Posts Tagged ‘sheet metal’

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

June 29, 2009

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

June 22, 2009

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 adjustable frame
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
Drilling holes in the enclosure top

Nibbling the outline between corner holes
Nibbling the outline between corner holes

Hole completed, nibbler and deburring tools shown
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 in enclosure
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
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

June 13, 2009

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.

sheet metal punch tools
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 and Klein nibbler tools
Adel Nibbler Tool (left), Klein Tools Nibbler (right).

holes milled in a sheet metal enclosure
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.
prototype and finished unit
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

June 6, 2009

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.

Assembly-Recessed-Top
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.

Assembled-CaseRev4
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

May 28, 2009

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.

Off-the-shelf enclosures –Part 3– Die-cast and injection molded pros and cons

May 22, 2009

The standard off-the-shelf electronics enclosures are made of die-cast metal, sheet steel, sheet aluminum, extruded aluminum or injection molded plastic. To examine the tradeoffs between these, we’ll look at the pros and cons of each. This part will look at die-cast metal and injection-molded plastic enclosures, and the next part will look at sheet metal and extruded aluminum enclosures.

Die-Cast Pros:
· Most durable
· EMI shielding
· Available with environmental sealing (gaskets, etc.)
· Some fastening features may be formed into the enclosure
· Holes, slots & other features can be punched and/or machined
· Some press-in threaded studs and similar fasteners can be used
· Good for thermal dissipation
· Low corrosion risk

Die-Cast Cons:
· Heaviest enclosure
· Draft requirement means sloped sidewalls
· Most press-in threaded studs and similar fasteners can’t be used
· Generally requires a painted finish

Plastic Pros:
· Widest variety of shapes available
· Most likely to have a battery door and/or compartment
· Holes, slots & other features can be machined
· Some available with environmental sealing (gaskets, etc.)
· No corrosion risk
· Color and texture are molded in
· Some available with transparent windows
· Molded-in pc board supports may be standard features
· Generally best for small and portable designs

Plastic Cons:
· Generally less durable than metal for the standard plastic enclosures
· Durability affected by ambient and operating temperatures
· Generally no EMI shielding without additional materials
· Very few press-in threaded studs and similar fasteners can be used
· Poor thermal dissipation
· Poor fastener retention without molded-in metal threaded inserts
· Angled sides due to draft requirements

See examples of new products built with modified die-cast metal and injection molded plastic enclosures at Design Innovation.

In the fourth part of this series we’ll examine the sheet metal and extruded enclosures.

Off-the-shelf enclosures -Part 2– Some of the manufacturers and a few tradeoffs

May 15, 2009

Some enclosure manufacturers include:
BOX Enclosures
Bud Industries
Hammond Manufacturing Company
OKW Enclosures
Pactec
Polycase
Pomona Electronics
Serpac

Most of these manufacturers make multiple types of enclosures and some make every type discussed here. Die-cast metal enclosures are durable and generally provide excellent EMI shielding, but also require a painted finish for appearance. Sheet metal steel enclosures provide these same benefits but the potential for corrosion may limit the environment in which they can be used. Sheet metal aluminum enclosures are less durable but also much less prone to corrosion and have the additional finishing option of anodizing. Sheet metal enclosures are also ideal for test fixture bases. Extruded aluminum enclosures are more durable than sheet metal aluminum enclosures. Injection molded enclosures are generally the least durable but have the greatest range of forms and features and don’t require any surface finishing. If the modifications needed for a sheet metal enclosure are projected to be sufficiently expensive, it may be worth pricing out a run of custom sheet metal enclosures.

Below are pictures of different types of enclosures. (Pictures copyright by Hammond Industries & Box Enclosures)

Environmentally sealed die-cast and plastic enclosures
Environmentally sealed die-cast and plastic enclosures

One style of sheet metal enclosure
One style of sheet metal enclosure

Another style of sheet metal enclosure
Another style of sheet metal enclosure

Extruded aluminum enclosures
Two examples of extruded aluminum enclosures

Injection molded plastic enclosures
Two examples of injection molded plastic enclosures

See examples of new products built with modified off-the-shelf enclosures at Design Innovation.

In the third and fourth parts of this series we’ll examine more detailed pros and cons of each enclosure type.

Using off-the-shelf enclosures for electronic products – Part 1 – The basics

May 15, 2009

The ideal enclosure design for an electronic product depends on a variety of factors, including the environment in which the product will be used, the range of user interactions with the product such as use of controls and display elements and the acceptable limits on size and weight, how the product is powered, and the desired aesthetics of the product. The production volume of the enclosure also plays into this decision. Having a knowledgeable designer assist with these choices can save you a lot of time and expense.

For most of these factors, extremes in any requirement may drive the design towards a custom enclosure. Fully custom enclosures can be designed to fit the product’s needs completely, but generally have the highest initial cost. Additionally, if a product is in a pre-production phase or if significant changes are still likely, it may be premature to lock in an enclosure design. This series of articles will focus on the use of off-the-shelf enclosures as an economical way to get initial (at the least) or small-run products out the door.

Off-the-shelf enclosures are generally a compromise between low initial cost per unit, added cost to customize, and any tradeoffs involving aesthetics and function. The materials for most enclosures are either metal or plastic or a mixture of both. The majority of metal enclosures are either die-cast, extruded or formed from sheet metal, and plastic enclosures are generally injection molded. There are both metal and plastic enclosures available with NEMA and IP ratings.

Modified die-cast and plastic enclosures
Modified die-cast and plastic enclosures

See more about how these two enclosures were used by Design Innovation in new products at our design of products and components page.

In the second part of this series we’ll examine some of the main enclosure manufacturers and look at a few examples of the different enclosure types.


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