Parting off

[John HaineParticipant @johnhaine32865]

Was it G H Thomas used to demo parting off coins of mild steel at 200 rpm with power feed at ME exhibitions? If it's good enough for GHT …

[Alan Wilkinson 1Participant @alanwilkinson1]

Right parting off, the tool must be on centre but be aware that as soon as you put on a cut the work rides up due to play in the spindle bearings so you need to set your tool to compensate for wear lift, if you are hand feeding the work goes up and down has you wind unless you keep the pressure on all the way, that’s why power feed is better, so set your tool higher by the amount of play in your bearings and then it’s like cutting butter. ATB Alan

[Darren BinghamParticipant @darrenbingham71373]

Thank you so much for everyone that replied.
I had no idea that this was a common issue, it’s restored some confidence.
best wishes to all

Darren

[SillyOldDufferModerator @sillyoldduffer]

Hinted at but not explicitly mentioned is parting off is easiest on big heavy machines and decidedly tricky on little ones! It's to do with rigidity. As parting requires more force than most operations and the tool is likely to be fully extended almost any unwanted movement due to the tool or tool-post bending, or the cross-slide twisting against the gibs, or the saddle moving will cause a dig-in. Everything has to be snugged up or locked, which may not be possible if the machine is worn or out-of-adjustment. Plus swarf jambs in the slot, the tool has to at a right angle to the job, lubrication is advised, and it needs a steady not-too-fast or slow feed, ideally under power because humans wobble and dither.

Darren didn't identify his machine, but parting off on a mini-lathe is difficult. My much larger WM-280 is far less fuss. It parts off OK from the ordinary tool-post, but it's happier cutting with a heavy rear tool-post. The rear post is considerably stiffer than the cross-slide and towering front tool post and it grounds the cutting forces directly into the saddle. Basically a rear tool-post removes several opportunities for unwanted movement. I'm sure a well-adjusted lathe in the Boxford, Colchester etc class would make parting off even easier; I wish I had a bigger workshop.

Dave

[old martParticipant @oldmart]

I mostly use Kennametal and Kyocera 26 and 32mm blades with 1.6, 2, and 3mm inserts. These are modern industrial tools and as I have mentioned before, both manufacturers state that tool height should be 0.004", 0.1mm above centre if the parting is performed on solid material.

Modern CNC lathes vary their spindle speeds according to the remaining diameter of work during the cut.

[Tony Pratt 1Participant @tonypratt1]

Posted by old mart on 10/06/2020 12:31:23:

Modern CNC lathes vary their spindle speeds according to the remaining diameter of work during the cut.

Called 'constant surface speed', definitely helps, you can do something similar by manually increasing the rpm on a variable speed lathe.

Tony

[Clive FosterParticipant @clivefoster55965]

Fundamentally its all about preventing the chip from jamming in the cut.

By the very nature of the cutting process the chip is wider than the slot so it either has to be folded or deflected sideways if it is to come out cleanly. Proper lubrication of the slot sides, preferably by spray or flood, will clearly be a great help.

The V shaped top of modern insert tooling effectively folds the chip for easy evacuation. Being carbide they can also run fast enough to soften the metal at the cutting point reducing loads. One thing to watch out for if not running at book speeds and feeds is the formation of a continuous coiled chip riding on top of the tool carrier blade.

If left to build up this will jam.

Often happens partway through the cut as the surface speed on the smaller diameter drops below optimum for that particular material. I find the effect is generally limited to a certain range of diameters and speeds for any given material. My Smart & Brown 1024 VSL has continuously variable spindle speed so hitting the speed change button usually sorts it. More of a problem with fixed speed machines. On my P&W B I switch to manual feed and can usually clear the chip by varying the thickness.

When using an Eclipse or similar tapered blade grinding the tip with a slight side angle helps. The chip starts off at a slight angle to the slot and will tend to continue that way twisting a bit more to accommodate the extra width. As the cut should be very small, Andrews 4 thou per rev is typical, maybe 2 thou on a small lathe, there is little benefit in grinding a top rake. Dead flat top and really, really sharp works fine. Due to the taper top rake produces a small clearance a little back from the cutting tip just right to trap a ragged edge and grab it. Sometimes tilting the balance between just managing to cut and dig in when things aren't going quite as well as they should.

Obviously a thin continuous chip really helps. Which can be difficult to achieve by hand. Especially if nervously grumbling a long at low speed. Most folk tend to over-cut when hand feeding. If, for any reason, the tool fails to cut next time round it will try to take a bigger cut needing more force. Doesn't have to happen for many revs in succession before you have serious dig in issues. Usually with the tool trying to climb over the work as the first step to disaster. The stiffer the machine and set up are the more likely it is that sufficient force can be generated to overcome a temporarily failure to cut before the problem builds up.

The rear tool post carrying the parting tool directly on the cross slide is a good engineering solution for small machines. Not just due to eliminating all the potential sources of flex in the top slide but also because it usually causes the feed screw to operate in tension.

Conventionally the feed screw pushes the tool into the work so, due to the screw helix, there is a slight sideways component to the forces causing it to bend fractionally. For practical purposes the push force goes through the core diameter of the screw so, on our small machines, it is significantly slimmer and weaker than it appears at first sight.

Effectively its a very stiff spring.

So if you are supposed to be cutting 4 thou per rev and it takes two or three knocking back revs to build up enough force to start cutting again the extra force is generated by the spring effect. Once it actually starts cutting again the tool will spring forward and get slightly ahead of your feed. Classic conditions for "pilot induced oscillation" and chatter. Hopefully there will be enough damping to get things back under control but this isn't always the case and a jam up will probably occur soon. Not always. My first SouthBend 9" would often happily settle into a steady 1/16" or so back and forth motion of the cross slide during parting off for this very reason. Jam up / no jam up about 50 – 50. Cured by an extensive overhaul. Don't recall ever finding the specific reason but it was well used and everything was slacker than ideal.

With a rear tool post the screw acting in tension is effectively inextensible so you don't get the spring effect. Everything is much more under control. On a small machine you have a more positive feel of what is going on in the cut too.

An further, often overlooked, benefit of a rear tool post is that its worth taking the time to set the tooling up just so for absolute best results. When changing tools, even with a QC post, close enough rather than just so is usually the norm. Sometimes close enough doesn't quite cut it.

Clive

Edited By Clive Foster on 10/06/2020 14:05:28

[John HaineParticipant @johnhaine32865]

As I've mentioned before, an Eclipse type blade in a rear tool holder with a built-in top rake angle can be improved by using a diamond needle file to produce a shallow "scallop" the full width of the cutting edge, just like you find moulded into a carbide insert tool. This encourages the swarf to curl in and reduced the tendency for it to jam. Takes only a minute or two to do by hand and well worth while. That means that the edge can be left square to minimise the chance of the tool being forced sideways as it cuts.

[egaParticipant @ega]

old mart:

Would the 0.004" above centre advice apply to the rear toolpost? ie should the tool tip be set low by the same amount?

[egaParticipant @ega]

Clive Foster:

As a confirmed RTP user I was reassured to read your very full post.

Can you elaborate on the circumstance that " "By the very nature of the cutting process the chip is wider than the slot"? I can see that this is so where the front of the tool is slightly angled to prevent a pip on the work but what is it that causes the straight-across chip to become wider than the parent "kerf"?

I have seen numerous explanations of the advantage of the RTP over the front-mounted tool but your point about the leadscrew being in tension was new to me and seemed persuasive. It did occur to me, however, that there might be a corresponding disadvantage in that the tool is free to advance by the amount of backlash in the nut.

[old martParticipant @oldmart]

ega, I did incorporate that offset into the rear toolpost that I made for the museums Smart & Brown model A and also added a rear lock for the saddle, so with two locks the saddle is pretty solid.

[Darren BinghamParticipant @darrenbingham71373]

Posted by SillyOldDuffer on 10/06/2020 10:13:50:

Hinted at but not explicitly mentioned is parting off is easiest on big heavy machines and decidedly tricky on little ones! It's to do with rigidity. As parting requires more force than most operations and the tool is likely to be fully extended almost any unwanted movement due to the tool or tool-post bending, or the cross-slide twisting against the gibs, or the saddle moving will cause a dig-in. Everything has to be snugged up or locked, which may not be possible if the machine is worn or out-of-adjustment. Plus swarf jambs in the slot, the tool has to at a right angle to the job, lubrication is advised, and it needs a steady not-too-fast or slow feed, ideally under power because humans wobble and dither.

Darren didn't identify his machine, but parting off on a mini-lathe is difficult. My much larger WM-280 is far less fuss. It parts off OK from the ordinary tool-post, but it's happier cutting with a heavy rear tool-post. The rear post is considerably stiffer than the cross-slide and towering front tool post and it grounds the cutting forces directly into the saddle. Basically a rear tool-post removes several opportunities for unwanted movement. I'm sure a well-adjusted lathe in the Boxford, Colchester etc class would make parting off even easier; I wish I had a bigger workshop.

Dave

[egaParticipant @ega]

old mart:

Thanks for the reply; that looks like a really rugged job.

My tipped blade is by ISCAR and so far as I recall the offset was not mentioned by them. I take it that like me you are relying on the repeatability of the tip setting. I will check mine to see that if I am off-centre it is in the right direction.

The saddle lock on my Willson is at right front of the saddle ie not the best place for this purpose; there is, however, minimal lift as the rear keep is closely adjusted – just as well as a lock at left rear would be somewhat complicated given the slant bed.

[Clive FosterParticipant @clivefoster55965]

Posted by ega on 10/06/2020 17:00:29:

Clive Foster:

As a confirmed RTP user I was reassured to read your very full post.

Can you elaborate on the circumstance that " "By the very nature of the cutting process the chip is wider than the slot"? I can see that this is so where the front of the tool is slightly angled to prevent a pip on the work but what is it that causes the straight-across chip to become wider than the parent "kerf"?

I have seen numerous explanations of the advantage of the RTP over the front-mounted tool but your point about the leadscrew being in tension was new to me and seemed persuasive. It did occur to me, however, that there might be a corresponding disadvantage in that the tool is free to advance by the amount of backlash in the nut.

 

I was told by one who certainly ought to have known that when a solid material is cut it always deforms during the cutting process. Chips end up slightly thinner and slightly wider than the material was when on the parent bar. Easiest way to think of it is to consider the underside of the immediately just cut material being squished by the tool under the force needed to push the next bit off. Or so said the expert when I failed to grasp a more proper explanation.

With the feedscrew operating in tension the tool would indeed be free to advance into the backlash region were there any significant forces promoting such movement. In practice a properly cutting tool will be pushed hard against the material by the tensioned screw.  The screw being pulled straight so it really can't go anywhere, any stretch in the screw is negligible. Unlike the opposite case where the screw is in compression and able to shift, or distort, sideways slightly to get out from under the load.

If things go seriously wrong and you have a jam-up causing the work to climb over the tool it can indeed pull the tool post and cross slide forwards into the backlash zone.

Been there, dunnit got the tee shirt with my old SouthBend Heavy 10. Needs fast reactions to get out of trouble before it all goes seriously pear shape. Fortunately the SouthBend was flat belt drive, very old belt at that, so so usually not too disastrous. The taper turning variant of the SouthBend Heavy 10, which I had, was unusual for small lathes in having proper telescopic cross-slide feed screw working in tension. Clearly better at parting off than the non taper turning version with a conventional push screw but not as good as a proper rear post system as the tool was still carried on the top-slide so inherently not as stiff.

Clive

 

Edited By Clive Foster on 10/06/2020 23:17:49

Edited By Clive Foster on 10/06/2020 23:19:22

[egaParticipant @ega]

Clive Foster:

Thanks for taking the trouble to explain this – I suppose it's an example of the maxim "everything is made of rubber".

[Anonymous]

Posted by Clive Foster on 10/06/2020 23:17:21:

Chips end up slightly thinner and slightly wider than the material was when on the parent bar.

Not sure I agree with that. For ductile materials at least, like low carbon steel, I think the chips are thicker than the feedrate might suggest and about the same width as the depth of cut.

Andrew

[David George 1Participant @davidgeorge1]

A rear toolpost is the way to go especially if you have a quick change toolpost. The forces on a tool when parting off have a different direction to turning and can be held better with a more solid rear toolpost. I made one for my lathe and wouldn't go back to front mounted one. Here is a couple of pictures of mine.

I can unclamp the top and just slide it in and out to suit job and it is still on center.

David

[Anonymous]

Posted by David George 1 on 12/06/2020 07:11:26:

The forces on a tool when parting off have a different direction to turning and can be held better with a more solid rear toolpost.

Can you explain that please.

Andrew

[Anonymous]

Not the same as turning but I've measured some chips created when milling with my new Arc face mills. In all cases the cutter was running at 900rpm and the feedrate was 430mm/min. With 5 inserts that gives a chip load of 0.096mm.

For aluminium (6082) I used a depth of cut of 5mm. The resultant chip measures 5.00mm wide and 0.35mm thick. It's interesting to note that the chip is tapered at one end. That implies that the chip gets thicker as the cut gets going and conditions stabilise.

For steel (hot rolled) I used a depth of cut of 4.5mm. The resultant chip measures 4.55mm wide and 0.19mm thick. The chip is also tapered at one end.

Andrew

[Phil PParticipant @philp]

I think the reason why a rear inverted parting tool works better is explained quite well here.

By working better, I mean a reduced chance of it digging in due to inadequate rigidity on a small lathe.

**LINK**

Phil

Edited By Phil P on 12/06/2020 08:45:47

[Clive FosterParticipant @clivefoster55965]

Posted by Andrew Johnston on 11/06/2020 08:03:48:

Posted by Clive Foster on 10/06/2020 23:17:21:

Chips end up slightly thinner and slightly wider than the material was when on the parent bar.

Not sure I agree with that. For ductile materials at least, like low carbon steel, I think the chips are thicker than the feedrate might suggest and about the same width as the depth of cut.

Andrew

Having put a good deal of thought into it, Andrews posts are always worth a good deal of thought, I'm going to stick with "my" experts opinion that the chip is a little wider.

If nothing else it makes such a nice, elegant explanation as to why parting off can be such a problem.

The marks left on the cut faces, if cutting dry, when the indications of a potential jam-up appear certainly look like scrape marks from the side of an over-wide chip. Lubrication of the slot is a known, effective, palliative which again suggests simultaneous scraping on both sides.

An explanation being elegant and sounding right is, of course, no proof of correctness.

Obviously the properties of a material have considerable impact on how a chip behaves. Looking at the standard diagrams for the forces on a chip as it is cut and, in greater or lesser degree, curled off the parent metal its entirely plausible that a ductile material could be scrunched up a little making the chip thicker as well as expanding sideways. Its not intrinsically impossible that any such scrunching could be enough to counteract sideways spread.

By the very nature of things homogeneous, or mostly homogeneous material, in solid lumps has internal forces pushing and pulling all the "bits" together in a manner vaguely similar to surface tension in a liquid. When you cut a chip the push forces are removed from the edge letting it expand a teeny bit. Usually chips are rather hot which will also contribute to expansion.

One thing is for sure when it comes to chip formation and behaviour. Not only is the devil in the detail but its also a very complicated situation so there are several metric boatloads of details involved. The scientific and engineering literature is well beyond enormous.

Clive

[Martin KyteParticipant @martinkyte99762]

My explaination is that with the toolpost at the front any give and there is allways give can draw the tool into the work and increse the depth of cut. At the back cutting forces allways push the tool out of the cut so you dont get the sudden changes in depth of cut. It's a feedback thing. You can allways get the chip jam ups etc at the rear but you don't suddenly go from the cut pushing the tool out to pulling it in.

regards Martin

Edited By Martin Kyte on 12/06/2020 11:38:38

[old martParticipant @oldmart]

I don't understand how the forces differ from front to rear parting. What matters is the stiffness of the setup, the removal of the compound helps, as does not having a QCTP on the front with the associated overhang and extra joint. Keeping the cross slide gibs properly adjusted and as near to zero leadscrew backlash as is possible helps. As the rear cutoff forces are upwards, the saddle must not be allowed to lift and it should ideally be locked down securely if possible.

[Martin KyteParticipant @martinkyte99762]

Posted by old mart on 12/06/2020 16:38:48:

I don't understand how the forces differ from front to rear parting. What matters is the stiffness of the setup, the removal of the compound helps, as does not having a QCTP on the front with the associated overhang and extra joint. Keeping the cross slide gibs properly adjusted and as near to zero leadscrew backlash as is possible helps. As the rear cutoff forces are upwards, the saddle must not be allowed to lift and it should ideally be locked down securely if possible.

The force geometry is not the same As you correctly said the rear cutting forces are upwards and tend to lift the mounting and rotate the tool out of the cut. At the front the forces are downwards and if the force vector on the tool passes in front of the point of flexure of the tool mounting this moves the tool into the cut increasing the force and so on. You are of course correct in saying that the rear arrangement is more rigid, there are less slides and generally less overhang.

regards Martin

[Alan JacksonParticipant @alanjackson47790]

With apologies for being flippant I wrote this some time ago

Alan

A while ago I tried out a vertical parting tool it worked ok but I must admit I did not give it exhaustive testing to destruction etc.

In order to clarify the operation of a vertical parting tool here is my rather primitive description of how I think it works.

Imagine that you are the parting tool holder whereby your two hands are held straight out from you body. You hands are gripped together and your fists are the cutting edge. You have a friend who represents the metal being cut and he (If you are a front parting tool) pushes down on your hands while you do you best to resist him pushing down. You can see that he can quite easily push your arms down. If you want to now become a rear parting tool you can turn round 180 degrees and your friend would now push your hands upwards still he can easily overcome your resistance to him pushing up. So in order to stiffen you up, say you are frozen solid or have rigor mortise and are wearing a large pair of lead diving boots to anchor you down. When your friend (or should I now say de-parting undertaker) pushes down on you hands he will not be able to move your arms down because they are rigidly fixed to your body and he have to apply more force until you tip forward on you toes. Note that as you tip forward you rotate about your toes moving your cutting tool hands deeper into the metal being cut. If you are now rotated 180 degrees to become a rear parting tool your friend (some friend) now has to apply more upward force until you tip backwards on your heels. Also note that your cutting tool hands now move away from the metal being cut as you rotate backwards on your heels. Now you have to play the part of a vertical parting tool as I am proposing, so you now can be thawed out or de- rigor mortised. So lay flat on your back and push one arm vertically upwards and clench your fist to form the cutting tool. Your friend now has to apply considerably more force to overcome your vertical arm. Your arm will be in direct compression and until your elbow or wrist give way you will have much less a problem resisting his downward cutting force. It will also not matter if you are a front or rear vertical parting tool as long as the rotating force is pushing down on your hands. You can now get up and go back to your work or whatever you were doing. I apologise for being so flippant but I hope it does explain the reasoning.

Regards

Alan

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