On Tue, 1 Nov 2005 12:38:26 -0500, "Proctologically Violated©®"
wrote:

Awl--

I'm sure I can "solve" this by being more vigilant, but that it happens at
all seems sort of strange.

Briefly, I'm drilling a 3/16 hole 3/4" deep in 1/4" round brass: face,
c-drill, then a 3/16 drill (tailstock/drill chuck, 5C collets).
Then, I'm turning down a piece of 1/4" round brass to 3/16" OD, for about a
3/8" insertion into the other piece w/ the 3/16 hole, for a "nice" fit.
Using a live center.
Have DRO, Aloris tooling, good for half a thou, I'm pretty sure--at least
the DRO says so!

Problem is, the 3/16 OD pc gets in only about half way before it hits some
kind of "step"!!!
To achieve the full 3/8" insertion, the fit winds up being far too sloppy!
Still works, but annoys the hell out of me.

Seems like a simple no-brainer, and I'm sure I could solve it w/ sequential
drill sizes, reaming, etc., but it doesn't seem to me like this should be
necessary, as there is nothing complicated or exotic here.

I would have anticipated any other type of problem--holes too big, holes too
small, ragged/jagged sides, holes off-center, etc.
But an invisible step??

Any idears?
Mebbe on my next have-at-it here, I'll test the female portion w/ a 3/16"
dowel, to at least try to identify the offending piece/operation.
It's pretty clear what the problem is (diameters!), but puzzling as to *why*
this problem is occuring.

TIA.
----------------------------
Mr. P.V.'d
formerly Droll Troll


Hey PV,

First, as to what it is, have a look at :

http://mathworld.wolfram.com/ReuleauxTriangle.html

then at :

http://www.integerspin.co.uk/polygon.htm

You can see why it occurs.

So, use the least chuck overhang you can, and if possible use steps of
drill lengths, as in center, spotting, screw machine, then jobbers,
taper, etc.,

One thing that will help with your particular problem is instead of
the general use drills for steels at 118 degrees, use drill bits
sharpened at 100 degrees with 12 degrees or less rake for use in
brass. And make the centre drilling step as deep as practical for
your purpose.

Take care.

Brian Lawson,
Bothwell, Ontario.
XXXXXXXXXXXXX

And here's one, right from the old horses mouth!! teenut said:


From: Robert Bastow
Newsgroups: rec.crafts.metalworking
Subject: Newbie: Drilling Brass/drill grind
Date: Tue, 14 Sep 1999 15:04:05 GMT

Hi George,

Unlike a lot of the problems/queries encountered on this NG yours is
simple
enough to explain and even simpler to rectify. 8^)

In days of yore, and even now if you care to track them down, there
were/are
available special STRAIGHT FLUTE BRASS DRILLS..No spiral therefore, no
positive
rake on the cutting edge of the drill lip.

It is this positive cutting rake that is causing your problems..Brass
being such
a (Relatively) easy and amenable material to machine, it always seems
in a hurry
to get it over with and tries to hurry things along by grabbing at,
and trying
to ride up that positive rake!!

The solution is simple..remove the temptation!! Take your drill to
the grinder,
align the axis parallel to the flat side of the wheel and gently
remove the
sharp positive rake edge of the drill lips, equally on both sides. For
your
3/4" drill, a flat, somewhat less than a 1/16th of an inch will be
perfect.

Voila! you now have a negative rake "Brass Drill"!

Now, you can go search out a special set of straight flut drills,
especially as
you do a lot of brass work. OR, do as I and alot of other machinists
do..Get a
new set of regular drills, modify the lot and keep them just for brass
and
bronze. In doing this, of course, as you get to the smaller sizes the
flat you
need gets smaller and smaller..to the pont that, under about 3/16"
diameter you
can achieve it with just a few wipes with a slip stone.

Quite frankly I recommend the latter course of action..If only because
it is
probably cheaper. But another good reason is because the remaining
spiral on
the drills does help clear chips out of the hole easier I think, than
a
straight flute drill.

Happy drilling

teenut

George Glines wrote:

I want to modify a new 3/4" MT2 (118 degrees) drill bit I have to drill 360
(free-machining) brass. The hole is 1 1/4 inches deep completely through
the material. I'm drilling a pilot hole but having some trouble with the
brass grabbing the drill bit.

My metalworking books, including Machinery's Handbook have been no help as
to the way to sharpen/grind the bit. I searched through DejaNews, and
posters mention either grinding a flat on the drill, or grinding a zero rake
on the helix???

Does this mean that I would hold the drill at a 90 degree angle to the
grinding wheel to remove all rake on the flutes? It appears that the drill
is grabbing on the flutes not on the point.

To make things more complicated, I've looked in all the big tool catalogs
for special purpose drills. Several show "fast-spiral" drills as good for
drilling deep holes in brass, then on the next page, recommend "slow-spiral"
as good for cutting brass. Since I work with brass quite a bit, I wouldn't
mind dedicating a group of drill bits just for that material, but since
these bits are all expensive, I don't want to buy the wrong ones.

Thanks,

George


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From: Robert Bastow
Newsgroups: rec.crafts.metalworking
Subject: Newbie: Drilling Brass/drill grind
Date: Tue, 14 Sep 1999 16:47:14 GMT

There is far more tendency to grab when opening up a hole in ANY
material..Brass
is no exception! The center of any drill..being a less than efficient
cutting
zone, helps hold the work down against the forces tending to make it
grab.

If you must use a regular drill and don't like the idea of zero rake
flats, then
just BLUNT THE POINT a bit, run slower than usual and feed steadily.

The blunt point, while not best practice, may at least reduce the
tendency to
grab. and save the job, your nerves..even your fingers!!

teenut

John Hofstad-Parkhill wrote:

What I have found is that the combination of a small pilot and a big drill
works very well.
If I need a 1/2" hole or larger, I'll spot, pilot with 1/8" then use the
1/2" jobber twist drill. My theory is that with more material to remove the
less tendancy to grab,


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From: Robert Bastow
Newsgroups: rec.crafts.metalworking
Subject: Newbie: Drilling Brass/drill grind
Date: Wed, 15 Sep 1999 03:34:51 GMT

Hmmm!! I KNEW you were going to ask that question...This is like
trying to
describe Gina Lollobrigida without using your hands!!

OK, you are familiar with a regular twist drill...Ok..It might help to
hold the
drill horizontally eyeball the cutting edge and imagine it is a
regular HSS
lathe tool. You can see that the cutting edge of the drill has a
distinct wedge
shape..just like your lathe tool

The angle of the "point" face is pretty well fixed..this is the
CLEARANCE
angle...It is like the FRONT FACE on a lathe tool!

However, the angle of the top face..the RAKE ANGLE..., is variable..It
is
determined by the HELIX if the drill...It corresponds to the top face
of a lathe
tool.

With a fast spiral drill, this angle is very acute, with a slow spiral
it is a
blunter wedge, and with a straight spiral it is at 90 deg to the drill
axis..In
other words George, you have a ZERO top cutting rake on your
wedge/cutting
edge...Just like a zero top rake on a lathe tool (Oft described as
NEGATIVE
rake..but it ain't really!!)

Now imagine what you would have to do to a positive top rake lathe
bit..to turn
it into a zero top rake bit? Grind a bit off the top! "Bust" that
sharp
angle... Blunt the wedge...Right!

This is what we want to duplicate! So what we do is "Blunt" the wedge
on a
regular drill by grinding or honing...INSIDE THE FLUTE and PARALLEL TO
THE
CUTTING EDGE. Just "busting" it enough to produce a visible flat on a
small
drill, is all that is necessary..so long as the flat is wider than the
depth of
the chip (depth of cut) then you are fooling that dumb 'ole brass into
thinking
it is being cut by a zero rake drill!!!

I hope to have my web page up soon so I will be able to post pictures
and
sketches to illustrate points..(no pun intended!!!)

That will save me a bunch of peck, peck, sh*t! peck, with my tired old
finger.

I hope this helps..please don't hesitate to ask again if all I have
done is
confuse you further!! It has been known to happen before!!

teenut

George Glines wrote:

Robert,

Many thanks!!!! I want to be sure I understand correctly. Am I grinding
the "side/flutes" of the drill or the "point/tip" of the drill? I'll
practice on some old bits first, but having a brass only set would be great.

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