'evening everyone,

I don't post terribly often, I'd rather build if I've got the time available.  However, I recently re-purchased a tool which I formerly considered common, but was actually quite difficult to find.  This caused me to wonder if many builders may not even know about them, but they're absolutely worth a little upfront cost when dealing with re-work of full size existing holes.  I'm specifically speaking of what we refer to at my day job as a "Gun Barrel Reamer"  Not truly a reamer, these are simply a drill bit with a couple unique differences. 1) the included angle (or point angle) is roughly 165 degrees.  This is very flat by comparison to a standard jobber drill, which are usually 118 or 135.  2) The flutes of the gun barrel reamer are also produced simply by removing the material from the cylinder of the bit, with no added backcut.  While the added backcut to the flutes on a standard twist drill certainly aids with speed and chip evacuation, that additional bevel also makes it easier for chips to wedge themselves between the sidewall of the hole and the bit, potentially enlarging the hole to a greater than planned diameter. 

These two differences in the gun barrel really pay dividends when drilling to match say, an existing 5/32 hole through something really thin like .025 into a brand new un-drilled sheet.  In something like .025thk a standard 118 degree 5/32 drill bit doesn't even have the full diameter of the bit in the hole, you will walk sideways at least a little, enlarging (although possibly still within limits) the final hole in the already to-size sheet.  By comparison, the very flat 165 degree gun barrel bit will have some straight sidewall of the bit in the hole, and already provide guidance before you touch the trigger for the first time. 

My personal (although I certainly didn't come up with it) standard process for match drilling involves simply circling inside the existing holes on the blank sheet with an ultra fine sharpie, then piloting with a #40 bit using the circle as a target.  Once that's accomplished chuck up the gun barrel bit, and drill to final size, using light feed pressure but relatively high RPM.  The guidance provided by the bit being full diameter in the hole from the get go, and the lack of a tendency to chase a slightly off center pilot "downhill" because the bit is nearly flat anyhow, will make match drilling within tolerance a snap.  The chips not catching between the hole and the bit completes the picture for nearly perfect holes every time.  For something like my 601XL, grab some in #30, #20, 1/4 and maybe a 5/16 for the wing bolts if you haven't done the "B" mod yet.  The only MFG that I've found that sells these in the quantities we'd be interested (down to a single bit, if needed) in is SC manufacturing, although there certainly might be others. I promise I'm not affiliated with them, just want to make higher quality rework more attainable if you're not aware these exist.  Hope this is helpful to somebody out there, keep building, keep learning, and let's all get just a little closer to some air under the tires today.