Companion DIY Drill Guide

Home shop machinist.

By
Bob Hadley
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1
DIY Drill Guide 01a
The Quantum drill jig (left) and the companion bolt shank driller (right).

A long time ago, in an issue far, far away…the May 2014 issue of KITPLANES, to be exact…The Home Shop Machinist covered the ins and outs of using the Quantum jig to DIY drill bolt heads for safety tying. Although not discussed in the article, the drill jig can also be used to drill bolt shanks, but only if the bolt is long enough. In the case of AN bolts, the minimum length would be a dash 12. In other words, a nominal length of 12/8 or 11/2 inch (note the actual specified length is closer to 1.28 inches). Bolts with shanks shorter than that can’t be drilled using the Quantum jig.DIY Drill Guide 03

Clearly, there are commercial jigs that can drill short shank bolts and, of course, drilled AN bolts are stocked by every aviation hardware supplier and are plentifully available in all sizes. Most builders probably have bin organizers full of them. So, why build this DIY jig? The answer is twofold: 1) It’s an easy project with minimal material, and 2) It’s for the day you need one bolt that’s not in your bin and, for whatever reason—you can’t wait, or the cost of one bolt plus shipping is crazy—you simply can’t justify buying one bolt.

DIY Drill Guide 04Drilled AN bolts, based on visual inspection, seem to be mostly drilled between four to five threads from the end. Instead of eyeballing the placement, Paul Dye suggested the jig design and hole locations be determined by the AN specifications. And so it was. I looked up the thread lengths and relative hole positions for AN3 thru AN6 bolts and sketched a jig that, with the bolt threads bottomed out against the jig face, the guide holes/drill bushings would be in the correct spot. Being to spec is always better than not. After extrapolating and averaging the various tolerances, the four-to-five thread observation proved to be accurate only for AN5 bolts or smaller.

DIY Drill Guide 05
Whenever starting with “rough cut” stock, use a machinist square to set the initial reference plane square to the mill (left). Fly cutting the rough end (right).
DIY Drill Guide 06
After confirming the end face was machined square (left), the top face was also cleaned up with a light pass of the fly cutter (right). See the February 2015 issue of KITPLANES to learn more about fly cutting.

The jig was made from a 1½x1½x1 block of aluminum scrap I had on hand. The material happened to be 7075, which machines really nicely, but any common aluminum will work.

Since I have a Quantum jig, I decided to have the guide holes to use the ¼-28 #50 drill (0.070 inch) bushings that come with the Quantum tool. You could get away without a guide bushing by drilling the guide holes to #50 drill size, but the risk of burrs snagging on and potentially messing up the threads in the jig is 100%. In any case, you need a relief hole on the back side to be able to blow out the swarf before unthreading the bolt from the jig. By using the drill bushing from the Quantum (or making your own; see drawing), I simply drilled all the way through the block with the #3 tap drill (0.213 inches) and then hand-tapped (¼-28) halfway down on the bushing side. This had the added benefit of being able to use a larger bit or chamfer tool to deburr and countersink both sides of the hole while the bolt is still in the jig.

DIY Drill Guide 07
The edge finder was used to set the X (left) and Y (center) zero position for the guide holes, as indicated by the crossing red lines (right).
DIY Drill Guide 08
Each guide hole was first spot-drilled (left), through-drilled with a #3 bit (center), and then hand-tapped ¼-28 at least halfway through the block for the Quantum drill bushing (right).
DIY Drill Guide 09
The process was repeated for the last guide hole (left). After threading, a spotting drill was used to add a small chamfer to each hole (center). To prevent backlash positioning errors when using the graduated dials on the mill hand wheels, it’s important to always slew the table from zero toward the desired coordinate. The same edge-finding procedure was used to reset X and Y to locate the holes for the AN bolts (right).
DIY Drill Guide 10
With the location marked, spot-drill, tap-drill, and hand-tap the holes for the AN bolts. Note the unthreaded side of the guide holes on the bottom face are toward the camera (left). The last operation was to drill the 10-32 fixture hole for AN3 bolts. The X and Y were reset again (center), and the hole was spot-drilled, tap-drilled, and then hand-tapped (right).
DIY Drill Guide 11
The drill press was set to the max 3,100 rpm and the #50 drill bit fed very carefully (left). A drop of cutting oil helped control the swarf. A ⅛ drill bit was used to add a small chamfer to the hole (right).

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1 COMMENT

  1. I hope Bob’s column will be continuing, somewhere, in the aftermath of the wanton destruction at Firecrown.
    It is one of the resources I will miss when Kitplanes self-destructs completely.
    Fortunately, many of the refugees seem to be sharing their expertise in other venues.
    I hope we don’t lose Bob.

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