A CAD Best Practice For Fixture Building

Your client wants a fixture built – it could be for assembly or test or EMI shielding or anything else, but there’s the issue of how much detail to include in the CAD model of their hardware. I recommend including more detail than is immediately needed, and if necessary justify it to the client as saving money in the long run.

Why should the client spend for more detailed CAD than they immediately need? Let me answer that question with three questions:
1. What are the chances that any fixture will have revision requests either during the design phase or sometime afterwards?
2. What are the chances that you’ll end up designing other fixtures for that hardware or revised versions of it?
3. What are the chances that any of these revisions or future projects will be less likely to have errors if more detail had been included in the model up front?

These are loaded questions but for good reason. The more detailed model provides immediate accuracy-enhancing (and oops-proofing) value to the client. It also better enables you to produce collateral materials such as assembly drawings and instruction manuals with more detail and thus more value. Plus, you’ll be able to produce those collateral materials faster, which the client will appreciate.

Make accurate and sufficiently detailed CAD models:
Include all features – all controls, displays, fasteners that protrude or are recessed or need to possibly be accessed, and all hardware. Fasteners and hardware can frequently be downloaded as CAD models from vendor sites. Features like fasteners, fan openings and vents, if they are flush or inset, can be adequately represented by a very shallow pocket, preferably made to be a contrasting color. Having a color that represents faked or approximated features gives you good control over this. Make sure to document it as such in the feature tree. Plus, this gives adequate information about something important being there, without excessive time being spent modeling the details of the feature.

Degree of feature detail should be proportional to the level or likelihood of mechanical interaction with the fixture, or proximity to the fixture. Generally the outlines of features like connectors and power inlets are sufficient unless there’s a likely need to interact closely with the feature. If you’re creating a model for a pc board and a fixture needs to fit closely around a connector for example, then it’s a good practice to fully model the connector. In many cases CAD models of connectors are available from the manufacturers and this saves time and can further increase the model accuracy.

For PC boards, the level of component detail to include really depends on whether anything about the miscellaneous component placement is likely to risk interfering with your fixtures in areas where there’s no aligning, clamping, probing or other activity that directly requires detail. I’ll generally approximate areas of maximum component height and label them with fake feature colors. This gives a worst-case crude interference check without spending undue time placing every single surface mount device.

Repeat clients will often need something new done with an old design and if you’ve got the data to make quick work of it, then you’re their first choice for future projects.

Rapid Prototyping With A 20X Price Difference

I’ve been lusting after some sort of office-friendly 3D printer for a while and now there are two offerings that are almost worth considering. The Stratasys Dimension uPrint 3D printer and the Makerbot CupCake CNC are both desktop rapid prototyping machines that produce Fused Deposition Modeling (FDM) plastic parts. The uPrint costs $14,900 and the CupCake costs $750. There are of course a few differences.

Always wanted to make your own toys? It just got easier.

Corporate Design versus D.I.Y. The uPrint is a proprietary turnkey machine and the CupCake is an open source kit. Both machines work by extruding a thin thread of molten plastic and effectively drawing a the part layer by layer, but they represent very different design philosophies and serve very different markets. The uPrint is the entry-level model of the Stratasys Dimension series, is a very capable machine within a few limits and is intended to serve serious prototyping needs in a corporate environment with a corporate-type budget. The CupCake is a kit although fully assembled machines can in theory be purchased for $2500 and is intended for the DIY/hobbyist market. The uPrint is a comparatively low-cost entry into a mature Rapid Prototyping technology. The CupCake is a work in process intended for users willing to do a lot of fiddling and fine adjusting, where one can expect continuing improvement and refinement of the hardware and software from both the Makerbot folks and the growing community of users.

uPrint vs. CupCake – finished & functional vs. open & ornamental?

Comparing some specs:
uPrint
Footprint: 635 x 660 mm
Build Area: 203 x 152 x 152 mm
Layer Thickness: .254 mm
Overhangs in parts: Yes with removable support structure material

CupCake
Footprint: 350 x 240 mm
Build Area: 100 x 100 x 130 mm
Layer Thickness: 0.3725mm (see below)
Overhangs in parts: Very limited (see below)

The CupCake has a smaller build volume than the uPrint but there are several more significant limitations:

Overhangs: On the uPrint overhangs are handled by incorporating a support structure which is removed after the model is built. On the higher-end Dimension printers this can be either a breakaway material or a material soluble in a heated caustic solution, but only the soluble version is available for the uPrint. Naturally, Stratasys will gladly sell you the solution and heated agitator tank. All it takes is money. In contrast, there are currently no good ways to handle overhangs with the Cupcake besides building in some support structures and then cutting them away afterwards. If you’ve had experience with raw SLA parts then you’ve seen these sort of scaffolds. The Makerbot website makes mention of some possible workarounds, and of research on a support structure extruder that could run in parallel with the regular material extruder as a future upgrade. Supposedly they’ve intentionally left room for such in the machine.

Resolution: Standard layer thickness for the CupCake is 50% greater than for the uPrint, which makes for a much coarser part. However, the Makerbot website makes reference to a “Nozzle v2″ and suggest that a layer thickness of .25 mm can be achieved by the Cupcake although there’s some deposition speed tweaking involved. Then again everything on the CupCake involves some tweaking but that’s supposed to be half the fun.

Accuracy: This is where a lot of hardware and software development are still needed, at least from the CupCake part images I’ve seen so far. Stratasys seems to have put considerable effort into building 3D printers which produce parts with a relatively smooth and consistent “skin” surface. The CupCake has a long way to go here, with some images of current parts showing random gaps and really rough surfaces. Then again, this is comparing industrial models in professional ads to largely raw parts shot by enthusiastic amateurs. And to be fair, I’ve only worked with FDM parts from Dimension printers, but even those took a lot of work with auto body filler and sandpaper to have something that looked injection molded as a result. But from the images I’ve seen so far, doing that with a CupCake part would be challenging.

Concluding Thoughts and Getting Yours

The uPrint can be purchased, leased or rented by the month. They’ll equip you with one any way they can. The consumables are expensive; a cartridge with 30 cubic inches (just over a pound) of high density ABS in Ivory (the only color available with the entry-level machine although other colors and materials are available for the higher end machines) costs $150. And you’ll need a cartridge of support material too at the same price. The company says to expect to use the ABS and support materials at slightly more than a 2:1 ratio.

The CupCake kits are currently selling faster than they can be built. Makerbot has apparently been swamped with orders which is excellent because makers like this deserve encouragement and success. Good news is that the next iteration of kits will come with assembled pc boards rather than the user having to do their own SMD component placement and then reflow solder on a hotplate. And some of the parts are apparently made on CupCake Makerbots. Their materials selection is ABS in white or black, and high density polyethylene (HDPE) where the natural ABS costs $50 for 5 pounds. No greedy printer industry consumables pricing ripoff here. There is discussion of other materials being available in the future, and a hack where the machine can be used to extrude actual cupcake frosting for custom designed cupcakes. Just imagine the possibilities, but also imagine having two different extruder heads. (I keep thinking of what could be done with chocolate.) Their plastic material comes on a spool rather than in fancy cartridges and from reading the user group forums, it sounds like some amount of babysitting is occasionally needed on the material feeding.

So if you have the money and a real need for slick industrial prototypes, by all means get the uPrint. But if you’re on more of a budget and feeling capable and just gotta have it, then the CupCake looks to be an interesting way to make your own stuff. Currently Design Innovation sends out CAD files for FDM or SLA parts, but this might change.

All photos in this article are property of Stratasys Inc or Makerbot Industries LLC. Makerbot photos may also be covered under the GNU Free Documentation License.

Some useful Solidworks related sites

There are a lot of Solidworks related blogs and websites, a few of which consistently have useful information. I’ve avoided sites with lots of dead links and/or too many links to articles that require info-grabbing “registration” to view. For each of the sites here I’ve selected a particular tutorial or article which exemplifies what’s best about that site. One of the advantages of reading these is finding information on CAD features and methods you didn’t even know to ask about.

CAD Digest Solidworks Tutorials
A fascinating random assortment of articles from a wide variety of sites going clear back to 2000, unfortunately with a miles-of-scrolling user interface. Here’s a clever technique for controlling part thickness.

Solidworks Tutorials
Most of these tutorials are a good assortment of beginner-level material. One notable tutorial is how to do a stress strength test using SimulationXpress, the rudimentary FEA tool that’s included with the basic version of Solidworks. A more comprehensive (and sales-intensive) tutorial on SimulationXpress is found at Solidworksmedia.

The Solidworks Geek
This is a good selection of tutorials, some on obscure but really useful commands. See the sidebar for directories of tutorials by general topic. See the Mates tutorial which includes a good introduction to SmartMates, which can be a useful labor saving method.

These tutorial sites haven’t been updated in a while but are still valuable:

About Solidworks
Another random assortment of articles, none very complex. One good example is Engraving Text On A Curved Surface.

DiMonte Group Solidworks Tutorials
If you only have time to follow one link in this whole article, download and read through the Rebuild Errors powerpoint tutorial titled Trees of Blood. It’s slightly dated but the core information remains completely relevant about how to approach fixing Rebuild Errors and some best practices for avoiding them in the first place, and is likely to save any Solidworks user many hours of grief.

A new topic will start next week.

Strategies For Mechanism Modeling – Part 2

Interference checking with fasteners and springs

Interference checking is one of the CAD functions where the effort of model creation really pays off in preventing some “back to the drawing board” design errors. It also highlights another example of the conflicting principles of Keep It Simple and Build A Complete Model.

Motion sequence for a desktop raising mechanism

Performing an interference check on a model with all the fasteners included will almost certainly generate a long list of interferences that you don’t need to care about, but likely with critical ones scattered through the list. Every interference really does need to be checked, but there’s a way to organize how it’s done which will reduce the odds of missing something critical.

Balance simplicity and completeness in interference checking:
1. While building the model, make a list of all the parts which will have interferences from fasteners. Holes that are threaded, or dimensioned for pressfit pins are prime examples. In fact, any threaded part is likely to have an interference condition when used in a model with other parts.
2. When the model is complete, suppress all parts except the fasteners and the other parts predicted to have interferences.
3. Run an interference check on this subset of the model.
4. Go through the interference list, view, verify and “Ignore” all the expected interferences.
5. Investigate any unexpected interferences and fix them if necessary.
6. Unsuppress the rest of the model and re-run the interference check.
7. Investigate and fix any remaining interferences – these are likely to be the critical ones.

Springs can create an extra set of challenges for interference checking. The two most commonly used springs in mechanisms are compression and torsion springs, but these strategies be used for any spring. I favor creating two models for a compression spring, representing the end conditions that the spring will experience in actual use. These should be independently added to the model, each with its own set of mating conditions. They can now be suppressed/unsuppressed back and forth in the CAD model and with properly set mating conditions will “move” parts and assemblies between end points. Compression springs can generally be modeled as a tube to further simplify the model while still having the part present. Torsion springs are a bit more tricky since their coil changes length and width in actual use. I find it essential to get samples of torsion springs and to measure them at the actual end conditions in which they’ll be used. This kind of real world measurement is far more trustworthy than what a data sheet will tell you.

I favor modeling torsion springs as a pair of coaxially mated cylinders with a spring arm protruding from the end of each as shown below.

Gas springs are best modeled as two pieces, thus allowing them the full range of motion in the model. Make sure to check for maximum and minimum extension and leave safety margins, and verify all of these in the motion model.

When to use collision detection:
Collision detection is used to find blockages in motion paths and to verify stopping points, but works best when used on a carefully selected group of parts in a model. I prefer to use it for a final check on motion paths. Trying to run it on the entire model at once can lead to confusing or spurious results. Start with a minimum group of parts and then add successively more parts that are near the motion path. Here’s a good tutorial on using collision detection in Solidworks.

This concludes the article about strategies for mechanism modeling. A new topic will start next week.

Strategies For Mechanism Modeling – Part 1

Getting your prototype to work right on the first try through good use of CAD

There are two principles of mechanism modeling which are in direct conflict and balancing them is one of the keys to design success:
1. Keep it simple.
2. Build a complete model.

Opening motion sequence for a concept-level laptop prototype.

Keep it simple: The more components that can move, the harder your CAD setup has to work at calculating and the slower it goes. Grouped components like fasteners, washers and spacers which can all rotate freely may really confuse the software (especially when they share an axis with components whose rotation is critical to the model) and should be replaced with distance constraints whenever possible.

Build a complete model: One of the classic “oops” moments is discovering that two parts are trying to occupy the same space at the same time. Even relatively simple mechanical assemblies in CAD can have this happen without it being obvious. If this makes it all the way to hardware sometimes it’s quick work with a file or grinder to fix, and sometimes it’s a fatal design error. Building a complete CAD model including all fasteners, washers and spacers and running interference detection through the motion path is critical to avoiding this.

A winning strategy that combines both:

1. Start with detailed sketches. I favor starting with a series of paper sketches, then moving to 2D CAD sketches and exploring the motion paths in CAD sketches before building anything serious in 3D. It’s fine to make quick “pretty pictures” to help people visualize the design intent, but rushing into a complex 3D model before fully understanding all the requirements and constraints will waste a lot of time in the long run. The CAD sketching phase is also a good place to do any critical force calculations, rather than getting further into the design and then discovering that something needs a significant revision.
2. Build a minimum model to test the motion paths. Once the 2D sketches have been proven out, build only as much of the model as you need to prove out the motion paths. Use distance constraints as necessary to take the place of missing parts in the assembly. Name your distance constraints too, so the model will still make sense weeks or months later. Use mate relationships and collision detection to drive the mechanism testing. (More about that in the next part of this article.) Fix anything that isn’t working properly.
3. Add parts to make a complete model. Add, replace and modify parts to create the complete model including all fasteners. Seriously, getting all the fasteners into a model is tedious but it can prevent a lot of unhappiness later if something collides or fails to fit. Be sure that your added parts match the distance constraints they’re replacing and also take worst-case dimensional tolerances into account. (You’d be amazed how often people get bitten by these.) Suppress the distance constraints but don’t delete them.
4. Run interference detection. Once you have a complete model, run interference detection and pay attention to the results. Modify and improve the model as needed. There are some fine points to working with interference detection which will be covered in the next part of this article.
5. Suppress all the “extra” parts. Suppress all those fasteners, washers and spacers and reactivate the distance constraints, updating them to match any part changes.
6. Test the motion paths in the interim model. This is where most of the actual motion testing will happen. Try to break the model or make it misbehave. See if there are any “wrong” or “stuck” states that it can get into. View the model from every possible angle and test it a lot. Check carefully for anything passing through where the suppressed parts are. If anything needs fixing at this point, repeat the process from step 3.
7. Final testing. Suppress the distance constraints, activate all the parts and run final motion tests along with interference detection at critical points along the motion paths. This verifies that nothing is colliding with the fasteners, washers or spacers. Make various parts transparent while doing this so you can see where things may be getting uncomfortably close even if no interference is found. Once again, view the model from every possible angle and test it a lot. With all the parts in place things may slow down and/or develop odd glitches from the excess rotatable components, but this is the final testing and by now you should have a good understanding of the model and how it behaves. As before, if anything needs fixing at this point, repeat the process from step 3.

In the second part of this article we’ll discuss interference checking with fasteners and springs including modeling springs in terms of components with changing dimensions, plus when to use collision detection.

Working with off-the-shelf enclosures for electronic products –Part 8– Finishing

There are several standard options available for cosmetic finishes on modified enclosures, all applicable to custom and production enclosures too. For any contract finishing option the price and quality will vary so shop around, get references and be sure to see samples of their work first.

Anti-corrosion coatings: These are strongly recommended for any steel enclosure and also for any die-cast or aluminum enclosure not being otherwise coated. The options for anti-corrosion coatings have become limited by the near-universal adoption of the ROHS (Restriction Of Hazardous Substances) directive which bans or strictly controls the use of many toxic materials. Cadmium, lead, mercury and hexavalent chromium are among the materials that are no longer acceptable. The most common acceptable anti-corrosion coating for steel (and aluminum if a conductive coating is required) is zinc chromate, which contains the acceptable trivalent chromium. Zinc chromate is most commonly applied as either clear or yellow chromate, where the yellow gives more corrosion resistance but the clear has a better cosmetic appearance if paint isn’t applied over it. Neither chromate finish will hide any cosmetic flaws.

Anodizing: Aluminum enclosures can be anodized, a process with a range of physical properties and many colors available. Anodizing is non-conductive, which can be an issue if grounding and/or an EMI shielding contact is required for the enclosure. Anodizing has a couple of significant limits – it will not hide any cosmetic flaws and no ferrous inserts can be present during the process. Ferrous inserts can be installed afterwards but this may be an appearance problem unless they’re covered by a label or something else.

Plating: There’s a wide range of plating options, with the most common for metal enclosures (aside from the trivalent chromates) probably being bright nickel and black oxide. Some useful information about the current plating options can be found here.

Paint: If you have access to a professional grade spray gun and booth, you may be all set. If not, there are still a lot of paint finishing options. Hardware store grade spray paint will yield less than professional results, just to get that out of the way. There are contract finishing houses that can paint a single piece, small run or large production run, whatever you need.

Paint can be divided into wet and dry types. The wet type is either solvent or water-borne and is the more traditional type of paint, generally applied by spraying. Filling surface defects and masking selected areas is straightforward but does add cost, and curing is accomplished either at ambient temperatures or with a fairly low temperature bake. (Note that this bake temperature is too high for most plastic enclosures.) Dry paint is applied as a powder held onto the part by an electrostatic charge and then baked at a fairly high temperature. The range of colors, color effects and surface textures available with this powder-coating method is impressive, but surface defect filling is more difficult due to the required baking temperature and masking is more difficult too. Talk to your vendor about what they can do. Also, some enclosures (often plastic) will have integral environmental seals, which must be masked for any painting process. Threaded areas, inserts and grounding contacts must also be masked, as well as any precision surfaces whose dimensions will be altered by the paint thickness. Some plastics can be successfully powder-coated with some of the lower temperature curing powder coatings, but few off-the-shelf plastic enclosures are suitable for this.

If contract paint finishing houses are beyond your budget, you can try making a deal with a local auto body place to do the spray finishing for you. In that case you may need to work on your own with any special masking materials such as thread plugs and caps, but the tradeoffs could be worthwhile.

This concludes the article about working with off-the-shelf enclosures for electronic products. A new topic will start next week.

Working with off-the-shelf enclosures for electronic products –Part 7– Modifying sheet metal enclosures

Safety Warning – The reader bears sole responsibility for having sufficient experience with metalworking equipment and understanding that working with metal and metalworking tools & machinery is inherently dangerous. Understand the equipment and materials you’re working with, use proper safety gear (always wear eye protection) and check your workholding one extra time before doing any operation.

Design Innovation uses a CNC milling machine for creating most holes in prototype and small-run sheet metal enclosures. However, complex hole shapes can be made in a sheet metal enclosure with a drill press and a nibbler tool. Working with sheet metal enclosures on a milling machine requires a couple of special workholding techniques and those are covered towards the end of this part.

Besides a drill press and a nibbler tool, (nibbler tool described in the previous part of this series) you’ll also want some fine-cut flat and round files and a deburring tool. Sandpaper in the range of 100 grit for edge smoothing and 220 grit for finishing is needed too. Some people favor tapered reamers for adjusting hole diameters but I prefer fixed-size reamers that can be used with the drill press. If holes too small for a nibbler tool (like D-sub connector openings) or too close to an edge are needed, then add a jeweler’s saw with an adjustable frame to the list. With good quality blades, it’ll do a fine job of cutting both aluminum and the sort of mild steel used in enclosures. Jeweler’s-scale needle files are needed for cleaning up holes this small.

Jeweler’s saw with an adjustable frame

Drilling accurate holes in sheet metal enclosures:
1. Lay out the complete cutting pattern accurately including all edges and radial centers. Make all corner and feature holes relatively small if you can.
2. Properly support the surface being worked on – flat pieces of plywood or MDF make good support blocks so that the surface won’t bow or distort when centerpunched and drilled.
3. Centerpunch the holes.
4. Use a drill press and drill bits that are properly sharpened. Drill bits with a 135° split point are the best to use for sheet metal and are less likely to “walk”.
5. Spot drill the holes first if they’re over about 1/8”.
6. Clamp the work or otherwise make sure it can’t be spun out of your hands. With larger drill bits this is critical and makes smoother holes too.
7. Drill at a smooth, even rate and pay attention to the sounds and vibration of the drill and the work.
8. The hole is likely to be rough when drilled. Use a drill bit one size smaller than the hole and finish the hole to size with a reamer.
9. Never drill over a support block that’s full of holes. Wood is cheap, enclosures and your time are not.

To drill on the side of an enclosure, clamp a board to the drill press table, swing the table off to the side, place supports on the board to reach inside the enclosure and adjust the pieces until there’s a minimum of overhang but all the areas to drill can be reached. In some cases it may be better and safer to use multiple setups for reaching different areas.

Radiused-corner holes are best done by first drilling at their radiused corners. Then the straight lines interconnecting them are nibbled, or sawn away if the area is too small or confined for a nibbler tool. Work a little bit in from the line, ideally about .020” (1/2mm) and plan to file to the line for the smoothest transitions into the radii.

Drilling holes in the enclosure top

Nibbling the outline between corner holes

Completed hole in enclosure top, nibbler and deburring tools shown

Cutting holes too small for the nibbler tool:
Lay out the hole, drill your corners and then use an adjustable frame jeweler’s saw to cut the path between the holes. Having the adjustable frame is critical since it enables you to rotate the blade in 90’ increments and work around the edges of the enclosure. Support the work as close as possible to where you’re cutting. Use a blade fine enough that at least two teeth engage the edge of the material at any time.

Drilling arrays of perfectly clean holes in thin materials:
If you need to produce an array of holes in very thin sheet material, particularly in a formed part, an excellent way to do it burr-free and very cleanly is to sandwich the material with two sacrificial pieces of aluminum, clamped flat to the part from either side. This way you’re drilling through the top aluminum, through the part and into the bottom aluminum and the material of the part has nowhere to distort to or produce a burr. This technique enabled us to produce a group of grille areas with a total of over 200 holes, each ~1mm in diameter, in a modified version of a small, delicate part formed of .010” tin-plated steel, with no cleanup needed. The setup was time-consuming but the work went smoothly and the result was a perfect prototype and a very happy client. See the production version of that part here.

Milling holes in sheet metal enclosures:
There are a couple of particular issues with using a milling machine on sheet metal enclosures – clamping without distortion and controlling vibration. Vibration leads to more frequent tool breakage and less accurate cuts. The solutions go hand in hand. Cut three pieces of plywood or MDF to fit as reinforcing blocks inside the enclosure. What is needed is for them to each easily fit inside the enclosure, but to be pressed into place so that when the enclosure is solidly clamped in a milling vise, the sides are held by the wood and no distortion occurs. This allows the face to be milled with less danger of the work moving, and a reduction in vibration. Plan on cutting about .010” depth per pass and having to cut several extra passes due to distortion in the enclosure top face. Smaller end mills and more flutes will also reduce vibration. For this kind of work I prefer a 4 flute centercutting 1/8” diameter regular or stub length end mill. A sprayed mist of coolant/lubrication is a major help for this kind of work too.

Reinforcing blocks inside the enclosure

Reducing vibration while milling sheet metal enclosure sides requires a careful setup. The key is to stiffen the enclosure as near as possible to the area being cut to reduce vibration. Clamp bars of wood, plywood or MDF across the back and front as shown, while rigidly connecting this clamping setup to the parts clamped in the milling vise.

Enclosure side milling workholding to reduce vibration

In the eighth and final part of this series we’ll examine the finishing options for modified enclosures.

Off-the-shelf enclosures for electronic products –Part 6– Strategies for modifying enclosures

Modifying an off-the-shelf enclosure can add significant expense to a product if not carefully planned. Most enclosure manufacturers will have detailed 3D CAD models available for download and these save a lot of engineering time. If any dimensions are likely to be critical it’s a good idea to get hold of a few sample pieces and measure them to see how closely they correspond to the “ideal” CAD model and any manufacturer’s drawings. Plan on not having any of these dimensions be more accurate than +/-.010”, particularly with sheet metal.

Adding features to an existing manufactured item can be challenging if the end result needs to still look cosmetically perfect. Painted, anodized and plastic surfaces can mar very easily during modification processes and accurate workholding often involves high forces and can leave permanent marks. (More about workholding in the next part of the series.) Surface touchup and repairs add expense, as does masking threads, ground contacts, windows, etc. before painting. Threaded inserts of incompatible material may need to be added after anodizing or plating. Threaded inserts that are male or blind (female threads and closed at the bottom rather than a thru-hole) for use in a painted enclosure frequently have overlooked expenses in regards to finishing, where a paint shop will generally want to fill and sand the surface to hide the insert back before priming and painting. Details like this must be considered when planning the project. Many manufacturers offer both painted and unfinished versions of their die-cast and sheet metal enclosures and the unfinished ones are easier to clean up after modifying. Any cosmetic or anti-corrosion finishing should be done after modifications whenever possible. Extruded aluminum enclosures generally come with either clear or colored anodizing. This can be stripped and redone after modifications but any surface imperfections must be dealt with before re-anodizing.

There are punch sets available for adding special connector shape holes into formed enclosures, particularly for shapes like DB (D-sub) connectors and holes with anti-rotation flats. These are very expensive but if you’re going to make a small run of enclosures needing one or more of these holes, the special punch set can pay for itself. Mouser has a good assortment of punches and McMaster-Carr has punches for round and special shaped holes and for DB connectors. These punches require one or more pilot holes to be drilled, then the punch is inserted and the nut tightened until the punch shears through the metal.

Two examples of connector punches

Drilling and milling openings accurately in a formed sheet metal part can require some tricky and labor-intensive workholding. Plastic parts may require holding nests with complex shapes and in some circumstances vacuum fixturing is needed to avoid clamp-induced distortion. This fixturing can have significant one-time costs, although for some applications a holding nest can be made with epoxy putty. Drilling and milling accurate openings in sheet metal enclosures is slow and often needs significant cleanup afterwards. Drilling and milling openings in die-cast metal enclosures is fairly straightforward and no more difficult than milling aluminum, though enclosure side draft must be taken into account for any side-mounted features.

Dremel tools with abrasive cutoff wheels are a possible option for making rectangular holes in enclosures, but it’s actually fairly difficult to do accurate work with this approach and a lot of cleanup is needed. One good option for producing rectangular and complex holes in light-gauge sheet metal without using a milling machine is to use a nibbler tool. I’m partial to the one made by Adel. This is a reasonable low-cost option for very small numbers of units. The nibbler requires a 7/16″ pilot hole and some filing afterwards to smooth the edges, but allows perfectly square inside corners and is surprisingly easy and quick to use. Practice with it before doing the actual work.

Adel Nibbler Tool (left), Klein Tools Nibbler (right).

This enclosure modification was done using a milling machine, but could also have been done with a drill press and a nibbler tool.

When adding features to an enclosure, more features = more work operations = more expense. Each time the part has to be handled and each face of the part that needs to be modified is at least one work operation. More sides being modified will often mean more fixturing costs, too. The costs and issues associated with hole deburring also need to be considered. The designer must understand the equipment and capabilities of the shop performing the work.

If the enclosure as purchased had a NEMA (National Electrical Manufacturers Association) and/or IP (Ingress Protection) rating, making holes in the enclosure will invalidate those ratings. If the hole and its contents are properly sealed the enclosure will still resist penetration by dusts and liquids, but without proper testing the modified enclosure cannot be advertised as formally having those ratings. Remember also that the EMI shielding properties of a metal enclosure can be compromised by holes if they’re not covered or filled with conductive material connected to the enclosure.

An example of a liquid-tight die-cast enclosure modified by Design Innovation and incorporating sealed high-frequency connectors and a DB connector sealed with a custom-designed foam gasket is shown below.

See more information about the raw prototype and finished product.

In the seventh part of this series we’ll examine some of the workholding and machine-use methods for modifying enclosures.

Off-the-shelf enclosures for electronic products –Part 5– Applying graphics with labels

Graphics may be applied to any type of enclosure with silkscreening and padprinting, or with an adhesive-backed label. While in some circumstances painted graphics will look better or be more cost-effective, labels are free from many of the quality issues of silkscreened enclosures and can also cover unsightly features in the enclosure surface, especially where holes have been drilled or machined in. Labels can be made with die-cut openings for buttons, knobs, displays and other user features, and may also incorporate transparent windows with or without color for displays. Labels have a range of available materials and surface textures including different levels of gloss and matte as well as various metallic finishes. Medium textured finishes are generally the best at hiding wear and skin oils. The eventual wear patterns around control knobs that are a problem with painted text and graphics are much less of a problem with labels.

Left: Enclosure with holes machined in recessed top
Right: Assembled with label having die-cut holes for buttons and transparent windows for LEDs

For cost and ease of assembly as well as a clean appearance, it’s best to have graphics on as few surfaces of the product as possible. Text and images near the edge of a surface can give necessary information about features on adjacent surfaces of the product. However, information about specifications, safety and regulatory compliance is best printed on a separate label since that usually isn’t desirable for the front panel.

Some injection-molded enclosures are made with a label recess as part of the top surface. One vendor of these is Box Enclosures. Be sure to leave some clearance between the walls of this recess and the outer edges of the label to avoid fit problems. Observe corner radii too. One critical requirement with labels is to have the label’s placement on the enclosure be inherently resistant to peeling and bubbling. This means having a controlled border area between the edge of the label and the edge of the surface it’s attached to. Never place a label flush with an outside edge of an enclosure. Most enclosures will have an edge radius, and the label must fit within the edge radii on each side or peeling will occur. Note that if the the surface is contoured, this may be a problem for label adhesion and pad printing could then be the best way to apply graphics and text.

Any features in the enclosure that are under the label must be taken into account. These features must be designed to be below the surface of the enclosure, never flush or above. Designing for perfectly flush mounting is a bad gamble because any tolerance problems will have at least a 50% chance of producing a lump or bubble under the label and even a very minor surface irregularity will be visible and can be felt. It’s also best to plan for the label to attach to a flat surface, not a curve, unless attaching to the curve is completely unavoidable, the curve is very slight, the label is made of very thin and flexible material, and there are adequate assembly processes for accurately aligning the label over the curve. If there are recesses or holes that the label must bridge, a thicker label material may be needed to avoid having a dimple result. Having a label that can conceal holes allows one enclosure with a “universal” hole pattern to be used for multiple versions of a product. One other issue to keep in mind for assembly is to avoid having fasteners captured under the label. There are circumstances where this may be unavoidable or the least bad option, but if at all possible, avoid any design where removing the label is a required part of disassembly. In some circumstances, the use of threaded inserts pressed into holes drilled in the enclosure can circumvent this problem. Threaded inserts are best installed to be slightly below flush with the surface.

Label for sheet metal enclosure with die-cut holes for buttons, LED light-guides and display. Blue features are threaded inserts for fastening the display and circuit boards.

Speaking of alignment, if there are features in the label that need to align with features in the enclosure, be sure to account for all the tolerances in both the enclosure modifications and the label manufacturing. There are quite a few things that can go wrong here and it’s best to work with a designer who’s experienced with all the requirements and a label manufacturer with good quality control. A responsible label manufacturer will insist on a mechanical drawing including all critical features in addition to any artwork needed. Additionally, the process of aligning and attaching the label during assembly needs to be worked out during the design phase. Are there physical guides to align the label? Visual guides? Is any fixturing necessary? It’s one thing to have the designer hand-apply a few labels for trade show models, and another thing to have assembly workers apply labels in higher volumes and under less ideal circumstances. One way to reduce the risk of problems in using a label on an enclosure is to model the label in your CAD system along with the rest of the design. Make sure to incorporate the actual thickness of the label in the model, too.

Find out more about the development of the products shown here and others too at Design Innovation. We work with high quality, cost-effective label manufacturers as well as silkscreening and padprinting vendors.

In the sixth part of this series we’ll examine the actual modification of enclosures.

Off-the-shelf enclosures –Part 4– Sheet metal and extruded aluminum pros and cons

The previous part of this series looked at the pros and cons of die-cast metal and injection-molded plastic enclosures. This part will look at sheet metal and extruded aluminum enclosures.

Sheet Steel Pros:
· Durable
· EMI shielding
· Holes, slots & other features can be punched and/or machined
· Press-in threaded studs and similar fasteners can be used
· Good for thermal dissipation
· Corners & edges can be welded

Sheet Steel Cons:
· Heavier than aluminum or plastic
· Risk of corrosion
· Generally requires a painted finish
· Least good for thermal dissipation of the metal enclosures
· Most difficult metal enclosure to punch and/or machine

Sheet Aluminum Pros:
· Lightest metal enclosure
· EMI shielding
· Holes, slots & other features can be punched and/or machined
· Press-in threaded studs and similar fasteners can be used
· Good for thermal dissipation
· Can be anodized instead of painted
· Low corrosion risk
· Corners & edges can be welded

Sheet Aluminum Cons:
· Least durable metal enclosure
· Poorest fastener retention of any metal enclosure without using threaded inserts

Extruded Aluminum Pros:
· More durable than sheet aluminum
· Some fastening features are formed into the enclosure
· EMI shielding (if endcaps are conductive)
· More visual interest than sheet metal enclosures
· Holes, slots & other features can be punched and/or machined
· Press-in threaded studs and similar fasteners can be used
· Good for thermal dissipation & some have built-in heatsinks
· Can be anodized instead of painted
· Low corrosion risk

Extruded Aluminum Cons:
· Very limited range of shapes without postprocessing
· Major constraints on assembly methods
· End caps must be processed separately

See various examples of new products built with modified off-the-shelf enclosures at Design Innovation. One of our specialties is test fixtures using modified sheet metal enclosures for the fixture base.

In the fifth part of this series we’ll examine the use of labels for displaying graphics and for other purposes on enclosures.