Everything for Electronics
Posted in: Developing Perspectives (June 2012)

Nuts & Bolts

By Bryan Bergeron

In most of the descriptions of DIY electronics projects that I come across, the fasteners used in construction are merely mentioned in passing, if at all. However, the size, composition, and configuration of the nuts, bolts, and other fasteners can be just as important as the electronic components. Skimping on fastener hardware isn’t limited to enthusiasts on a shoestring budget. The practice extends to companies that assemble thousands of devices a day.

In my work with teardowns, I often have a good idea of the quality of components and wiring in a device by the time I remove the first screw from the case. Companies that cut corners on nuts, screws, and other fasteners cut corners on components as well, which typically results in a product that fails prematurely because it can’t stand up to normal use.

Similarly, your choice of fastener hardware can significantly affect the performance and longevity of your next electronic project. Let me give a few examples. Let’s say you need to fasten a sheet of 1/4” thick plastic — an acrylic faceplate with countersunk holes — to a metal enclosure. Let’s assume you want a nice flat finish, with no protruding hardware. The best fastener for the job is probably a flat-head machine screw — either Philips or slotted — and a matching hex nut. There are still several variables to define including size, composition, and length of the screw and the type of nut. There is also the issue of English vs. Metric measurements. For simplicity, let’s stay with the more common English standard.

Given an acrylic faceplate isn’t going to put a great stress on the screw, a 1” stainless steel screw with #4-40 threads should do, paired with a hex lock nut with nylon inserts. The 1” refers to the total length of the machine screw, from the surface of the flat head to the tip of the threaded end. Note that the measurement of screw length is determined by the head configuration. A non-countersunk screw would be measured along the length of the thread, and not include the height of the head.

The #4 refers to the diameter of the screw, including the threads; #4 screws are approximately 1/10th of an inch in diameter. Larger diameter screws have correspondingly larger size designations. I use #8 screws when I need something a little more secure than #4 screws. In addition, I like to use lock nuts in my projects instead of fumbling with lock washers and standard hex nuts.

Now, let’s say we need to fasten a 1/8” thick metal arm to a piece of hardwood, with the proviso that the arm should be easily removed and replaced as needed. Let’s say it’s a metal support attached to the hardwood body of a guitar. Assuming the metal bar has countersunk holes, you might be tempted to use a common flat-head wood screw with a 1/2” clean shank. In fact, I’ve seen this combination.

The idea of using a wood or other screw with a clean shank is that it presents a clean, friction-free passage for the first material, but that it’s in full contact with the underlying wood when the screw is fully engaged. Unfortunately, manufacturers often cut corners by stocking only one size machine screw, and use excessively long clean shanks resulting in defective joints.

In our example, the clean shank extends into the wood by over a 1/4”. The clear shank is too long and extends into the wood, providing a less secure hold than would be possible with a screw with threads that engage along the entire depth of the wood. This not only puts excess stress on the wood, but ruins the threads in the wood for repeated assembly and disassembly.

If you’re working on a project in which metal or plastic is repeatedly attached and detached from a hardwood surface, consider using E- LOK threaded inserts. Simply drill a hole in the wood, screw in the insert, and then use a standard machine screw to fasten another surface to the wood. I keep a supply of 6-32 brass inserts on hand for working with my electric guitar projects. The inserts take #6 machine screws which are thicker in diameter than #4 screws, with 32 threads per inch. Amazon sells the inserts, as well as the optional insertion tool.

As with electronic devices in general, you can develop an intuitive feel for fasteners by tearing down commercial equipment. Note the depth and angles of the countersunk holes in different materials, the thread count, and composition of fasteners. In addition, take a look at online catalogs and sources of information on fastener selection and use.

My favorite source for supplies and information on common nuts and bolts is Bolt Depot ([url=http://www.boltdepot.com]http://www.boltdepot.com[/url]). The prices are reasonable and selection is good, but they don’t carry what I consider miniature hardware — something with a smaller diameter than a #4 screw. For specialized, miniature, and exotic hardware — from brass to titanium and ceramic — I usually turn to McMaster-Carr ([url=http://www.mcmastercarr.com]http://www.mcmastercarr.com[/url]). If you can’t find it there, it probably doesn’t exist. Even if you don’t order anything from the company, the site is worth visiting for the hundreds of nicely laid out images of the various types of fasteners, including screws and head styles.

The most economical approach to acquiring a library of fasteners is to tear down every discarded electronic device you can get your hands on. Consider investing in one of those plastic compartmentalized containers for small parts. It’ll save time later when you’re looking for just the right fastener for the job. NV