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Written by Jim Ryan
Basic shop tools to the rescue
Feature
As seen in the January 2020 issue of Model Aviation.

As modelers, we occasionally find ourselves needing to do precision machine work, but we lack the tools normally required. However, a little ingenuity and forethought can enable us to achieve accurate results with only basic shop tools.

A friend of mine was building an Aviation Concepts 1/5-scale PT-19, and he was having difficulty fabricating the elevator joiner assembly. He’d made a couple of attempts but had been unable to make a part precise enough for his needs.
I offered to make the parts for him to show what’s possible with basic shop tools. Although this project is particular to a specific model, the same processes can be used for any number of machining operations, such as fabricating custom rotor shafts for helicopters or machining scale landing gear struts.
The main stumbling of this project was drilling perfectly centered holes in the 3/8-inch aluminum tube that makes up the body of the elevator joiner. The generally accepted fixture for drilling centered transverse holes in round stock is a V-block, but I’ve always felt that V-blocks are a poor choice for a number of reasons. It’s difficult to clamp a V-block to the drill press table perfectly centered relative to the chuck, and the work piece isn’t positively locked in place. Perhaps worst of all, a thin drill bit can walk a little before it starts to cut into the surface.
The technique I’m sharing enables anyone with a drill press to drill perfectly centered holes every time, whether it’s in aluminum or even hardened steel.

01 These are the materials and tools for this particular project. I’ve made copies of the relevant section of the plans, and have heavy-wall 3/8-inch aluminum tube, 3/16-inch aluminum rod, and a 2-inch long 6-32 socket-head cap screw for the control arm. Blocks of medium-density fiberboard (MDF) will be used to hold and locate the aluminum tube during the drilling operations. I also recommend (although it isn’t absolutely required) a set of inexpensive digital calipers and a small machinist’s rule.

02 I first cut the 3/8-inch tube to the precise length that I needed. Lacking a metal lathe, I cut the tube by spinning it in reverse with my cordless drill (so that the two tools are running in opposite directions) while I cut it with a reinforced cutoff wheel in my Dremel tool. This made it easy to get a square cut without the cutoff wheel catching in its kerf. I cut the tube slightly long then carefully ground it down, checking often with the digital calipers, until I reached the precise 5.5-inch finished length. This is how to cut precision lengths with only a couple of basic hand tools. Be sure to wear approved eye protection during this and all machine operations.

03 I’m drilling 3/8-inch holes in the MDF blocks. Be sure to set the height of your table so that you can bore the 3/8-inch hole with the blocks lying flat and the smaller holes with the blocks up on edge. You don’t want to move the table at any point in the drilling operations.
A long piece of poplar is clamped to the table to set the Y-axis of my drilling operation, and the small block of scrap MDF sets the Z-axis. I always use MDF for temporary fixtures. It’s cheap, dense, machines easily, and has no grain structure to interfere with accuracy. I like to cut 2-inch square blocks on my table saw, but they don’t even need to be precisely the same size, as long as they each have one true 90º corner.

04 Here’s where the magic happens. Because all of the holes are bored through the MDF blocks while they’re pressed against the fence, their centerlines will always precisely intersect. Also, the MDF prevents the drill bit from walking as it starts to drill into the tube. The result is that the holes in the tube will be perfectly centered. Note that before boring the holes, I marked each block with an arrow so that they’d all have the same edge on the table and the same edge against the fence.
The 3/8-inch tube should fit snugly in the guide blocks. Because this project required the middle hole for the 6-32 socket head to be 80º relative to the 3/16-inch aluminum dowels, I scribed a line on the tube and spray mounted an 80º angle template to the end block. (Regular glue would work as well.) I’ve clamped a stop block on the table to locate the tube’s precise midpoint for drilling the first hole.

05 After drilling the 9/64-inch midpoint hole, the tube was carefully rotated 80º using the end template as a guide. Because the two dowel holes were both 1.91 inch from the midpoint, I locked my calipers at the distance between the stop block and the first guide block, zeroed the display, then opened them to 1.910 and locked them again.
The calipers could then be used to move the drilling setup the prescribed distance. After drilling the first dowel hole, a piece of 3/16-inch aluminum rod was inserted into the hole to lock the tube in place while the second hole was drilled, also using the locked calipers for location.
During the drilling operations, it’s critical that you hold the blocks hard up against the Y-axis fence. For harder material, such as a hardened steel rotor shaft, I clamp the blocks sandwiched between a second piece of poplar. This isn’t necessary for aluminum. After drilling down to the tube, I always dribble in a little light oil as cutting fluid before drilling through the tube.

06 Here’s the finished assembly. I polished the parts by mounting them on my trusty cordless drill and wet-sanding them with 400-grit sandpaper. The finished assembly has the 3/16-inch dowels perfectly parallel, and all lengthwise hole locations are correct to plus-.005 tolerance. The same general techniques can be used on any number of precision-machining operations. Who needs a lathe or vertical mill when you have a drill press and some MDF?

2 comments

Thanks

Nice work Jim.

Check out Big Gator V-Drill guides. Makes it easy to drill/tap straight holes.

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