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Parting on the Lathe

Commodus

Aluminum
Joined
Oct 22, 2007
Location
Vancouver BC, Canada
Hi all, I have a hard time parting things off accurately and just generally well on the lathe.

I use an carbide insert-style parting tool, angled (slightly, maybe 5 degrees or less) so as to leave the burr on the stock and not the part. With 4140, I have the most success running at about 600rpm (with around 1"-2" diameter parts),and just feeding it in by hand, fairly forcefully. I've played with tool height a little, finding it works best if I'm a little below centre. The part (I'm talking fairly small parts here, usually less than a half inch long) heats up quite a bit, such that it will sizzle coolant sprayed onto it, so I flood the cut with coolant, thinking that perhaps it prevents possible binding of the tool in the cut?

Still, the whole thing seems a bit more 'dramatic' than I would like - the process seems very sensitive to feed rate and likes to squeal and chatter unless like I say, I'm quite forceful with the feed.

Also I have difficulty doing it accurately. I usually just touch on the face and zero my dial, then move in my desired distance, plus the width of the tool, of course. I can achieve accuracy within about .008 either way easily enough, but anything past that is a bit of a crap shoot and I'm not sure why? I guess maybe it's because I don't get a very positive touch on the face - the tool flexes a lot from side to side. Clearly that's not enough accuracy, half that variation would be too much!

I'm wondering if anyone has any tips for me...I was thinking tonight that maybe I could put some masking tape on the face of the part and move in until I tear that, and use that minus a few thou for my zero. But I can't try that until this bloody long weekend is over so I'm in suspense until Thursday, heh.

Thanks in advance!

Commodus.
 
Commodus,

When you say you angle the tip how are you doing that? The blade needs to be perpendicular to the axis of the job or the side of the tool will rub. Parting tips come in flat, and left and right hand angles for recessing and parting leaving the pip on the left or right.

To bring to length, I start a groove at around the position required plus about .050" of extra length. Make a groove wide enough to get the tip of my caliper in and adjust on the compound to bring to size leaving .002" up. withdraw the tool with about 1/4" left on the part before it separates, take up the .002 and take a light face cut with the parting tool to clean the face then fully to depth to separate.

I always use coolant when parting due to the wide chip load normally about 250 RPM but I'm generally conservative. I don't force the tool, but I don't let it rub either always by hand so I can back out if it starts making nasty noises.

Hope my method works for you, othes will have better ways for sure.

Best Regards
 
Commodus,

When you say you angle the tip how are you doing that? The blade needs to be perpendicular to the axis of the job or the side of the tool will rub. Parting tips come in flat, and left and right hand angles for recessing and parting leaving the pip on the left or right.

To bring to length, I start a groove at around the position required plus about .050" of extra length. Make a groove wide enough to get the tip of my caliper in and adjust on the compound to bring to size leaving .002" up. withdraw the tool with about 1/4" left on the part before it separates, take up the .002 and take a light face cut with the parting tool to clean the face then fully to depth to separate.

I always use coolant when parting due to the wide chip load normally about 250 RPM but I'm generally conservative. I don't force the tool, but I don't let it rub either always by hand so I can back out if it starts making nasty noises.

Hope my method works for you, othes will have better ways for sure.

Best Regards

I'm angling the tool with the compound rest, which of course will make it rub...yeah that makes perfect sense.

Also I really like that idea of making the groove wide enough to check! Why didn't I think of that?!

Thanks so much!
 
Commodus

Another advantage of ERA's excellent advice is that, for parting off large work, it puts you in good shape to follow up your parting groove with a wider groove, for chip clearance. Every time you sense things are getting a bit squeezed, back out and skim .020" off each wall of the narrow part of the groove.
Then recentre in this widened groove, and have at it afresh. Naturally in doing this, you need to stay clear of the reference or witness face established at step 1.

The toughest job in all of turning, I tend to think, is parting off material which has a high propensity to work-harden. Any momentary dwell on the infeed (in the case of something truly horrible like manganese steel or pure nickel) means you can pretty much say goodbye to a successful op.

Power hacksaws or bandsaws are a great option, then turn it round and put it back up in the lathe to clean the rear face, but you need to be ingenious sometimes if there's not much stock to hang onto in the vice, especially for a big hacksaw.

I've seen people grip a beaten up old lathe chuck in the vice, but better would be to make an adapter out of an offcut of angle steel (say 180 x 180 x 12mm for a 200mm diameter chuck, maybe 210mm long), gripped in the vice on the ends of the angle, and with the chuck sitting over the foot flange and bolted to the vertical flange.

Having said that, in many commercial shops you'll get a hard time if you are judged to be avoiding confrontations with the Minotaur...

If you avoid sloppy lathes, follow careful procedures (eg per above), minimise overhangs (work and tool), lock up unused slides, stop and rectify the moment chatter is sensed, keep feeds decisive and vigorous, swap out tips before they chip, and generally do your bit to avoid irritating the gods, parting off can be just another op.
 
Further to what I posted before, I support the job with a live centre in the tailstock wherever possible. If you're worried about what happens when the part separates, don't be, it just drops, but you can back the tailstock out just before the split if needs be.

Best regards
 
Parting is such sweet sorrow

This question has been posed and answered repeatedly and enough to make me yawn.

I part off on a 7" swing lathe which is long past its first or second bit of youth- with a 3/32" blade. That is 2" round MS!

Way back in the midst of time, an Ian Bradley wrote of a rear tool post and the late George Thomas further modified the Bradley casting to be used on a treadle lathe originally.
HemingwayKits carries the kit and details appeared in Volume 142 of Model Engineer and now are in GHT;s Model Engineers Workshop Manual.

The tool is mounted at the rear of the saddle and the cutting action is with the tool upside down and inclined at an angle of 7degrees. The blade is the conventional HSS variety but has a male vee ground at 140 degrees at the front and female vee of 140 degrees on the inverted top.
I run my lathe at normal turning speeds and part off in one cut.
The trick is that that the blade(s) cuts but the turning is narrowed to avoid binding and falls off in a continuous ribbon.
There are other designs- one of which is Ken Metcalf's which follow the principle so GHT is not alone here.

I like my own choice of lubricant and use neat lard ol. After all, the smell of cooking is one of the joys. Get the book or the kit or whatever. I have the sawcuts in my lathe bed where the previous 'expert' mis-used his hacksaw!

Cheers

Norman
 
Parting is such sweet sorrow

Greg,
When one has reached the tender age of 77+, it is reasonable to expect that some knowledge is going to be gained.

When you reach that stage in life, will you pass on that knowledge for a younger generation to benefit? Or will you have anything worth passing on?

My regards

Norman
 
Setting a parting tool at an angle seems like asking for trouble to me.

I don't know much about your machine but carbide does need some pretty decent speeds and feeds to work. HSS might be easier to handle with your machine.

The nipple you get on the part being taken off is a fact of life. The support is all at the chuck end so that end finishes off as clean as but there is nothing to support the smaller piece being parted off so it ends up with a nipple.

This might not be an issue if you have to turn it around in the machine for a second op or you might find that you can rub it off with a file pretty easily.

Stephen
 
I agree, a parting tool should be set as near to perpendicular to the axis of the work as posible. I prefer to use the inserts that cut on both sides because the ones that cut to left or right tend to pull the insert that way giving a convex or concave surface on the part.

If the part has to be a certain length I part off long and face or surface grind to size.
 
Parting is such sweet sorrow

Stephen,
I have been making some bits and bobs on my Myford this morning.
I was using HSS because carbide tooling in a shaky old lathe( like me, I suppose)
can break the carbide edge. Rigidity, fragility etc., sort of thing.
Inclining the top edge the classic 7 degrees means that one doesn't have to constantly lose height in the regrinding process- and all that is required is to move the tool a fraction onto centre height. The 140 degrees 'concave' vee was put in with the aid of my home made tool and cutter grinder. However, this can be altered by using a rounded arc from a small abrasive wheel in the chuck. This I haven't tried but it seems quite feasible.

The 'pip' is questionable because if the front edge of a parting tool is ground obliquely, the pip disappears.

Concluding, I part off either with a rotating back centre or without but near the chuck.

I have a delightful article of a test cutting 2.5 square MS in a 7" lathe faster than he could do it in a powered hacksaw.

So, there you have it

Cheers

Norm
 
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Yes, do a search, as this is old ground. I have a carbide parting tool on the way, but in the meantime, I've been getting by with my hss .5" blade. For my particular set up, cutting 1.25" 4130, setting the blade below center helped very much.


bb
 
You mention the tool flexing L/R a lot. I assume there is no question about the rigidity of your toolshank clamp and toolpost. To underscore a point briefly noted by Troup, it is important to minimise overhangs. Don't have any greater length of the tool sticking out than is necessary to part off to the central axis of the work. Any length of tool sticking out will flex to some degree, but the shorter the stickout, and the beefier the tool, the less the flex.
 
Getting 8 thou repeatablity for lengths for cutoff tools
isn't bad.

I take it you are touching off on the end of the stock
and then cranking over the width of the blade, plus
what you want to take off.

1) blades do cut a teeny bit oversize. One or two thou.

2) get a good face on the end you touch off on. Then
mark it with blue sharpie pen. Then sneak up your
cutoff blade till it just removes the sharpie ink.

Jim
 
Rigid. Rigid. Rigid. Your set-up must be rigid. I do light production work and the first step is parting off in the lathe. Current job is 7/8 dia. 303 SS. about 700 pcs. I could make these in a second operation lathe, but it is faster to part off in the lathe and drill and counter bore in a collet fixture in the Bridgeport.
My lathe is 17 inch swing on a D1-8 spindle. Rigid. I'm running 400 rpm and power feeding 0.0035 per rev. with a HSS “P” blade and a shop made holder mounted upside down on the back of the cross slide. Much more uniform and stable then hand feeding. With soluble synthetic lard oil, of course. Sulpherized heavy cutting oil would be better but that black stuff gets around better than 5th wheel grease.
This yields 10 parts in less than 8 minutes.
Usually tolerances are 0.003 to 0.005, with some squealing and a little chatter. Some times it just bes that way. If I'm to net $50.00 per hour I've got to hump my ass. Measuring is out of the question, so I use a stop set up in the tail stock, while holding a ¼ inch gage in one hand to allow the part to drop free.
EngineRoom Bodger you have been lucky if the part has never hung between the bit and the center, resulting in shrapnel whistling about ones ears.
Lets repeat what the others have highlighted. Get the speeds and feeds up. Cutter perpendicular to the part. If you can grind a “v” relief o top of the cutter it will help the chips fold up so they can exit the cut. Having the cutter upside down lets the chips fall out better, but restricts your view of the action.
What Troup said about giving yourself a fresh cut when going deep. When parting off large diameter aluminum above 2 ½ to 3 inches I use a Portaband saw hinged in the back of the cross slide. Rotation of the lathe is in the forward direction.
What Norman A said about searching the archives.
Grind your cutter square. A tit is part of the job and will be taken off on the belt sander.
Excessive overhang is death to the process. For carbide your set-up must be rigid.
Having said all that I'm in the process of setting up an Eisele cold saw with hopes of getting the hitch feed working. Cutting to length automatically while I drill will make Carlos a happy boy. Some call me Carlos
 
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Heh, I belong to a lot of forums and somehow this is the only one that I can rely on to give me accurate, useful advice. You guys are great!

Honestly, I'm a bit shameful...some of these things you guys have mentioned are pretty common sense, I'm a bit disappointed that I didn't just immediately figure it out on my own. Oh well, I know now! I'm sure on Tuesday I'll be parting like a pro :D
 
Stephen,
I have been making some bits and bobs on my Myford this morning.
I was using HSS because carbide tooling in a shaky old lathe( like me, I suppose)
can break the carbide edge. Rigidity, fragility etc., sort of thing.
Inclining the top edge the classic 7 degrees means that one doesn't have to constantly lose height in the regrinding process- and all that is required is to move the tool a fraction onto centre height. The 140 degrees 'concave' vee was put in with the aid of my home made tool and cutter grinder. However, this can be altered by using a rounded arc from a small abrasive wheel in the chuck. This I haven't tried but it seems quite feasible.

The 'pip' is questionable because if the front edge of a parting tool is ground obliquely, the pip disappears.

Concluding, I part off either with a rotating back centre or without but near the chuck.

I have a delightful article of a test cutting 2.5 square MS in a 7" lathe faster than he could do it in a powered hacksaw.

So, there you have it

Cheers

Norm
Cheers Norm

Did do a bit of mucking around with the pips produced by parting. Was able to grind tips at an angle as you suggest but was advised against that as you can get the tool running off at an angle. Fair enough I said and just decided that it was one of those things I would have to accept. Of course this was when I was younger and the machines we had were only so rigid, HSS parting tools were only so rigid and so on. So at that time in that instance we chose what seemed like the best compromise.

I do my work on CNC machines now and so the fixture is carbide with a disposable carbide tip. They might make them angled but I haven't come across one as yet. I am pretty sure the machine and fixture could handle it though.

Stephen
 
Was able to grind tips at an angle as you suggest but was advised against that as you can get the tool running off at an angle.
Hi Stephen,

You can always grind the tool to a symmetrical point, like a threading tool. No run-off.

Of course, then you would have two pips to deal with rather than one. :D

- Leigh
 
Are you initiating the part off on rough od? I found that unless you turned off the hardened OD crap from the foundry, the blade would tend to wander a few thous at first...
 
Hi Stephen,

You can always grind the tool to a symmetrical point, like a threading tool. No run-off.

Of course, then you would have two pips to deal with rather than one. :D

- Leigh

I did even try grinding a vee in the middle but that didn't achieve much. The sharp points tended to get knocked off.

A six axis lathe that has two spindles makes it easy. Just hold the job with both chucks as you part synchronise your speeds and no pip! *VBG* too bad there aren't more of those machines around.

Stephen
 








 
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