Doing layout work on square tubing is easy. You need a decent machinists
square and a scribe.
When you mark the location of a hole on one side, set the square with the
straightedge perpendicular to the length of the tubing on the first mark,
and scribe around to the opposite side.
When the line is scribed all the way around, the ends will meet if some
attention is expended during the exercise.
With a little care, the holes on opposite sides of the tubing should be
within/less than .010" difference. That will probably be as close as just
drilling thru the tubing can attain.
It's a good practice to pilot drill holes with a 1/8" split-point drill for
holes of 1/2" or larger.
When the layout work and the pilot holes are placed accurately, the larger
holes are generally (nearly) perfectly located, and cut substantially
Everyone working with metal should have a variety of split-point drills in
sizes up to about 1/4". The split-points start cutting immediately upon
contact, unlike a conventionally ground drill point.
The web in the center of a conventionally ground twist drill is similar to a
cold chisel with a slightly crowned edge. This edge isn't a cutting edge, it
just displaces metal as force is applied, before/until the cutting edges
contact the metal.
The trouble is, the drill continues to require a significant feed force to
For larger sized drills, the force required to get the cutting edges into
contact with the metal just generates unnecessary heat, then continues to
generate more heat until the hole is finished.
It should be obvious that a good cutting lubricant is going to be a major
Application of a small amount, as little as a quick swipe, with a small
paint brush (or disposable acid brush) before starting each hole is
Laboratory wash bottles with a dip tube and an extended spout are a handy
method of applying cutting lubricant for drilling, milling, sawing and
turning on a lathe.
Again, cutting lubricant doesn't need to be flooded on, just enough to keep
the cutting edges wet, so it's not going to be an expensive practice..
instead, it will reduce cutting time and extend the time of the sharp edges
on cutting tools.
For large holes, a good guide for the size of the pilot drill is the width
of the chisel edge of the web. Then the pilot hole will allow the large
drill to start without a lot of force.
The objective is to generate chips, not watch a drill spin around.
As I've mentioned before, creating split-point drills isn't a difficult
task. By examining a factory-made example of a split-point drill (a larger
one is easier to see), one may notice that there are flats ground at a low
angle to the point, on the trailing/back sides of the flutes.
The back side grinds are made first, then the point is ground in the usual
manner, and the split points will magically appear.
"Jon Danniken" < snipped-for-privacy@yahSPAMhoo.com> wrote in message
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