Turning A Simple Box Into A Chassis...

Prototyping is all about testing experimental products. Cheap. Chassis often present a challenge because of the mounting needs that some components may have. Circuits, Power Supply, Connectors, Controls. Holes need to be made to mount and interconnect all these. That's easier then it sounds with a very limited set of tools. I only have about 4 drill bits in my arsenal and an electric drill. Plus a reamer (hole enlarger), a Nibbler (Metal carver), a round file (rat tail) and straight file (small), a center-punch and hammer and an Exacto knife. I Hacked out some pretty good metal chassis with nothing more.

Honestly... If you are only making a 'one of' prototype for demonstration or personal use does it really matter if it takes two days rather then 2 minutes and a small fortune.

Besides... You never want to make multiples without a trial first. Prototypes are a learning experience. I have 'never' built a 'first of' without silly mistakes. Simple mistakes. By about the third iteration I was usually ready for limited production tests.

Carving Holes In A Chassis...

There are two types of holes. Round and square. Both very easy to make. If possible use Aluminum. Even Tin cans can work in a pinch. So... Small one-of holes are always easy. Point and drill. Usually though holes need to be in some pattern with some precision to them. This is where a center punch comes in handy. Only remember... When punching thin metal you will leave a crater around the point unless you use a solid surface below the metal under the punch. This leaves a nice guide divot for the drill. I have always kept a small chunk of brass around for only this purpose. Finally... When drilling holes with this method always drill a small pilot hole and then drill larger holes, progressively, until the desired size is reached. Trying to drill a large hole with a small punch-mark is only going to lead to poor alignment. If the hole is still too small. Use a Reamer to hone it in. Literally.

Beyond that the Nibbler and files are the best way. The Reamer will get to a point where the Nibbler can be used. Just nibble away until close to the final shape then use the files finish up.

Marking A Chassis For Machining...

When a chassis gets to the point that it has a front panel with displays/controls and a back panel with various I/O it is helpful to mark the whole thing out. On the chassis. Before making a stupid mistake. My favorite way is to cover the surface with wide 'masking-tape'. Any type will do as long as you can write on it. This creates a temporary surface which can accept layout marking. It's also provides a good opportunity to test fit different items. And... If the layout fails (usually clearance problems) then just rip the tape off and start again. Many times I'd wished I had moved that one hole a half inch over.

Routing Power And Signals...

When mounting sub-assemblies to the chassis treat every screw and every wire like it will be vibrated beyond imagine going on a rocket into space. That's an extreme example but it is very frustrating when a wire feeding through a hole with chaffed insulation shorted because of a lack for a simple rubber grommet (or bulkhead connector). Or how about the loose screw and washer that shorted out the circuit below it. , Screws fail, connectors fail, potentiometers fail, sockets fail. Connections, mechanical and electrical, are generally unreliable. Minimize them. Your prototype will behave more accurate, repeatable and reliable for a much greater time. Always.

When you need to mount or interconnect assemblies clamp them down. Tight. Cable Clamps/Straps... Loctite... Safety Wire... Try to use locking connectors and avoid sockets. And by all means use lock washers and flat washers under screws. You can even use nail-polish as thread lock in a pinch. Remember it may have worked fine on the bench for months 'undisturbed' but when migrating to a chassis the intent is to demonstrate it. Or use it. And that means moving it. If it falls apart when it's moved it failed. If it doesn't behave the same as it did on the bench then it is probably because noise or loss of signal flow has been introduced by a bad connection and again it failed.

User Control And Feedback...

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Demonstrating A Packaged Prototype...

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