If your pinewood derby race rules allow axle holes, then you have likely drilled the holes using a drill press, or a Pro-Body Tool or Pro-Axle Jig(1) from Derby Worx. For Cub Scout axles, a #44 bit(2) is recommended, while a 3/32 inch bit is typically used for Awana axles. For a few other kits, a #43 bit works well. But regardless of the bit size, have you considered the best type of bit to use? The type of bit, as well as the technique used, both affect the accuracy of the result. In today’s shop talk we will discuss the various types of drill bits and some techniques to help improve accuracy.
Small Drill Bit Types
The most common small drill bit types are high-speed steel (HSS), carbide, and cobalt.
HSS drill bits are flexible and strong. They are inexpensive and used where long-term durability is not important. Most drill sets are HSS. The flexibility of the HSS bit is helpful in minimizing broken bits, but the flexibility is a hindrance where accuracy is concerned. For drilling axle holes with a Pro-Body Tool/Jig, a HSS bit is fine, as the tool minimizes the flexing of the bit; but when drilling holes with a drill press the flexibility really hinders accuracy.
Carbide drill bits are extremely hard, so flexing is virtually eliminated. However, because they do not flex, they are prone to breakage if careful technique is not applied. Carbide bits often come with a shank larger than the bit. So if you purchase one, make sure to get one that is long enough to drill axle holes – many Carbide bits are too short for drilling axle holes.
Typical Carbide Bit
Cobalt bits(3) are also extremely hard, so flexing is virtually non-existent. But Cobalt bits have a big advantage over HSS and Carbide bits — Cobalt bits have a “split point” tip that is specifically designed to keep the bit from “wandering” (i.e., not entering the wood at the location you want).
Wood is a relatively soft medium, but it is not consistent in density. Depending on the grain, wood will change from a hard to soft density over a small fraction of an inch. This change in density affects the way the drill bit goes into the wood. The drill bit will seek to go into the softer part of the wood. With a HSS bit, the bit may wander seeking a soft spot, and then when it has entered the wood it will tend to flex away from the hard grain. This results in inaccurate holes. Carbide bits also wander, and if they wander when in a drill press, due to the rigidity of the bit either the wood will move, or the bit will break. It seems odd, but I have broken more carbide bits when drilling into wood than any other type of bit.
Cobalt bits, with their split point, are virtually wander-free. Like other bits, once the bit enters the wood it will want to follow the softer grain, but this can be compensated for with proper drilling technique.
Cobalt Split Point vs. Typical Bit
When drilling into wood, there are three techniques that greatly improve accuracy. First, expose only the amount of the bit needed to drill the hole — leave the rest inside the drill chuck. This minimizes the opportunity of the bit to flex (or break). Second, run the drill at full speed (1,500 rpm on a drill press)(4), but enter the wood slowly. This helps to make a clean entry hole, and minimizes drill bit wander and flexing. Next, drill about half way in, pull the bit out enough to clear the debris from the drill bit flutes, and then finish the hole. The pine sap limits the ability of the flutes to clear the debris. If you don’t clear it, it can jam up, resulting in an inaccurate hole and/or an overheated bit.
When drilling holes with a Pro-Body Tool/Jig, make sure the tool is clamped tightly to the block and the block is clamped in place. Then, making sure that the bit is parallel to the hole in the tool, spin the drill up to speed and enter the wood slowly. Drill about halfway in, pull the drill bit back to clear the flutes, and then complete the hole.
When using a Drill Press, use an accurate fence to stabilize the block and set the offsite from the bottom of the block. Squeeze (or clamp) the block to the fence, then enter the wood slowly, clear the flutes at the half-way point, then complete the hole.
Drilling accurate axle holes is a challenge which can be greatly simplified by using the right equipment. When selecting equipment, the type of drill bit is one decision you don’t want to overlook.
(1) While there are other drilling guide products available, these two Derby Worx tools are the most popular tools on the market today. You can find them Here.
(2) Drill bits are available in four size classifications. The first class, the type in most people’s tool box, is “fractional” (1/16, 3/32, 1/4, etc). The second size class (most popular outside the US) is “metric” (1mm, 2mm, 3mm, etc). The third class is “numbered”. Numbered bits have very small increments and range from “80” (.0135 inches) to “1” (.228 inches). The #44 drill bit used for Cub Scout axles is 0.086, which is about halfway between the 5/64 and 3/32 inch fractional bits. The final drill bit class is “letter”. These bits start after the number “1” bit and range from “A” (.234) to “Z” (.413)
(3) The #44 bits sold by Maximum Velocity are Cobalt Split-Point drill bits. You can find them Here.
(4) For Dremel type tools, run on a low to medium speed.
From Pinewood Derby Times Volume 11, Issue 2
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