Precise Drilling of the Tracker Bar Holes

by Matt Gatlin

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Precise Drilling of the Tracker Bar Holes
Even with an accurately measured template and a drill press, it can be challenging to precisely drill the tracker bar holes.  Here is my method of doing it with just a cheap $10 digital slide caliper which I bought off Ebay.

This step isn't really necessary, but first I drew up an accurate template for the holes and glued it on top of the hardwood tracker bar top piece.

Then I got a piece of scrap wood to serve as a baseboard, and attached a strip of wood at the back to serve as a guide.  The idea is that the tracker bar will slide along this guide, and this will keep all the holes in line.

Now for the spacing: I attached the digital slide caliper to the baseboard, making sure that the caliper was securely fastened in a position where it could slide freely and extend the entire length of the row of holes.  I also made sure that the base/back board was firmly clamped to the drill press platform and that all locking screws on the platform and column were securely tightened.

   

I drilled pilot holes using a 1/16” bit, lowering the bit very slowly each time to prevent wandering caused by grain irregularities in the wood. After the first hole was drilled, I “zero set” the caliper, selected “mm” as the measurement, then proceeded with the rest of the holes.  With “zero” as the setting on the first hole, the row of holes (from smallest pipe to largest) would have caliper readings of 0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, and 96 for the small holes, then 105, 114, and 123 for the larger holes. I tried to go for accuracy within a range of +/- .05mm.  Do not zero the calipers between each hole, but measure off multiples of 6mm (9mm for the last 3 holes).  This avoids cumulative errors, and even if you make a mistake with one hole, the holes after that won't be affected.  As a check, the last hole should read 123mm.

After the pilot holes were drilled, I repeated the procedure with the final 4mm and 6mm bit drilling.

Upon Melvyn’s suggestion, I went with Walt Lysack’s method of construction rather than with John Smith’s honeycombed cardboard method.  In a variation of Walt’s method, I started with a 7/8” thick, solid piece of hardwood, drilled the 4mm and 6mm holes about 1/2” down into the block, then sliced off the top 1/4” of the piece for the tracker bar top.

I then glued the bottom half to the base piece, and drilled my offset 1/4 “ and 5/16” holes all the way through both pieces.

I used a rotary cutting bit on a Dremel tool to gouge out a connecting passage between the center, half-deep holes and the offset tubing holes.

With this method, there are no alignment issues with any of the pieces, and no risk of splintered drilling between the layers.

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