Subject: lathe thread cutting tool question

From: Terry Mayhugh <[email-masked]>
Date: 1 Mar 1995 14:51:15 GMT

Should a 60 deg single point cutting tool ground for
threading contain any relief or rake angles other
than end relief when fed in on a 29 deg compound? The
text I have says no, but I'm getting tearing on the
leading edge (mild steel) with feeds even as low as .005".
It seems like the tool is wanting some side relief or
maybe some side rake?

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From: [email-masked] (Mike Rehmus)
Date: 2 Mar 1995 17:52:53 GMT

In <[email-masked]> [email-masked] () writes: 

>
>Terry Mayhugh ([email-masked]) wrote:
>: Should a 60 deg single point cutting tool ground for
>: threading contain any relief or rake angles other
>: than end relief when fed in on a 29 deg compound? The
>	(SNIP)

Try a little top rake . . . about 2-3 degrees should do it.  It will not 
distort the resulting threads and you can pass a go/no-go thread gage 
test.  Also, make certain you use an adequate amount of the proper 
cutting fluid or oil.

Make certain the workpiece will allow a smooth surface finish in the 
first place by taking a cut along the length or a facing cut.  If you 
can, use a leaded steel, brass, or hard aluminum (e.g. 7075).

If you must use metal that finishes poorly, a good solution is to leave 
the thread few thousandths oversize and run a tap or die over it to 
finish off.

The most usual cause of poor finish on threads is a poorly formed cutting 
tool, a dull tool or a tool with a build-up on the cutting edge.  
Contrary to popular belief, unless you are cutting plastic, you do not 
need a tool finish any smoother than that left by a properly operated 
60-80 grit grinding wheel to get an acceptable surface finish.  In fact, 
stoning the tool is likely to get you into trouble because, as Ed Haas 
points out, it is very easy to round over a corner that should be left 
sharp.

At school, they suggested that we touch up our thread-cutting toolbits on 
the "China" grinders which were 6" grinders from Taiwan that had white 
wheels mounted (how's that for a grinding wheel designator?).  These were 
about 120 grit wheels and while they cut very slowly, they did allow one 
to sort of polish the toolbit.  The "real" grinders were 12" three phase, 
5-7 HP monsters with 2" wide wheels.  It was very easy to grind bits on 
these machines as compared to my 8" x 1" grinding wheels at home.

Now before I get a lot of flames, look at the term "acceptable".  As 
amateurs, we probably strive for a "just ground" look on all our machined 
surfaces.  OK, that may be fair.  But in the gotta-make-a-buck world of 
machine shops, striving for a better surface finish than spec is wasted 
time and money.  Besides, I've seen "real" machinists (I am NOT one) 
consistently turn out work that looks "just-ground" with HSS toolbits 
that are free-hand ground on 60-80 grit wheels (freshly dressed 60-80 
grit wheels mind you).  On my luckiest days, I can sometimes get as good 
a finish as they do.

Mike

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From: Bruce Olson <[email-masked]>
Date: Wed, 1 Mar 95 21:55:22 -0500

>It seems like the tool is wanting some side relief or
 
Mild steel can be difficult to get a good finish on when thread cutting.
B1113 or Leadloy will give much better results with less effort.  You will
get better results with a positive rake angle which should be ground on
the tool before you grind the 60 degree point.  I also grind a little side
relief even though it is not necessary.  The positive rake should favor
the side of the tool that does the most cutting, the one in line with the
compound rest (usually set at 29 1/2 degrees).  If this is a standard
size thread leave a few thousandths and run a die over it to clean it up.
Thread files are also available for non critical fits.  Good luck.

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From: [email-masked] (James Kirkpatrick)
Date: 2 Mar 95 13:29:07 MST

In article <[email-masked]>, [email-masked] (Mike Rehmus) writes:

|>Now before I get a lot of flames, look at the term "acceptable".

Which reminds me of something I read in The Machinist's Bedside Reader.
Something like "Perfection is not necessary here, mere excellence
will do just fine."