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From: (JMartin957)

I have to drill a number of flat bottom holes, 7/8" diameter and 1/10"
(.1") deep in some 2/10" (.2") thick aluminum. Is there a source for
flat bottom drill bits of this size that will fit in the 1/2" drill
chuck that's on my drill press?

First, drill a 7/8" hole THROUGH a piece of steel plate, say 1/2" thick.
Clamp
the plate on top of the aluminum piece. Drill with an end mill or a flat
bottom drill bit.

I like this. It's a drill jig. But the OP implies all he has is a drill press.

I'd take a 7/8 Forstner bit, and change to Mg plate instead of Al if I could
get away with it. Then I'd use the method I developed the other day for reaming
an 11/16 hole in mild steel to 3/4 with a shell reamer.

Apply a large disc to your drill press drive pulley. Fit the belt for maximum
speed reduction. If you want, make the disc the same size as a PSA sandpaper
you use. It can serve as a disc grinder. Glue the disc to the pulley with
silicone, make sure it's on center, let it dry, and trim to center and smooth
the edge while it's running, with a block plane and a file, then some
sandpaper. The larg mass of the block plane will tend to catch the high sector
and shave it down. Take very light cuts.

Then, modify your Forstner to have less than a 1/10 inch pip, preferable no
more than 1/10 inch wide, as well.

Lay out your

number of flat bottom holes,

with center marks, and find the center mark with the pip. CLAMP THE WORK.
Remove the up-spring from your drill press's feed handle, with gloves on, and
put it in backwards, so it automatically feeds down, as hard as possible.

Then you can rotate the disc by hand and if the spring pressure is sufficient,
the Forstner will cut.

The proper setup for doing this, if a mill is available, is:

cutting speed 300 fpm
diameter 7/8 inch
cs * 4 / d = less than 1300 rpm
cutting time: Let's say 3 sec
depth: 1/10 inch
cu inch: 0.72 / min
HP: about 1 HP milling machine

The reason it's not working on your drill press is there's no way it's going to
deliver 1HP at 1300 rpm. Also, at that speed and feed, a standard drill chuck
will slip. At the hand speed, you can arrange things so it will not slip.

So you can save $10,000 on a new mill by doing these holes by hand, or give it
to a machine shop and it'll be done in 5 sec + move time, times how many holes,
billed at about $45 (at least ) per hour. If you're lucky you can be in and out
in a quarter hour for $50 minimum. The reason they charge so much is that when
the machine sits over two shifts at night, not being used, the interest on
their loan really piles up, because interest is 24/7.

It's much easier with a disc, and preferable a finger hole in the disc, because
the drive pulley is not made to be rotated by hand. Make everything on the disc
smooth, it must never hurt someone. If needed, make up a guard for it, or
remove it when done.

The problem I was faced with about a half a week ago was that while the HP and
speed were OK, the stiffness was inadequate. As soon as I hear the chamfer on
the reamer start to chatter and groan, I quit for the day. That **** scares me.
With a fresh start later, I was able to start the chamfer and even proceed into
the material a bit, then power up and ream right through it. Getting a 7/8 bit
started in Al with a drill press is similar. You might get it started by hand,
but the depth is so shallow, why not do the whole thing while staying in the
safety zone.

CLAMP THE WORK.

UNPLUG THE MACHINE.

HAVE A NICE DAY.

Yours,

Doug Goncz (at aol dot com)
Replikon Research

Replikon Research researches replikons, which are self-reproducing
configurations of non-living matter in environments that support replication,
analogous to organisms living in ecologies.