normal speed and the metalworker did not give any advice regarding how the holes were actually drilled.  So in case this article inspires you to make your own caliper in aluminum, I'll let you know how the drilling went for me.  I used a 3/4" Irwin Speedbor bit (as it happens, an Irwin Speedbor spade bit was also used in the video), but first drilled a small pilot hole with a normal metal twist bit.  It takes quite a few minutes to drill each hole.  The aluminum gets very hot - if you had an egg handy, you could certainly use the aluminum plate to fry it!

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For this particular tool, the aluminum was 1/2" thick.  None of the holes were drilled in a single operation.  I would drill about a quarter of the way through and then let the material cool for about twenty minutes - even after this "cool down" period the plate was still very hot to the touch.  The last hole was drilled a little more aggressively, with only three separate operations.  The key to drilling aluminum with a woodworking spade bit is to find the most efficient pressure point; too much and the bit comes to a stop, too little and the bit doesn't cut very effectively.  With the right pressure, the bit is almost on the verge of stalling, but while the bit is cutting very well, the side walls remain very clean.  It seemed to me that the cutting was most efficient when the metal had heated up.  If you're doing it right, there will be a constant thread of metal being removed by the bit.  Believe me when I say it's a very messy process and your shop vac will see a lot of action!

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The first few holes were made "dry" but for the next two holes I used some WD-40 to aid the cutting.  This has to be used sparingly and with caution as it is very volatile - did I mention that the aluminum gets very hot?  You alone are responsible for safety in your shop, but in any event I didn't use any oil for the final hole and this hole was cut much quicker than any of the others, I suppose because I had experience determining the most effective pressure point.  After all eight holes were drilled the Irwin spade bit was still reporting for duty, even if one edge had been a little "blued", so you shouldn't expect to sacrifice a bit to do similar work.

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Given the relative softness of aluminum, I suspect any common woodworking round-over router bit would work to bevel the hole edges, but of course carbide bits would work best and in my case I used a small 1/8" shank carbide-tipped bit from Lee Valley that was perfect for the job (I also used a 1/8" to 1/4" bushing adapter to use the bit in a full-size router).

 

The shape was cut with ease from the plate using a small bandsaw; since I had a metalworking blade I have no idea what results you might get if you try this with a woodworking blade, but I suspect it would work.  I beveled all edges with the same bit used on the holes.

 

The next task was to make the hole that secures the dial gauge.  Measure