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From: Robert Bastow <teenutNOSPAM@home.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: 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


From: Robert Bastow <teenutNOSPAM@home.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: 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,


From: Robert Bastow <teenutNOSPAM@home.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: 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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