Friday, January 25, 2013

Drilling Axle Holes With Precision
By Randy Davis

Excellent wheel alignment is a key factor in creating a competitive
pinewood derby car, and using drilled axle holes (instead of slots) is
a key way to improve wheel alignment. Of course, this assumes that
the axle holes are drilled accurately.

Today's article will share how to drill accurate axle holes (and some
inferior techniques that you want to avoid) with either a hand
drill/Pin Vise or a drill press. But before drilling axle holes make
sure to check
your local rules to make sure they are acceptable for your race.

DRILL BITS

The first question that arises with axle holes is, "What drill bit do
I use?" (1) Generally, to have a snug fit you want to use a bit that
is essentially the same size as the axle. The chart below shows
typical bits for common kits:

Kit Axle Size Bit
BSA 0.086 to 0.087 #44 - 0.086
PineCar 0.088 #43 - 0.089
Awana 0.092 3/32 - 0.938

HAND DRILL/PIN VISE

To accurately drill axle holes with a hand drill or a Pin Vise you
must use a drilling guide. Do not attempt to freehand-drill axle holes
- the results will likely be much worse than using the axle slots.

The Pro-Body Tool is a drilling guide designed specifically for
drilling axle holes in pinewood derby blocks.


Figure 1 - Pro-Body Tool

It is placed over the bottom of the block, and then clamped into
place. The drill bit is then run through the holes in the Pro-Body
Tool. The metal of the tool ensures that the drill bit goes straight
into the wood. Full instructions for using the Pro-Body Tool are
located Here,
however, here are a few additional tips.

1. Keep the drill bit aligned with the hole in the tool. Don't flex
the drill bit - it can break.

2. Make sure the Pro-Body Tool fits snuggly on the wood. If it is
loose, use paper to shim it; if it is too tight sand the sides of the
wood block.

3. The Pro-Body Tool is equipped with a separate guide hole for
drilling a raised hole for one of the front axle.

DRILL PRESS

The Pro-Body Tool is highly accurate, and in some ways is preferable
to a drill press. But for more flexibility in hole placement, and for
drilling larger quantities of blocks the drill press is a good option.

The principle employed by the Pro-Body Tool is that all holes are
referenced to the bottom of the block. Thus, an out of square block
will not affect the accuracy of the axle holes.

This same principal must be employed when using a drill press. Thus,
when using a drill press, an accurate, vertical fence must be present.
(2) By pressing the bottom of the block to the vertical fence the
holes will be referenced to the bottom of the block, eliminating any
issues due to an out of square block.

However, most people with a drill press do not use a vertical fence.
Instead, they place the left side of the block on the drill press
table and drill the right side holes. Then they flip the block over,
and drill the left side holes. This is okay if the block is perfectly
square. But if not, the resulting holes will not be parallel to each
other, leading to poor alignment. This is shown - in an exaggerated
fashion - in Figure 2.


Figure 2 - Inaccurate holes due to out of square blocks

Some people attempt to resolve this issue by using a long drill bit to
completely drill through the block. However, since the drill bit is
narrow and long, it will flex, leading to inaccurate holes. This is
especially true for dense pine such as the Southern Pine used in many
BSA kits.

Instead, it is best to use a short bit, and drill half way through the
block. As mentioned earlier, issues due to non-square blocks can be
resolved by using an accurate vertical fence. The key is to make sure
the block is clamped firmly to the fence, even if the side of the
block is not flush against the drill press table (see Figure 3).


Figure 3 - Vertical fence creates accurate holes regardless of block shape

CONCLUSION
Axle holes can mean significant improvements in alignment, however,
they must be drilled accurately to provide any benefit.(3) So use the
proper tools and techniques to make your car go as straight as
possible.

(1) Regardless of the size, cobalt split point bits create more
accurate holes than any other type of drill bits. For more
information, check out: " Drilling Small Holes" from Volume 11, Issue
2.

(2) Meaning that the side of the fence is perfectly parallel with the
drill bit.

(3) Maximum Velocity offers pine blocks with accurately drilled axle
holes. Both standard and extended wheelbase versions are available Here.

From Pinewood Derby Times Volume 12, Issue 9

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