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Parting off on Myford lathes
| Billy Mills | 17/01/2011 19:30:18 |
| 377 forum posts | Gentlemen Everyone has different experiences and equipment and therefore opinions. Lets ask what is happening under normal conditions. The tip of the parting tool is fed into the rotating workpiece so that the tip penetrates into the rotating cylinder. The material above the tip is compressed beyond it's yield point and is detatched. You could say that the rotation of the workpiece and the presence of the tip steadily working toward the centre of rotation propagates a spiral crack. The forces involved in making the material detatch are very high and act vertically downwards on the tool for front mounting. If the cutting edge becomes rounded then the forces increase. If the tool is forced into the workpiece then the amount of work needed to remove the material above the tip increases rapidly. The amount of force acting on the immersed tip of the tool is not far short of destructive for the tool. So we can suggest several failure modes. With zero clearance we could have chip jamming causing the swarf to jam the tool into the sides of the cut. We could see a detatched wedge of material of larger than normal thickness combining with another part detatched chip wedging against the tool causing an impulse to the tool with an elastic rebound causing a dig in where the tool breaks. There is also the chance that the tool+toolholder+mounting+slack slide system has a resonance. Once the mean free length of the chips times the surface speed hits the resonance then the tool starts oscillating, heats, blunts and digs in. This latter concept does fit the noise before dig in experience. It would not be too dificult to fit sensors to a machine to find out why although we may already have all of the clues needed! Regards Alan. Edited By Alan Gray 1 on 17/01/2011 19:59:45 |
| chris stephens | 17/01/2011 19:31:54 |
| 1049 forum posts 1 photos | Hi Terry,
Glad to hear you did not waste your youth on booze and birds, but I was not impugning your qualifications, which I am sure are impeccable. How could I doubt them as I was unaware you had any. I would however like to see your stress analysis to support your views, and what form of modelling you would favour to prove your point of view. I have visions of a perspex toolpost and blade and polarized light, what about you? I suppose it is all done on computers these days. Without a suitable model I would like to know why you think a tool post is more likely to bend than a parting blade. I would have thought that the hold down bolts were stronger in tension than a parting blade is in bending. As for the toolpost itself, I would doubt that it would bend significantly, unless of wholly inadequate design. But then what do I know, I was asking for proof not conjecture.
I completely accept that IF the drawing is correct that the rear position would structurally be superior and could cause fewer dig-ins, but I am still waiting for the evidence.
All other things being equal, the rear parting brigade would have you believe that it was not possible to part from the front, which is clearly false. If some people can part from the front but other can't, what does that say? Could it be that they just don't know how or maybe that their machinery was in need of some attention?
I have just reread GHT on parting, well worth a few moments study. He comes up with many more reasons for problems than can be fixed with just a rear toolpost. I particularly liked the bit about a young man, a student at a technical college, being given naff instruction on parting off by his tutor! I wonder how many of his pupils passed this misinformation through the generations that followed?
I am sorry if you thought my general queries were a personal slur, but as you should realize by now they could not possibly be.
Happy partings
chriStephens
PS Would you agree that if the pivot point were to be at the point where the parting tool met the toolpost, and not as the drawing, that it would not matter a jot if it were front or rear mounted, as far as dig-ins were concerned, and that some other cause is likely to be the reason for having problems?
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| chris stephens | 17/01/2011 20:17:51 |
| 1049 forum posts 1 photos | Hi Guys,
I have renewed sympathy for people who can't part off. Last week I wanted to part some phosphor bronze and having lent my usual small parting tool, I was stuck. Then I remembered that ten or more years ago I had bought a parting tool holder of the 45 degree bend type, that I could not get on with at the time. I thought I could find it and as I now know so much more than I did then, I felt sure I could make it work. Well I dug it out, and after sharpening it, I put it on the lathe. Blow me down it was still crap, it would not bite at all. Two seconds of thought and I realized that it was made upside down, and that all the cutting loads were on the clamp screw not the body of the holder. So I turned the blade over and after sharpening the thing again I reversed the lathe and hey presto it worked. Now, I quote this as an example of lack of knowledge on my part at the time (must be as I said ten years since I bought it) and not a fault in the tool or front versus rear parting. If I had put it in a rear holder it would have worked, and some would have said that it was a clear example of how a rear holder was better, more fool them. I checked with the supplier's web site and the holder is now no longer sold, I wonder why?
chriStephens
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| Terryd | 17/01/2011 22:09:34 |
 1946 forum posts 179 photos | Hi Chris Trice, You say: "Trying to understand all the theory is good but testing, as with all experimentation, leads to a set of results from which to draw a conclusion." I said that I analysed with a combination of the theoretical and empirical. I suggest you re-read my posts and look up the meaning of "empirical knowledge." Terry Edited By Terryd on 17/01/2011 22:10:59 |
| Chris Trice | 17/01/2011 22:59:46 |
 1376 forum posts 10 photos | Er... my comments weren't aimed at you? They were more general to all the participants in the thread who offered their opinions. If a comment is directed at a particular person, I quote them or name them. I've always found you cool and agreed on many occasion but that's the second time you've taken a pop after assuming it's all about you. Starting to feel officially miffed now. |
| John Stevenson | 18/01/2011 00:02:01 |
 5068 forum posts 3 photos | Posted by Terryd on 17/01/2011 22:09:34: and look up the meaning of "empirical knowledge." Terry Edited By Terryd on 17/01/2011 22:10:59 . Used to watch the news reels at the old Empire Cinema before it got knocked down for a new Teasco's is that the same as Empirical Knowledge ? John S. |
| Billy Mills | 18/01/2011 02:16:13 |
| 377 forum posts | Yes but what about the parting off on bendy lathes with top slides? Perhaps we could get back to the shear plane sometime? All of this persecution is just too complex. Alan. |
| Ian S C | 18/01/2011 12:16:05 |
 7468 forum posts 230 photos | If it works, its right. Ian S C |
| michael cole | 18/01/2011 16:14:46 |
| 166 forum posts | Who cares, follow the advice of not much sticking out, and as solid as possable. I use flood coolant on steel when parting off and it makes a difference. Parting off yesterday 3 inch dia eccentrics. We know back tool posts work and that front mounted upside down works just as well. I moved from a inport 5 inch lathe to a newish British lathe again 5 inch swing and just part off front toolpost mounted right way up. This leads to to believe that the head bearings have a lot to do with it.
Mike |
| Terryd | 18/01/2011 16:17:38 |
1946 forum posts 179 photos | Hi John S, The short answer is no, Empirical knowledge comes from watching too many Star Wars Films too often.  .Hi Ian SC, I agree entirely, I usually part off from the front but for many who are not so experienced the rear toolpost is often better. You probably, like myself, have developed an instinctive feel for when things are going well and visa versa. That's a skill that cannot be taught easily it comes from that great teacher 'experience' (empirical knowledge). Best regards Terry |
| John Stevenson | 18/01/2011 19:18:40 |
5068 forum posts 3 photos | Why don't you say 'Experience' then instead of using two words we have to waste shop time going to look up ? John S. |
| Alan B | 18/01/2011 22:58:07 |
| 4 forum posts | Hi All,
It's all about mechanical feedback due to geometry of the machining setup, especially the fatal dig in one can get with a front mounted parting tool. The front tool gets deflected downward and into the work, the rear is deflected upwards and backwards away from the job. It's quite surprising how much a parting tool can deflect under cut, especially noticable on an interrupted cut, eg on a square bar.
Alan B. |
| Tony Jeffree | 19/01/2011 10:09:09 |
 569 forum posts 20 photos | There's a further variant on the front vs rear toolpost debate - front toolpost with the topslide removed. When I converted my ML-7 to a hybrid manual/CNC machine, I decided that the topslide was now surplus to requirements, and have replaced it with a toolpost machined from solid aluminium (I guess CI would be better but that is what was to hand) with the QC toolpost mounted atop that. The result is a far more rigid mounting than a front toolpost mounted on the cross-slide. I have a Q-cut indexable carbide cutoff tool fitted to one of the QC carriers and routinely use this front-mounted; it works like a dream. (Actually, it worked pretty well front-mounted on the topslide too, but the current setup is noticeably better.) The improved rigidity gained by losing the topslide significantly improves normal machining behaviour as well. Regards, Tony |
| Tony Jeffree | 19/01/2011 10:29:46 |
569 forum posts 20 photos | ...and some observations on the elastic belts issue. The drive train of a lathe, whether it is geared obelt driven, is elastic to some degree; obviously, gears have less give in them than belts, but they do give nonetheless. So the difference, and the difference between different types of belt, is one of degree only. So yes, the lathe spindle is at the end of an elastic drive train, and the kind of rotary oscillation that the OP talked about is certainly possible and if it occurs will certainly affect cutting behaviour and surface finish. This will probably be more obvious on parting off, but if it is there, it will also affect normal cutting too. Nicholas Farr made a comment earlier in this thread that "I can't imagine Myfords design with cheap belts in mind." Cheap or not, the standard V-belts used in Myford lathes are elastic, and have other issues due to the fact that they are jointed and the join is never as flexible as the rest of the belt; I found with my ML-7 that fitting replacement "T-Link" belts resulted in a significant reduction in vibration and a corresponding improvement in cutting performance and surface finish. They are also easier to remove and replace, as you don't need to disassemble the headstock bearings. Regards, Tony |
| Gordon W | 19/01/2011 10:40:52 |
| 2011 forum posts | I've tried to hold back on this recurring theme, but:-I'm only a raw beginner at model eng., did apprenticeship in mech. E. Parting off was never thought about, just whack 'em off, always assumed parting tool was at the rear because there was more room for it there, maybe that's what started all the theories? I agree with Tony, above, I've just checked my little lathe with top slide and 4 way toolpost, there are 7 horizontal joints between top of cross-slide and tip of tool, any sort of analysis is pointless. One other thing, how do you line up the tool square? I do it by sighting against the chuck face, not really accurate, maybe the rear post is set better to start with, and not disturbed between jobs. On cuts deeper than about 6 mm I always back out and widen the cut, seems to work, and must make up for some of the , inevitable, off squareness. |
| blowlamp | 19/01/2011 10:52:26 |
 1885 forum posts 111 photos | Myford drive belts are flexible, but they aren't stretchy - if that is what people are meaning when they say they are elastic.
I would still like an explanation as to why this theory doesn't apply to lathes that have geared heads and primary drive belts from the motor.
Taking a second cut creates a wider slot and simply gives more room for chip clearance.
Martin. Edited By blowlamp on 19/01/2011 11:02:34 |
| Terryd | 19/01/2011 10:52:36 |
1946 forum posts 179 photos | Posted by John Stevenson on 18/01/2011 19:18:40: Why don't you say 'Experience' then instead of using two words we have to waste shop time going to look up ? John S. Hi John, because as you must know, experience does not always equal knowledge. Unlike most, if not all of the intelligent contributors on this forum, such as yourself, who obviously have gained from their experiences, not everyone is so blessed or can even bothered to learn. I could have said 'knowledge gained from experience' but that would have been four words instead of two.  I have always been taught and instructed to use precise and carefully chosen language to convey meaning as exactly as possible without ambiguity and the phrase 'theoretical and empirical knowledge' best summed up what I meant. Even at my advanced age I still enjoy learning, including new words as well as the engineering know how from the fount of knowledge on this and other forums, and I assume others do as well. I apologise if I assumed wrongly. I am also sorry if I have wasted your shop time by inviting you read this message. Best regards Terry Edited By Terryd on 19/01/2011 11:04:16 |
| KWIL | 19/01/2011 11:11:29 |
| 3681 forum posts 70 photos | Tony Jefree's toolpost is just like the "Gibraltar" toolpost, in that case a CI lump, I cannot remember whether it was Tubal Cain or GHT. Either way, the only advantage I can see of a rear toolpost is that you do not have to change the front tool to part off when doing rep work, if you do not have QC toolholders. As yet no reply from the originator of this lengthening sage as to what clearance his front bronze taper bearing was set to, can be critical with a Myford. |
| Tony Jeffree | 19/01/2011 11:20:49 |
569 forum posts 20 photos | Posted by blowlamp on 19/01/2011 10:52:26: Myford drive belts are flexible, but they aren't stretchy - if that is what people are meaning when they say they are elastic. I would still like an explanation as to why this theory doesn't apply to lathes that have geared heads and primary drive belts from the motor. Firstly, ALL belts, Myford or otherwise, stretch. Some stretch more than others, but they do stretch. And yes, that is what I mean when I say that they are elastic - if you want it in mechanical terms, they have a Young's modulus that is finite. So, if you apply a tension to the belt, it will stretch. Maybe not by much, but it WILL stretch. End of. Secondly, given the above, and given that the gear train itself is also elastic (although not so much as the belt), yes, it does all apply to a greared train with or without a belt for primary drive. As I said earlier, the principle is the same; the variable is the degree of elasticity - a gear train is (generally) less elastic than a belt drive. Regards, Tony |
| blowlamp | 19/01/2011 13:34:56 |
1885 forum posts 111 photos | Quote:
"Firstly, ALL belts, Myford or otherwise, stretch. Some stretch more than others, but they do stretch. And yes, that is what I mean when I say that they are elastic - if you want it in mechanical terms, they have a Young's modulus that is finite. So, if you apply a tension to the belt, it will stretch. Maybe not by much, but it WILL stretch. End of. Secondly, given the above, and given that the gear train itself is also elastic (although not so much as the belt), yes, it does all apply to a greared train with or without a belt for primary drive. As I said earlier, the principle is the same; the variable is the degree of elasticity - a gear train is (generally) less elastic than a belt drive.
Regards,
Tony"
End Quote:
I was trying to keep the discussion reasonable, by speaking in practical terms of this particular topic, but if we are now at the microscopic level, yes drive belts are elastic. They are elastic in the same sort of way that a lump of granite or wood is, but not in the same way that a rubber band is.
If we're talking about belts slackening whilst in use, it will be seen as stretch by some, but I think it is more likely to be down to wear on the vee surfaces, which will allow the belt to sink deeper into the pulleys.
Any manufacturer worth his salt will choose materials that are appropriate for the job and a belt that progressively stretches, wont hold it's tension and will start to slip.
Martin.
Edited By blowlamp on 19/01/2011 13:35:50 Edited By blowlamp on 19/01/2011 13:37:02 |
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All Forum Topics > Manual machine tools > Parting off on Myford lathes