Like many pilots of electrically-challenged aircraft operating in congested airspace, I depend on a handheld radio. Since it’s basically impossible to hear or talk on a handheld while in flight without a headset, it’s common to see them stuck to the panel with Velcro or clipped to a tube or other fuselage component. In my case it was on the right side of the cockpit with Velcro. Since handhelds have to go home between flights for charging, nothing beats Velcro.
While I never expected it to perform like a panel radio, I was surprised to learn in Jim Weir’s April 2011 Aero ‘lectrics column how much an external antenna would improve things. According to one test he mentioned: “Simply switching from the standard antenna to the rubber ducky on the handheld cut the range of the radio by a factor of 10.” To make matters worse, your aluminum skinned flying machine (well, mine at least) doubles as a Faraday cage to further obstruct radio signals.
That week, I set about installing an antenna and figured the most it would take was a few holes and some wiring. Wrong. It turned into (What else?) a full-fledged home shop project. The upside is: it was worth it because the radio is securely mounted and, with the external antenna, I now have no trouble contacting a tower 20 miles away or more.
My Icom IC-A6 transceiver is still mounted to my right with the knobs and keypad within reach. I can still easily remove it for charging between flights, which, depending on my schedule, might be a day—or a week or two. It’s very convenient. Loosening the hold down, pulling all the plugs (push-to-talk, microphone and headphone) and the antenna, which is a standard BNC bayonet connector, takes less time to do than reading about it.
Making the new mount incorporates some of the sheet metal techniques described last month (so I won’t re-visit those techniques), plus an intro to the bending brake. The mounting lugs are easy lathe work and we’ll also do some threading with a tap and die set. But the pièce de résistance are the two unique drilling tools that I promise will make your life easier.
I should probably explain why the mounting scheme came to replace the Velcro. The radio was originally attached with the rubber ducky antenna pointing straight up. With the new antenna, having the radio standing upright did not seem the cleanest arrangement for the coaxial cable. Uncertain as to the best way to reposition the radio, I decided to get in the cockpit and do some pretend flying (proving that it is sometimes necessary to just sit behind the stick—engine noises optional).
I settled on laying it down 90 degrees from the original position, but found that the display needed to be angled about 45 degrees to improve viewing. Unfortunately, that placement put the radio floating in space and meant no Velcro.
A cardboard cut-out confirmed the angle and where to position the radio from the bend. Only then it was off to the drawing (key) board (figure 1). Yes, CAD is to drafting what the spreadsheet is to accounting, but you don’t need a computer to make simple sketches. But it sure is nice! There are a number of free 2D CAD programs available. Google “free 2D CAD” and you’ll find downloads from Solid Edge, Solid Works, LibreCAD, and others.
My solution was simple to the core: an angled bracket with slots and two anchored lugs with thumb nuts. But to arrive there required several revisions and related contemplation. You will notice the lugs have a flange at the base whose diameter corresponds to the keyhole socket on the bracket. This indexes the radio in the exact spot every time. If it was just a slot and thumb nuts, the bracket could slide back and forth or completely disengage. That won’t do in an airplane: knobs, buttons and switches need to be where you expect them. The socket arrangement creates a nice, secure, engaged feel when in place.
The original prototype was not slotted and required the thumb nuts to be completely removed. This proved to be a lame idea during the first test-fit, when I dropped one of the nuts under the seat. Opening the holes into slots was easy enough, but that experiment proved the bracket material, 0.062 aluminum sheet—which was adequate for a closed hole—proved too fragile when slotted. Prototype number two was made from .083 and that worked out fine, but the holes were too close to the edge. When parts don’t work, they’re prototypes, right?
The Mounting Bracket
Although you could just as easily bend the bracket in a vise between two blocks of wood, I’ll use this opportunity to introduce the pan-and-box brake. There are two basic brake machines for bending sheet metal: the brake press and the pan-and-box brake. Wikipedia has excellent explanations and animations of both, so I won’t describe either in any detail. The brake press style is more a production machine and the pan-and-box brake is for (you guessed it!) bending pans and boxes. Examples would be drip pans, trays and tool box drawers, etc. Typically, pan-and-box brakes are limited in the thickness of material they can bend and the bend radius is more or less fixed. Bend radius is very important for structural components shaped from sheet metal. A knife edge bend or a too sharp a radius invites cracking. Assuming that you’re working with a bendable material—6061-T6, for example, is not bendable—and you’re getting cracks, scratch marks or scars, something’s wrong.
My bracket has two 45-degree bends. The very short reinforcing flange is the first bend. You’ll notice the clearance notch on the drawing 1.187 inches from the baseline. The second 45-degree bend is at that notch. Simple!
The keyholes should be drilled first (1/2-inch) and then slotted with a fine-tooth hacksaw after the lugs are riveted in place in the airplane. (You’ll see why later.) The best way to make a 1/2-inch hole in sheet metal is with an annular tooth saw-type cutter (unique drilling tool number one). The cutters made by Blair and sold at Aircraft Spruce are called Blaircutters and a company called Hougen makes a version called RotaCut. Both are high-quality brands. These tools will make a clean, round hole every time, even in very thin, unsupported sheet. It is possible to drill thin sheet with a regular twist drill, but the chances of it grabbing and tearing the material are high. You can reduce that chance by drilling a very small pilot hole and then clamping the sheet between some wood to drill the hole to size. If I were talking to anyone other than aircraft builders, that would be OK make-do advice. But I am not, so get a set of those cutters. They will come in handy for years to come.
Threaded Lug (two required)
The mounting lugs are basic lathe work. Instead of using the lathe gearbox to make the threads, I used a hand threading die. While it is true that one of the lures of the lathe is threading capability, it’s not always the fastest or most convenient way. I use thread dies most of the time because I’m usually making threads 3/8-inch or smaller. When you start to get down to ¼-20 or metric M6 or smaller, it gets harder and harder to maintain accurate tolerances when chasing a thread by hand. (Chasing is the term used for thread cutting with a point tool on the lathe.) Compared to making threads with a quality set of thread dies, it’s no contest. The trick to a good thread is starting square and on center. You can get a tailstock die holder for around $30 and I recommend it. However, it’s just as accurate to include, when possible, a threading guide pilot when roughing out your part. This pilot, turned to the minor diameter of the thread, will provide dead-on alignment for the die.
Thread dies are chamfered on one side to facilitate starting. The chamfered end faces the start of the thread. The process of die-cutting external threads is the same as tapping internal threads: apply pressure and turn. When the cutting starts, advance the die in 1/8- to 1/4-turn increments, and reverse 1/3 of a turn to snap off any chips that can obstruct the cut. Once you are two or three turns into the process, the thread will start self-feeding and only rotational force is needed. Keep threading, reversing and threading until the die just kisses the flange. Unscrew the die and flip it over and feed it carefully onto the fresh-cut threads. This second pass with the die reversed will finish the partially-formed threads that were left near the flange (the chamfered starter threads on the die).
Once the thread is cut, turn the pilot to 0.245 and part it off at the thread shoulder. This waste piece will be used as the spacer for the rivet tool when we install the lug. Face the end of the thread square and then use a spotting drill (a short, rigid spade-type drill for creating a chamfered spot for later drilling) to create a center-guide for the 1/8-inch rivet hole. After drilling the 1/8-inch hole, counter bore the opening to the 0.325-inch depth with a ¼-inch square-end mill cutter. Any type (2, 3 or 4 flute) is fine. I am compelled to say that, as a general rule, never put an end mill cutter in a drill chuck. The exception is for plunging, and only then when the cutting forces are light or moderate.
Using a boring bar to chamfer the shoulder. The chamfer helps the die to start threading on the major diameter.
With the flange gently clamped in the three-jaw chuck, hand chamfer the backside hole with a small center drill.
Thumb Nut (two required)
You can purchase thumb nuts in a variety of styles, but because our application requires a low profile and very low tightening torque (hand tight only), it’s better to make them. The thumb nut is turned from 1-inch Delrin rod. Delrin is a tough plastic that is very easy to machine. Because the final thickness is only 0.288, you need to turn a pilot about 1½ inches long and ½-inch in diameter to hold the part for fluting in the spin indexer collet. A spin indexer is a simple device that is rotatable on a center axis and can be locked in 360-degree graduations. Although primarily used for grinding, they are very handy for light-duty milling jobs that require rudimentary indexing, such as bolt heads (60 degrees) or flutes. In this case, the 12 flutes required for the thumb nut equal 30 degrees rotation per flute. Since Delrin cuts easily, the full form of the flute can be made in one pass, even on my meekly-powered benchtop mini-mill!
With the flutes cut, remount the knob in the lathe and finish according to the drawing.
Installing into the aircraft
I promised two special drilling tools: the second type is called a Vix-Bit. I used it in combination with the bracket to self-fixture the lugs into position. A Vix-Bit has a retractable spring-loaded sleeve that self-centers the drilling bit. It’s used primarily in cabinet making to precisely center pilot holes for hinge mounting.
Position the bracket exactly where you want it with the threaded lug in position and drill the pilot hole with a #5 Vix-Bit (7/64-inch). The spring-loaded sleeve provides positive hold-down pressure to the lug and prevents it from drifting or slipping, which could happen if you tried to drill the hole using the lug as a guide without the Vix-Bit.
Shown here is the second prototype. It featured slots that lacked positive engagement, so it was back to the drawing board.
Before drilling the next lug, use a 1/8-inch drill bit and carefully open the hole to fit a 1/8-inch pop rivet. Using a pop rivet spacer tool made from the scrap cut-off from one of the lugs, I riveted the lug in place. As an extra measure, I daubed 5-minute epoxy on the lug face before riveting it down. Rivets are not great in tension and, although the thumb nuts never need to be torqued more than finger tight, I wanted some extra holding power to prevent the lug from coming loose and free-spinning.
As I was getting ready to rivet the sample for the photo, my meatatarian friend Bryan (as in vegetables are “food for food” from last month) asked if I was going to rivet this from the backside. When I popped the spacer into the counter bore and pulled the rivet up tight, his eyes grew wide open.
“I can use that on all kinds of places on my race car,” he said.
Remount the bracket and tighten it in place with a thumb nut. Make sure the bracket is aligned and, using the second lug and Vix-Bit, drill the rivet hole the same as you did for the first one and then, again using the counter bore spacer and a daub of epoxy, rivet the lug down securely. When you’re done, you’ll have located the lugs with ultra-precision without measuring or marking.
All that is left is to saw the slots for the keyhole opening. Obviously they need to be a hair over 3/8-inches wide to slide over the thread. Once you’re satisfied with the fit, make sure all the edges are deburred and then clean up the bracket and paint it flat black so it won’t reflect any glare.
The techniques, sequence and tools selected for this project reflect just one way to make the described parts—the way I did it! As you gain experience as a machinist, you will discover there are many paths to the same end. Just remember to be safe, and when in doubt, ask questions. The machinist community is a friendly and helpful one.
Got a question for or job that needs assistance from the Home Shop Machinist? Send us a description of the part and where it’s used. We’ll consider it for a potential topic for a future HSM column. If we pick your request, be prepared to supply the raw material and, if it’s part of a published plan, access to the original design drawings. Send inquiries via e-mail to: [email protected] and put HMS in the subject line. Due to liability, certain items may be limited to the construction of a demonstration part for instructional use and display only.