My time as an engineer in the aerospace business taught me that using the right fasteners can have a huge impact to the functionality and serviceability of an item. Sure, a common pan head machine screw will work to hold an access panel in place. But using a custom screw with an oversized Rosette head allows an astronaut in a spacesuit to quickly remove the panel without tools. I’m sure you can imagine what a tremendous advantage that is.
I often find that specialized hardware can provide similar benefits with my RC projects. The main difference boils down to a question of availability. If the specialty fastener that I want is even manufactured, it is usually prohibitively expensive or is only sold in large quantities. Other times, I need a special fastener “right now”. Ordering online or even driving to the hardware store just won’t cut it. More often than not, I end up making my own specialty fasteners by modifying common nuts and bolts that I already have in my workshop.
Making a thumb screw out of a Phillips head screw requires purposeful cutting of a plastic tab.
In this article, I will illustrate my techniques for creating three different types of custom fasteners.
Whether I’m going on a week-long vacation or just an afternoon trip to the flying field, I tend to be a minimalist when it comes to packing. I take only the bare essentials and try not to weigh myself down with accoutrements to deal with “what if” scenarios. This probably means that I’ll be among the first to succumb to a zombie apocalypse. Until that brain-eating day, I’ll live happily unencumbered.
Translating my streamlined packing approach to RC means that I aim to carry very few tools with me. If a specialty fastener lets me do a job without tools, then I’m game. That’s why I often find myself turning normal screws into thumb screws. If you’re not familiar with thumb screws, they are fasteners that are designed to be turned by hand rather than with tools. Thumbs screws are super-convenient as long as you don’t have high torque requirements…which I almost never do for RC applications.
Transforming a slotted screw into a thumb screw is often as easy as gluing a scrap piece of plastic into the slot.
Converting a common slotted machine screw into a thumb screw is very easy. You just make an appropriately-sized tab out of scrap material and glue it into the slot of the screw. The tab becomes your grip for turning the screw. I have a sheet of 1/32″-thick Kydex plastic that I typically use to make tabs. I have also used craft sticks, thin plywood and scrap aluminum for the same job. You’re bound to have something that will work. GOOP adhesive is great for gluing the tab to the slot.
Only recently have I begun converting Phillips-head screws to thumb screws. It’s actually not much more difficult than converting slotted screws. The trick is cutting the tab with the right profile to mate with the shape of the screw head. A couple of strategically-shaped notches should do the trick and you don’t have to be terribly precise. The photos describe the final shape better than words ever could. Apply glue everywhere that you can.
Adding heatshrink tubing over the screw head and tab can make your modified fastener more presentable. Here, the tubing has been shrunk and is ready for trimming.
In the example shown here, the thumb screws that I created are being used on a very visible area of a model boat. The home-grown appearance of my tabs didn’t really jibe with the otherwise attractive look of the model. So I gave the thumb screws a makeover by covering the screw head and tab with heatshrink tubing. I used an oversized piece of tubing and trimmed off the excess after shrinking it. Not only does the thumb screw look better, but I think the tubing actually strengthens the joint between the screw head and tab.
There are few things more frustrating than searching for a dropped screw when that screw is vital to making something work. It’s amazing how easily such a screw can just vanish from sight…never to be seen again. I like to mitigate this situation for any screws that I have to manipulate at the field by making them captive whenever possible. Basically, this means that the screw stays physically attached to the component that it is intended to fasten.
Your plastic piece should thread onto the screw with some resistance.
In my example, the loose component is a hatch that covers the radio compartment of an RC boat. Four screws are used to fasten the hatch to the upper deck of the boat. Ideally, there would be a gap between the inside surface of the hatch and the bosses on the upper deck that receive the screws. If that were the case, I could have simply threaded a locknut onto each screw to keep them attached to the hatch. The locknuts need to be slightly loose so that the screws can rotate freely for attaching them to the upper deck.
But alas, there is no gap between the hatch and the screw bosses on this boat. So using standard locknuts to capture the screws isn’t feasible. The good news is that it is easy to emulate the function of the locknuts using scrap plastic. Even the 1/32″ Kydex is usually too thick for this application. I typically get the ultra-thin plastic I need from the vacuformed blister packaging found on many items. The specific donor in this case was the packaging for an SD card. The plastic from such packaging is almost always clear, which doesn’t photograph very well. So I used thick, bright yellow plastic as the stunt double just for the photos.
The completed plate keeps the screw captive on this boat hatch while still allowing the screw to turn for attaching the hatch to the hull.
The first step is to cut a square (or circle, or hexagon, or…) of plastic that is larger than the diameter of the screw. Then, you must drill a hole in the plastic that is slightly smaller than the screw. The goal is to be able to thread the plastic piece onto the screw with some resistance.
Drill bits can sometimes be difficult to use on thin plastic. If the bit is not very sharp, it will do more tearing than drilling. You can use an X-Acto knife (carefully), grinding bits in a Dremel, or whatever you’re comfortable with. I usually drill a small pilot hole with an X-Acto and then enlarge the hole with a hand-held tapered reamer.
I use a tapered reamer to enlarge the necessary hole when creating a captive screw.
Once the hole is the correct size, you then place the screw into the thru-hole of the removable part. Next thread your plastic retainer all the way up the screw. That’s it. The screw should still be able to turn in the thru-hole but the retainer will keep it from falling out.
Depending on how tightly your retainer fits onto the screw, it may work itself loose from time to time. You can tack it into place more permanently with a small drop of super glue. Just be careful to avoid getting glue into the threads that engage the other component. I use medium-viscosity super glue and apply it using the tip of a needle.
Weight is almost always a consideration with my models. I try not to get crazy about shaving weight, but fractions of an ounce can be significant in some cases. One way that I am able to reduce weight is by substituting in nylon hardware for the steel fasteners that are typically provided.
Nylon fasteners have been popular on RC planes for a long time, but not necessarily for the mass reduction they provide. The nylon parts can actually lessen the damage caused by a crash. These fasteners are most often used to attach a model’s wing and/or landing gear to the fuselage. The relatively low shear strength of the nylon bolts makes them a sacrificial joint in the event of a crash. Ideally, the bolts will break on impact and lessen the energy that is absorbed by the rest of the airframe.
You obviously don’t want to use plastic fasteners for any high-stress or high-torque applications. If you’re desperate for lower weight, but don’t trust nylon, aluminum fasteners are an alternate higher strength (but fairly expensive) option.
You can often find nylon counterpoints for standard metal hardware. If, however, the hardware that you want to replace is already a specialty part, you’re likely going to be stuck making your own lightweight version. I recently ran into this situation when mounting cameras to my models.
I was able to save a little weight by replacing the stock fasteners on a GoPro mount with nylon hardware.
No matter which of my action cameras that I’m mounting to an RC vehicle, I almost always use the plastic mounts designed for GoPro cameras. Each of the rotating joints on these mounts is tightened using a 1-3/8″-long 10-32 bolt with a plastic slip-on knob. The bolt threads into an acorn nut that is press fit into one of the mount pieces.
Since I was going to be placing multiple cameras on this particular model, I wanted to shave weight wherever I could. I emulated the stock bolt and knob using a 2″-long 10-32 nylon screw coupled with a 1″-long threaded spacer. The spacer acts as a convenient grip to make this a thumb screw. The spacer also provides a shoulder that presses against the interlocking parts of the mount as the screw is tightened.
The closed-end acorn nut limits how deeply the thumb screw can be inserted. This situation demands careful positioning of the spacer to ensure that the rotating joint is tight before the screw bottoms out. I had to leave a gap of about 1/8″ between the top of the spacer and the screw head. A drop of thin-viscosity superglue keeps the spacer locked in place on the screw. I later figured out that I could have just replaced the acorn nut with a standard style (metal or nylon) 10-32 nut. I’ve since made that change and saved another gram in the process.
I saved 6 grams of weight for each replaced screw. That is, admittedly, a small amount and probably not worth the effort in most cases. However, when the savings from multiple mounts were added up, I felt it was worthwhile. That is something you’ll want to consider each time before investing your time and money in alternate hardware. Sometimes is makes sense and sometimes it doesn’t.
Small differences in hardware can make a big difference in how something performs. Don’t be afraid of swapping out fasteners, even if it means you have to make them yourself.
Terry is a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.