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Pinewood Derby Shop Talk: Drilling Small Holes
By Randy Davis


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

HSS drill bits are flexible and strong.(3) 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

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
(Source: www.carbidespecialties.com)

Cobalt

Cobalt bits(4) 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

DRILLING TECHNIQUE
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)(5), 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.

Pro-Body Tool

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.

Drill Press

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.


FINAL THOUGHTS

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) We have our remaining HSS #44 bits on sale
Here.

(4) Beginning in October 2011, the #44 bits sold by Maximum Velocity
are Cobalt Split-Point drill bits. They are on sale for the next two
weeks
Here.

(5) For Dremel type tools, run on a low to medium speed.

From Pinewood Derby Times Volume 11, Issue 2

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