Subject: How do you part off?

From: [email-masked] (D.Diamond)
Date: Fri, 10 Sep 1993 03:56:52 GMT

>From time to time, I have to do a parting off operation in the lathe. With 
material larger than about 1/2" diameter, I often strike trouble, no 
matter how slowly and carefully I feed the tool in, the tool 'digs into' 
the work, sometimes with ruinous results.

I have seen in some texts where it is suggested that the tool should be 
placed on the back of the slide, so that the work tends to push away from, 
rather pull into the work. Is this method worth following up, perhaps by 
making a special tool holder for this job? Any suggestions?

Kind Regards, Drew.

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From: [email-masked]
Date: 11 Sep 93 09:56:20 -0400

> making a special tool holder for this job? Any suggestions?

Are you sure that the tool is exactly on the centerline?  The only time that
I've had trouble with parting off is when I miss the centerline a bit.

The other thing that I do is lock the saddle to keep the tool from moving
sideways.

eln

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From: russ@bbxrbk (Russ Kepler)
Date: Sat, 11 Sep 1993 15:13:15 GMT

>making a special tool holder for this job? Any suggestions?

[Mail to the poster bounced, so I'm posting my reply:]

Make sure that the cutter is sharp - I usually regrind mine every few
cuts.  Set it up with about 5-7 deg back relief just below center on
the work.  If you setup above center it'll do what you describe above.
Exactly on center is best, but I don't trust myself to do that, so I
setup just a little bit below.

I usually keep a brush with cutting oil handy, and hold it against the
work in the cut.  This minimizes the chatter from the tool, but it
will chatter no matter what you do.  A lot of chatter means you're
running to fast, slow the RPMs.

Were I you I'd chuck up some large stock and start cutting disks off
it - try different setups and see which works best for you.  When it's
running right it's magic.

-- 
Russ Kepler, posting from home  [email-masked],[email-masked]
"Sale of this posting without attached front cover may be unauthorized"

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From: [email-masked]
Date: 11 Sep 93 09:56:20 -0400

eln

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From: russ@bbxrbk (Russ Kepler)
Date: Sat, 11 Sep 1993 15:13:15 GMT

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From: [email-masked] (Peter Northway)
Date: Mon, 13 Sep 93 11:18:27 +1300

> >matter how slowly and carefully I feed the tool in, the tool 'digs into' 

Maybee too slowly. In parting off faint heart never won fair shavings :-)
The fead must be steady but deliberate.

> >making a special tool holder for this job? Any suggestions?
I had a back tool post on my Myford ML7 lathe. Worked well on this light
machine.

The best parting tool I have ever used was one with an inserted High Speed
Steel cutter, held in a holder with a spring neck.

                       ------
                      |      |  <- Spring neck
                      |  ||  |
     /----------------   ||  |-/--/   <- Cutter held in clamp.
    /____________________||___/  /
                             /  /
                            /__/

Excuse the crude C.A.D. representation.

And no I don't know where you can get one of these tools.
I suggest a visit to your local engineering suppliers.
Note: unless you have a big lathe with plenty of grunt and tight slides
      don't be tempted to buy one of the expensive Tungsten Carbide inserted
      tip tools.

Summary - Tool on or below centre (on or above if held in rear toolholder)
          Slow R.P.M.
          Steady Deliberate Feed.
          Plenty of _good_ lubrication (Cutting oil not old crankcase waste)

Hope this helps.

     Peter Northway        | Disclaimer : This is just my opinion. Well more
  Wanganui New Zealand     | like a tiny ripple in the fabric of the universe.
[email-masked] | Well would you believe just another beer break.

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From: [email-masked] (Tim Kirby)
Date: 13 Sep 93 13:07:42 CDT

D.Diamond ([email-masked]) wrote:
: From time to time, I have to do a parting off operation in the lathe. With 
: material larger than about 1/2" diameter, I often strike trouble, no 
: matter how slowly and carefully I feed the tool in, the tool 'digs into' 
: the work, sometimes with ruinous results.
: 
: I have seen in some texts where it is suggested that the tool should be 
: placed on the back of the slide, so that the work tends to push away from, 
: rather pull into the work. Is this method worth following up, perhaps by 
: making a special tool holder for this job? Any suggestions?

Usually the problem is a mix of

(1) Is the parting tool ground correctly. If it is a flat edge (i.e. parallel
    to the axis of rotation) you may hit problems. Excuse the limitations of
    ascii graphics...

            _
           | |             /|
           | |            | |
           | |            | |

           Bad           Good

    Needless to say, it wants to be as sharp as you can make it.

(2) Are you speeding up the work as you get closer in? Don't forget the work
    speed in relation to the cutting edge is slowing significantly as you get
    to the center.

(3) If at all possible, large items should be supported in some way,
    particularly as you get close to the end

(4) Feed that last bit *really* slowly.

Leastways, that's what I recall. I'm sure others will have something to
contribute...

Tim
--
Tim Kirby --------- Cray Research Inc., Eagan, MN, USA ----------- [email-masked]
Disclaimer: I disclaim, therefore I am. Be warned ...
-------------------------------------------------------------------------------
When all else fails, Immortality may always be assured by spectacular error(JKG)

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From: [email-masked] (Tim Kirby)
Date: 13 Sep 93 16:05:17 CDT

D.Diamond ([email-masked]) wrote:
: From time to time, I have to do a parting off operation in the lathe...

I ([email-masked]) wrote:
: Usually the problem is a mix of
: 
: (1)...(2)...(3)...(4)...

And forgot a couple...

(5) When feeding in to part off, it is usually advisable to take off more
    material from the chucked work than is needed to clear the tool... it
    doesn't have to be a lot, but if you just feed in the parting tool
    without providing any side clearance then there may be a tendency for
    the work to pinch the tool. You work your way in until there is not much
    left to part off, then take a finishing pass across the back of the
    turned item, feeding all the way through until the parting of the ways (sic)
    Uh, lousy graphics again...

          |          ||     |
          |          \|     |
    ---- -| ---- - ---- - ---- - ---- CL
          |          /|     |
    Chuck |          ||     |
          |         / |     |
          |         | |     |
          |_________| |_____|
     _____|
                     /|
                     ||
                     ||
                     ||

(and the other really obvious one)

(6) Your parting tool really is spot on center, isn't it?

Tim

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From: [email-masked] (Stephen Tolhurst)
Date: Wed, 15 Sep 1993 03:04:44 GMT

>> making a special tool holder for this job? Any suggestions?

I used to have lots of trouble parting off till I changed to the rear 
tool post arrangement and now it works fine.  This is especially the 
case if you have any wear in your headstock bearings as if there is the 
job will climb the tool when you try to part off the normal way. Low
speed and lots of suds also helps.
--

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From: <[email-masked]>
Date: Wed, 15 Sep 1993 11:24:13 CDT

Could someone please explain the concept of a rear toolpost?  Is the
tool monted inverted on the back side  of the crossfeed?  I don't
understand how this would improve the process of cutting off.  Any
clarification would be appreciated.   Thanks

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From: [email-masked] (Tim Kirby)
Date: 15 Sep 93 23:50:55 CDT

([email-masked]) wrote:
: Could someone please explain the concept of a rear toolpost?  Is the
: tool monted inverted on the back side  of the crossfeed?

Exactly so. The toolpost is "behind" the work and is fed by winding out the
cross slide. It's main advantage is that it can be set up and left there
(though one or two mishaps have been known where the operator forgets that
it's there and clears the "regular" toolpost a little too enthusiastically,
embedding the parting tool in the just turned work. Never done it myself, but
I have seen it done)

I have not seen any evidence that it is any more stable/effective/whatever
than a regularly mounted parting tool; if the headstock bearings are so shot
that it makes a difference then I would have thought the "regularly" turned
work wouldn't be worth the material it was turned out of...
(I'm assuming some accuracy is involved here...)

Tim

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From: [email-masked]
Date: 16 Sep 93 09:25:27 -0400

> An article in the current Home Shop Machinist magazine shows a cutoff
> holder that tips the blade for a 5 degree back rake. All the other
> info I've seen shows the blade going straight in with no back rake.> 

Everything that I've read about parting tools shows 5 degrees front clearance
and 0 degrees of back rake.

When I bought my tool holder (J&S) it came with a cutter preground with 5
degrees of back rake; it works well like this.

Maybe this is UK practice.

eln

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From: [email-masked]
Date: 16 Sep 93 09:57:18 PDT

> than a regularly mounted parting tool; 

The best explaination I have seen is: 

With "Conventionally-mounted" parting tools, all the points of 
flexure between the bed and the tool are such that the tool will
tend to move *towards* the work if it should chance to dig in.
This often results in chatter, broken tools or worse.

With a rear-mounted parting tool, the tendancy is for the tool
to move *away* from the work.   Thus, digging in is eliminated.

It's sort of like the difference between positive and negative
feedback.   :-)

I have not yet machined the casting for mine, so I can't testify
from personal experience.

--  Alan E. Frisbie               [email-masked]
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
--  Los Angeles, CA 90065         (213) 258-3585 (FAX)

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From: [email-masked] (John Morton)
Date: 16 Sep 1993 22:26:30 GMT

>to move *away* from the work.   Thus, digging in is eliminated.

I disagree.  The only design I know of which has this advantage
is that in which the tool shank is off center, so that the
motion of the tool when digging is in an arc away from the work.

My feeling is that digging should simply not happen, and indicates
that something is wrong.  It never happens to me unless I'm in
a great hurry and neglect to squirt enough juice or let the
tool cool off now and then.

The reason for the problem is the same as the reason for 90%
of all setup problems:  the setup is not rigid enough.  The
reason it has come up so much in this newsgroup is that we
mostly have light-duty machines at home.  To put things in
perspective, look at a typical commercial engine lathe
setup, in which you use a 2000 lb. machine to turn a 1" diam.
job.  Now scale that down to where your 200 lb. machine is
turning a .1" diam. job:  everything works well, nothing
digs in.  Unfortunately the machinist is not 6" tall and
therefore cannot see well enough to set the tool correctly
to center height!  :-)

John Morton					University of California
[email-masked]			Mechanical Engineering
{decvax,cbosgd}!ucbvax!euler!jmorton		Machine Shop

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From: Gregory P. Widin <[email-masked]>
Date: Fri, 17 Sep 93 17:16:36 GMT

>work wouldn't be worth the material it was
turned out of...

In some things I've read (primarily English,
since they are the main advocates of the rear
toolpost), it has been suggested that the
parting slot is cleared more readily when
mounted upside down (and hence on the rear),
since then those long curly chips tend to fall
out, rather than getting jammed up in the slot.
Can't say myself, because I've never used a rear
parting took, but it seems plausible.

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From: Gregory P. Widin <[email-masked]>
Date: Fri, 17 Sep 93 17:15:15 GMT

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From: [email-masked] (Tim Kirby)
Date: 16 Sep 93 22:46:10 CDT

Greg Lewis ([email-masked]) wrote:
: An article in the current Home Shop Machinist magazine shows a cutoff
: holder that tips the blade for a 5 degree back rake. All the other
: info I've seen shows the blade going straight in with no back rake.
: What say you experts? 

I dug through the HSM for the article. IMHO it was OK but not particularly
good, since the tool they described was really a grooving tool (appropriate
for the work in question but not so for general parting off).

Having made one or two vaguely contentious statements in my earlier posting
about parting off (no-one called me on them, though. You are all too kind :-)
this continued thread prompted me to wander through my bookshelves to my copy
of "Machine Shop Practice" (K.H. Moltrecht, Assoc. Prof Industrial & Systems
Engineering, Ohio State U.), pub: Industrial Press Inc. The copyright is 1981,
this is the 10th edition. It's a useful book becuse it has a lot of the basic
stuff in it that has faded in the 20 years or so since I was actually being
taught any of this stuff... :-S

Anyway, to the matter in hand; Parting (or, as KHM saya, cutting) off...
I paraphrase...

"The end cutting edge is ground to a 3 to 15 degree angle in order to cut off
the part without a burr. ... the blade has a back taper of 1 degree on each
side to provide clearance for the tool with respect to the sides of the groove
made by the tool... side relief 2 degrees, end relief 10 degrees... the face of
the tool is made flat, although a very large radius (equal to the radius of the
grinding wheel used to prepare this surface) is sometimes ground on the face...
this provides a very small back rake angle."

There is nothing more useful in that particular book as to when a back rake
is appropriate. I have very, very vague memories of being taught to put a small
back rake on a parting tool for soft materials (i.e. those for which you would
have ground a significant back rake on a "regular" tool.) It's a very vague
memory, though.

Ian, are you following this thread? Any words of wisdom to add?
                   (oh, we're not threading, though, we're parting off...
                                             sorry, I couldn't resist that one)
Tim
--
Tim Kirby --------- Cray Research Inc., Eagan, MN, USA ----------- [email-masked]
Disclaimer: I disclaim, therefore I am. Be warned ...
-------------------------------------------------------------------------------
When all else fails, Immortality may always be assured by spectacular error(JKG)

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From: [email-masked]
Date: Tue, 28 Sep 1993 15:15:49 GMT

In article <[email-masked]>, [email-masked] (Greg Lewis) writes:
>info I've seen shows the blade going straight in with no back rake.
>What say you experts? 
>

The tool holder allows setting the tip to be "on center".  In the ALORIS tool
catalog, they set the angle to 4 degrees.  The HSM article should also note the
blade is a "T" type ($25) and not the cheaper ($5) beveled blades.

I am in the process on making the HSM cutoff holder to fit my EMCO C5, I found
a "t" blade and am also going to modify to hold (I hope) a beveled blade too.

Steve