For you non-professional steel fabricators out there, here is the latest
method I use for drilling holes between 1/2" and 1 1/16" in steel plate,
up to 5/8" thick.
A lot has been written on TBN over the years about holes, but this is a
surprisingly economical way to go. I expected to see more on annular
cutters on MachineBuildersNet, but there is very little there. So here
I post.
First, a little history. I rarely drilled any big holes in thick steel until I
made my CADDigger back in the mid 90s. Back then, I had an O/A cutting
torch, but I was not very effective at cutting holes accurately enough
for my purposes. Some of the holes required were as big as 2" in 1/2"
A36 steel plate. I tried bi-metal holes saws, but it was very slow and
my Delta 16" drill press really complained. And, at 240RPM, it was WAY too
fast. I also had some of those cheapo Silver & Deming reduced-shank
twist drills for the 1" and smaller holes. I got a set from HF tools for about
$25, but they chipped easily, even in mild steel.
I also paid folks I know with milling machines to use end-cutters. I have also
paid a guy with a punch press $1/hole to do some 1-inchers in 1/2 plate.
They were OK, but inconvenient, and there was some distortion of the plate.
I upgraded my drill press to the biggest and slowest one I could find that
was not a geared-head unit. This is the 20-inch DP from HF, which is about
325#, and much sturdier than my old Delta. At 180RPM, it was still too
fast for some of these bits, however. I also moved up to Morse Taper 3
bits, which the new press supported. The bits in the photo are very nice
US-made units with MT-3 shanks that go right into the DP's quill. Much
nicer.....for at least the holes up to 1". Better US-made twist drills all worked
better than the Chinese S&D units, too. The larger bi-metal hole saws were
still not a very good solution.
I now use my plasma cutter and templates or a circle guide for holes
bigger than 1 1/16" diameter, and use weld-in DOM tubing to make up for
sloppy holes. I still need to drill anything smaller than that.
Enter annular cutters. Those familiar with magnetic drill presses have
probably seen these, and these DPs can be purchased for as little as
$500 now, less the cutters. I used a mag drill back in the mid 70s when I
was a technician in a lab, but it was many thousands of dollars. I now have
a way to use annular cutters in my drill press.
