From: nwaero@northwest-aero.com
Subject: Re: Brought to tears, drilling 4130!
Date: 05 Jul 1998
Newsgroups: rec.aviation.homebuilt

Might want to get yourself a solid carbide center drill. T15 cobalt would be
the next step down, and then M42. #4 is a good all around size (.125 for
CD's). Sometimes a bullet proof center drill will help to get through the
hard outer surface, and then you can drill easier with the finished size
drill. It also makes the hole location more accurate, and allows you to push
like hell with the finished size drill without worrying as much about the
drill walking or slipping off.

Cutting speed for drilling hardened 4130 (assuming Brinell Hardness around
275 or so) is 60 fpm, which translates to about 1800 rpm for the center drill
and about 600 rpm for the .375 drill. Feed is in the neighborhood of .0015
per rev for the center drill and about .005 per rev for the .375 drill. This
translates to 2.7 and 3 ipm, respectively.

However, once you slam the textbook closed, and get out the hand drill, you
will find it difficult to apply enough pressure with the .375 drill to
actually get the optimum 3 ipm feed out of it. The result will be
insufficient chip depth, which will lead to work hardening, and then nothing
happens after that.

To avoid this, you need to slow the speed down to the point where you can
actually get the .005 thick chip going. One technique I use when drilling
hard shit with a hand held drill is to press as hard as possible, and then
slowly start increasing the speed until I hit that sweet spot where the steel
is peeling outta there like a hot snake, then maintain that speed and
pressure for as long as I can (this applies to the larger drills, not the
CD). You get a feel for it after a while. Same with using manual drill
presses. You set up the correct cutting speed, then you can feel when you hit
the right amount of pressure on the handle to keep the cutting going and not
work hardening. In that scenario, the problem is that you run out of hp
before you can reach the optimum amount of feed once you get past about a
.375 drill. Here again, you will be forced to slow down the cutting speed
below optimum to maintain the correct chip thickness without stalling the
drill motor. And again, this is in reference to cutting the harder materials
like 4130, 4340, 304, 15-5, etc.

If you have already work hardened the hell out of it, even a new carbide CD
will seemingly 'just bounce off'. In that case, get out the die grinder and a
small stone and grind through the hard outer material and into something you
can drill. Likewise, if you had a torch, you could anneal that area, if it
was something that didn't matter in terms of heat treat or hardness.

As a general rule, when drilling alloy steels and stainless, the worst thing
you can do is run the drill too fast and the feed too slow.

-j-


> >Help!   I find myself unable to drill a couple of little old holes in my
> >horizontal stabilizer spar tube.   The area to be drilled is in the
> >heat-affected zone of a completed MIG weld, and therefore pretty
> >hard to start with.  On top of that I've probably work hardened the
> >area more by my futile attempts to pilot drill it.   It just will
> >not drill!  Ultimately, I need to make 3/8" holes in these two spots,
> >completely through the tube, to install bushings for hinge bolts.
> >
> >Can somebody share their tricks for drilling holes in work-hardened
> >4130?
> >
> >Any suggestions that I use my welding torch on it will be to no avail.
> >I do not have a welding torch, just a MIG welder.