the BOM. The purchasing database probably lacks the resources to store information like this. Do you want purchasing to methodically send out all assembly drawings? This process could be a lot of effort.
If fabrication requires the assembly drawing, the entire order must go to one shop. This is not very flexible for purchasing, especially if the fabrication processes are varied. Shop A may be good for your machined parts, and shop B good for sheet metal.
Both shop A and shop B can cope with the order, and both receive purchase orders. Do they solve assembly problems the same way? Are the resulting parts interchangeable?
Your assembly drawing may contain information that the fabricator does not need to know. It could be your proprietary technology. It could identify your customers. It could be something classified as a national secret. Do your fabricators and their employees understand your data security?
All of this hassle goes away if you are good at drafting.
Do your job.
Be the go-to person for getting your job done, and for providing your expertise. If someone else has to step in and do it, don’t go looking for respect.
Out in the real world, when you ship out a poorly documented, non-functional design, production does not appear at your desk to make puppy-dog eyes at you. They generate the documentation themselves, and they debug the design if they have to. Having done all of this, they take control of the design and documentation, and they will have very little concern about any opinions you may have. It is too bad if, as design engineer, you are responsible for quality and safety. Mechanical design is a popular target for micro-managers. Don’t encourage them.
As mechanical designer, you are mostly in control of your work and of your reputation. You can set an example of how work ought to be done. Any control you have over production and sales is due to the respect they have for you.
If mechanical design, the other engineering departments, manufacturing and accounting respect and trust each other, there are all sorts of opportunities to do things more efficiently.
Lean Manufacturing is manufacturing’s job. Much lean manufacturing requires design to be done properly. Lean principles can be applied to engineering and design.
You need to understand what adds value to your product, and what doesn’t. If the process or materials do not add value, you should not be doing it or using them.
Transportation
moving products that are not actually required to perform the processing
Inventory
all components, work in process, and finished product not being processed
Motion
people or equipment moving or walking more than is required to perform the processing
Waiting
for the next production step, interruptions of production during shift change
Overproduction
production ahead of demand
Over Processing
resulting from poor tool or product design creating activity
Defects
the effort involved in inspecting for and fixing defects.
Kenneth Leung adds not using your employees Strengths, and he provides the mnemonic TIMWOODS[14]
Lonnie Wilson lists the wastes in a different order. Any inventory that does not support sales, is waste. Waiting means simply that your workers are not working. Defects are scrap, upon which you expended time and cost.[9]
Careful study of manufacturing during design creates opportunities to plan an efficient production with minimal transport, inventory and motion around the shop.
Waste | Cost | Mech design |
Transport | time | too many steps |
Inventory | interest charges | long lead items, tooling |
Motion | time | too many steps |
Waiting | time | |
Overproduction | inventory | tooling |
Over processing | time | too many steps |
Defects | material & time | accurate tolerances |
Table 5 shows how mechanical design forces waste in production. If there are too many parts, or parts require too many setups, time is wasted in transport, extra motion, and over processing.
If your manufacturing process requires expensive tooling and set-up, there will be a need to do large production runs, creating lots of inventory, and the above mentioned overproduction. Any production tool is part of somebody’s inventory.
Over processing happens when your design requires extra manufacturing steps. Try to design your castings and weldments so that they do not require clean-up machining. Minimize the number of set-ups on your machined parts.
Defects happen when the manufacturing process cannot reliably meet your tolerances. Do your tolerance stacks. Open up clearance holes and loosen critical tolerances. Difficult tolerances require the manufacturing process to run more slowly and carefully. If a significant number of parts do not conform, they all have to be inspected. Inspection is not a value added process.
Defects also happen when your documentation is bad, and people have to work without clear instructions.
Continuous improvement is a useful concept for design and engineering, as well as manufacturing.
According to Lonnie Wilson[9]…
What is the present state or condition?
What is the desired future condition?
What is preventing us from reaching the desired condition?
What is something we can do now to get closer to the desired condition?
What is our expectation when we do this?
What will happen?
How much of it will happen?
When will it happen so we can “go see”?
[1] NPD Solutions http://www.npd-solutions.com/dfmguidelines.html
[2] Product Design for Manufacture and AssemblyGeoffrey Boothroyd, Peter Dewhurst, Winston Knight. Marcel Dekker Books
[3] Institute of Scrap Recycling Industries http://www.isri.org/about-isri/awards/design-for-recycling https://www.google.ca/search?q=design+for+recycling
[4] MIL-STD-1472, Department of Defence Design Criteria Standard Human Engineering. As of 2016/07/13, this seems to be at revision G. You can Google and find PDFs of this online.
[5] Tips on Designing Cost Effective Machined Parts by Joe Osborn, OMW Corporation, http://www.omwcorp.com/tips-on-designing-cost-effective-machined-parts/
[6] Cumberland Diversified Metals http://www.cumberlandmetals.com/aluminum/minimum-bend-radii/
[7] ASME Y14.5-2009 Dimensioning and Tolerancing. American Society of Mechanical Engineers This is the ASME standard for geometric dimensioning and tolerancing (GD&T). There are textbooks out there, but this official standard is very readable. Earlier versions of the standard are ASME Y14.5M-1994 and ANSI Y14.5M-1982.
[8] Design News article The Quest for Imperfection, by Charles J. Murray. 2005/10/10. https://www.designnews.com/automotive-0/quest-imperfection/8576051453366
[9] How to Implement Lean Manufacturing, by Lonnie Wilson, McGraw Hill
[10] ASME Y14.100-2004 Engineering Drawing Practises, American Society of Mechanical Engineers. This an updated of the old DOD-100 standard.
[11] Eng-Tips.com post and discussion http://www.eng-tips.com/viewthread.cfm?qid=322065 Critical Dimension[s]. “… what is considered as ‘Critical Dimension’ and how should I select it?” The author participated in this discussion as Drawoh.
[12] Machinery’s Handbook 26th Edition, Industrial Press
[13] Vendor communication with Christie Digital.
[14] Lean White Belt Certification by Kenneth Leung, PEng.
[15] The Case of the Treadmill Tragedy https://www.ecmweb.com/fire-amp-security/electrical-forensics-case-treadmill-tragedy Michael Leshner, P.E.