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From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Need help trepanning D-2
Date: Tue, 18 Apr 2000 14:59:16 GMT

Hi Tom,

Helluva first project to cut your teeth on trepanning!!

Obviously you have a few to make...and D2 is neither cheap..Nor a material with
which to take liberties.  So here is how I would go about it on a methodical

Soft jaws are GOOD..Make them deep enough to get a real good hold, and make the
bottom step WIDE enought to support, not just the outer ring...But the INNER
circle as well.  A step that reaches inwards at least 3/8" past the cut is

Face one side, flip and face the other.  Now, either center drill..Or if you
don't want to "spoil" the cutout..use a pressure pad..A short stub of round bar,
faced both sides and center drilled, so that you can apply pressure with the
tail stock while you do the actual cutting.

Go right through from one side!  The pressure pad will take up end play in the
spindle and it will prevent the inner disc from canting and jamming the tool as
you break through.

No need to break right through..As you come to the back face, the tool nose will
cut right through and slightly INTO the soft jaws.  However, BETWEEN the jaws,
it will push out a thin web of D2 that will hold the disc in place.  You can
then remove the tool and tailstock and break the inner disc out with a pry bar.
leaving the outer disc in place to finish the bore.

The tool is critical.

Ideally it should be custom made for THAT Diameter!  The external radius and the
internal radius, should match the external and internal radii of the cut..less a
bit for clearance. Make it as DEEP as possible. Grind it from a chunk of HSS
(Some on EBAY as we speak..1/2" x 3/4" deep! For cheap!)

Or..Machine it from a chunk of 4140, and braze on a "Thinbit" carbide tip.  If
you have a lot to do..GO BUY ONE!! ;^)

The tool should have a good chip breaker groove..Curl up and break those little
suckers..And use pleanty of HP cutting oil.  LUBE is what you want..NOT
coolant.  Run at about 50 ft/min and keep up a gentle but steady feed pressure.

LOCK every slide and gib not actually moving!..Even put a bit of drag on the one
that is!!

If the Lathe is at all old or worn..Consider taking off the top slide and
substituting a solid tool block, mounted directly on the top slide. In any
case..reduce all overhangs to a minimum.

Finally, Grind the trepanning tool with a SLIGHT "Bull Nose" and break the sharp
edges on the corners slightly..The bull nose will produce a chip that cups in on
itself (like the chip from a round nosed gouge)..this will make it slightly
narrower than the tool and the groove..easier to clear out and less likely to
rub and gall the sides of the groove.

Happy trepanning!!


Tom Gardner wrote:
> I need to make more cutter knives from D-2 sawn disks that are 9" OD x 7/8"
> thick.  I couldn't get hollowbar cheap enough.  The final ID will be 4".  I
> made soft jaws.  I turn one face then trepan the ID to about 3-7/8" half way
> through, flip the blank, face the new side and trepan the other half then
> bore to final ID .  I hand grind the cutter bit from 3/8" HSS cobalt so the
> cutter land is about 1/8" wide with a few deg. of rake and the nose with
> just a little rake to controll the "pull" into the cut.  This works ok but
> it's ssslllllloooooowww and I get only three blades done if I am lucky
> before the trepan bit breaks and imbeds into the disk. (I'm good at getting
> the shattered pieces out...practice)   I am mist cooling and the speed feels
> right.
> I have never seen a real trepanning bit so I am guessing at the geometry.
> Is there a formula for the bit?  Hell, I don't know if I'm even spelling it
> right.  Is there a better way?  I can't just bore the blades and turn all
> that D-2 into chips, I guess I'm cheap, but the slugs are too handy for
> small dies and other stuff.   The job is like dental work and we have to do
> 18 more.
> Tom Gardner

From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Need help trepanning D-2
Date: Tue, 18 Apr 2000 17:57:29 GMT

Chip breaker should be a groove, across the nose, about 1/8" wide and .030" to
.040" Deep..leave a TINY flat on the extreem tip, so as not to weaken the tip
too much.

"Bull Nose" means slightly radiused..even a shallow vee nose..either should be

teenut wrote:

> > The tool should have a good chip breaker groove..
> What would your preferred chip breaker look like on a tool
> like that?
> > Finally, Grind the trepanning tool with a SLIGHT "Bull Nose"
> By bull nose, do you mean a concave front to it?
> Jim

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