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Parting off on Myford lathes

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Tony Jeffree	19/01/2011 14:07:15
569 forum posts 20 photos	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. I was not talking about wear or about belts slackening in use due to permanent deformation. I was talking about elasticity. Like it or not, when you drive a spindle via a drive train, there is a measurable amount of elasticity in the drive train which, when coupled to the various spinning masses involved (motor armature, spindle, chuck, workpiece, plus any intervening shafts/pulleys/gears) forms a mechanical system that can be made to oscillate at one or more resonant frequencies. You can shift the resonance to a different frequency by changing the stiffness of the drive train and/or changing the inertial load on the drive train. You can dampen the tendency to resonate by introducing friction into the system (and applying a cutting tool to the work does this). Hence, incidentally, the rationale for one of the maxims for getting rid of tool chatter: "Decrease speed and increase feed". The decrease in speed shifts the rotational speed away from the point where it may be contributing to the resonance, and the increase in feed increases the amount of frictional damping. Regards, Tony
blowlamp	19/01/2011 15:08:16
1885 forum posts 111 photos	"I was not talking about wear or about belts slackening in use due to permanent deformation. I was talking about elasticity." As I tried to point out, you are talking about the elasticity (acting as a spring) of the belt at a minute level, which in comparison to belt slip in this context, is inconsequential. You also stated that belts stretch, which I took you to mean as a permanent deformation. "Like it or not, when you drive a spindle via a drive train, there is a measurable amount of elasticity in the drive train which, when coupled to the various spinning masses involved (motor armature, spindle, chuck, workpiece, plus any intervening shafts/pulleys/gears) forms a mechanical system that can be made to oscillate at one or more resonant frequencies..." It's not a matter of if I like it - it's just fact. If you take a look at my previous posts on this, you'll see I asked for clarification on this very point, by posing the question about how a geared-head lathe with a motor belt drive in the system, differs from the original poster's example (Myford). As almost all lathes have a belt drive somewhere in the train and therefore have an 'elastic' transmission setup, it's strange that apparently these aren't susceptible in the same way as a Myford. Martin. Edited By blowlamp on 19/01/2011 15:11:30
Richard Parsons	19/01/2011 15:40:27
645 forum posts 33 photos	Long ago and far away in a rain soaked land I ‘grumped’ about being asked to part-off an item without running the lathe in reverse (as my Dad did) or having a rear tool post.I was given a verbal ‘thick ear’ by my ‘teacher’. "Remember the details of tool rake I taught you! Choose, sharpen and hone the parting tool and do not forget the side clearances, pick a tool with a slight side angle depending where you want to leave the ‘pip’. Use plenty of cutting oil (we did not use ‘slurry’ and take it gently". He then added “if you bug*er it up I will rip your etc. I will beat you to death with the soggy end”. He stressed ‘concentration’ and ‘care’. The only time I have had problems were when I was distracted by SWAMBO, a “skilled man” (which is another story) or was in too much of a hurry. Edited By Richard Parsons on 19/01/2011 15:43:04
KWIL	19/01/2011 16:43:06
3681 forum posts 70 photos	Thank you Richard, we are back to my entry at number 3 in from the start of this saga, most of it is up to the nut driving.
Tony Jeffree	19/01/2011 16:50:58
569 forum posts 20 photos	Posted by KWIL on 19/01/2011 16:43:06: most of it is up to the nut driving. ...or maybe the half-nut <grin> Regards, Tony
Alan Jackson	19/01/2011 17:42:04
276 forum posts 149 photos	I generally agree with TerryG' s summation that a rear parting tool post works best. I have spent many hours being petrified at parting off on lightly constructed lathes with average head bearings etc. Now the rear toolpost on my chipmaster works easily without fear. I have also experimented with front and rear cutting tools and reverse rotation upside down mounted tools and believe that rear inverted cutting position works best on light lathes because the cutting tool does not seem to get to the dig in position so easily, it becomes less resonant. The cutting forces seem to reach a more stable situation and this shows up in a better finish. Alan Edited By Alan Jackson on 19/01/2011 17:44:19
WALLACE	19/01/2011 18:17:43
304 forum posts 17 photos	So how about someone for the good of lathe users everywhere doing a test with two identically ground parting off tools on the same bit of steel using a rear and front mounted tool post and seeing which is best. You could steadily increase the feeds until they break Even better, video it and stick it up on YouTube for all to enjoy - I would offer to to the dirty deed myself - but alas - I don't have a rear mounted tool post on my lathe . . .. w.
Chris Gunn	19/01/2011 19:36:16
459 forum posts 28 photos	I have read through the thread, and up to now no one has commented on the bed design of the Myford. I remember reading a long while ago in the ME that this was felt to be the reason for poor parting off in the Myford. As the bed is of rectangular construction, with no vee sections, it is difficult to get the cross slide rigid and chatter free. Parting off in a front tool post in a lathe with a vee bed surely pushes the cross slide down into the vee, and will tighten things up. When I worked at British Steel in the DRTD, we had aworkshop for small jobs, with Myfords, Boxfords and Kneller round bed lathes, everyone would pick the Knellers first to do a job, the Boxfords second, and would avoid the Myfords apart from doing a bit of polishing. I know this will upset the Myfordfans but thatis how it was. I have a Bantam, and use front and rear toolposts, and it will part off using the feed with a sharp tool and plenty of suds, something else no one has mentioned. I was always taught to use copious amounts of suds when parting off when I did my apprentiship, ithelps keep everything cool the tool sharp and flushes away the chips. Chris Gunn
blowlamp	19/01/2011 20:51:15
1885 forum posts 111 photos	Michael. Thanks for this very interesting reply, but in it, you seem to be concentrating on chatter, rather than the topic of parting-off. When I've had parting-off cram-ups in the past, I don't particularly remember chatter being part of the problem. It was more a sense of the lathe having a 'rough' feeling coming through the feed handle, accompanied by an unpleasant sound before the inevitable crack of the tool breaking. Another comment I might make, is that if springiness in the drive system is the problem with these lathes, then I can't see how using a rear mounted toolpost can cure it, as we will still be using that same drive system along with all it's supposed inadequacies. Did you find that using backgear eliminates the problem? Martin. Edited By blowlamp on 19/01/2011 20:52:17
Nicholas Farr	19/01/2011 23:26:28
3988 forum posts 1799 photos	Hi, Tony, I haven't said that belts are not elastic quite to opposite if you read my first post. My "stretchy"terminology was obviously not understood, what I ment was akin to a soft rubber band where it will be obvious that it is significantly stretching under load and then relaxing when the amount of load decreases. While this does happen, it is not significant enough for the operator to notice, being so small when the belts are driving. The driving side of the belt is always in tension during rotation, while the slack side is constantly taking up the differance in the whole system by the phenomenom known as certrifugal force. Michael, you can't compare a static situation of holding the motor pulley still while seeing how much your chuck will turn, with a rotating system for the reasons I have metioned above. You can't introduce the forces that keep the drive balanced. These are my opinions based on being hands on involeved with belt drives for all my working life. Wallace, I was thinking filming the events might shine some light on the situation, but I think you would need high speed filming in conjunction with known refferance points, and maybe data from strategically placed strain gauges. Regards Nick. Edited By Nicholas Farr on 19/01/2011 23:26:53 Edited By Nicholas Farr on 19/01/2011 23:27:34 Edited By Nicholas Farr on 19/01/2011 23:30:36
Ian S C	20/01/2011 09:10:01
7468 forum posts 230 photos	Although the V belt will fractionally stretch, I feel that most of the apparent stretch is in the fact that as the load goes on, the belt pulls down in the V (the belt normally should not bottom in the V). I use link type belts, and have no problem. Ian S C
KWIL	20/01/2011 09:25:17
3681 forum posts 70 photos	Have we now come to the conclusion that it is nothing to do with it being a Myford or not, merely down to technology, materials and technique?
John Stevenson	20/01/2011 09:48:54
5068 forum posts 3 photos	Getting back to a Myford. Posted by Michael Williams "(1) Elasticity of belts : Under safe conditions get someone to hold the motor pulley still and see how much you can rotate the chuck by . Make a distinction between initial taking up of the slack and the actual springiness ." Better still chuck a decent sized bar up 3/4" or 1" about 10" long protruding from the chuck , put a dial gauge on the bar about 2" out, roughly where you work the most, with the base on the cross slide. Now grab the end of the bar and wobble it about and watch the gauge. I'll leave you to draw your own conclusions and none of these relate to belts, gears etc John S. Edited By John Stevenson on 20/01/2011 09:49:39
The Merry Miller	20/01/2011 10:16:31
484 forum posts 97 photos	Can I take it as read that everybody ensures that the parting-off tool is dead at right angles to the headstock spindle axis and the saddle is locked before the tool starts cutting. I may well have missed these points in earlier posts, please forgive me if I have!
Tony Jeffree	21/01/2011 11:02:09
569 forum posts 20 photos	Posted by MICHAEL WILLIAMS on 21/01/2011 10:17:44: (3) I would really like someone other than me to decode those experimental results and explain what they are saying . OK...I'll bite. The experiments you describe are basically measuring (changes in) rotational speed of the spindle/chuck/workpiece. In the first one (strobe), you set the strobe so that it fires exactly once per revolution; the indicator line appears to be stationary because he strobe light catches it in exactly the same position on each rev. Applying cutting force to the work will slightly slow the rotational speed of the spindle, so the indicator line will appear to rotate slowly clockwise when viewed from the tailstock end, because the spindle is taking longer for each full rev than the interval between strobe pulses, so each pulse "catches" the indicator line slightly too soon. (Hence, clockwise rotation shows speed rediction, and anticlockwise would show speed increase, which you could easily demonstrate with a variable speed drive.) In the chatter case, the average speed is still reduced, but because the motor/drive train/spindle assembly is now resonating (i.e., the instantaneous rotational speed is varying above and below the average rotational speed) because of the elasticity of the drive train, the indicator appears to jump back and forth either side of the "average" position. In the second experiment, the explanation is basically the same, and what it is showing is (a) the average speed of rotation (the first peak) and (b) the frequency of the resonance in the drive train (the second peak). John S's experiment was pointing out that the workpiece/chuck/spindle assembly is itself flexible, and can therefore also be made to vibrate in other modes under the right conditions, and that will also induce chatter. However, if you eliminate that factor from the equation (large diameter bar, cut close to the chuck, massive/rigid tool/toolpost), then you can still create chatter via the kind of rotary oscillation that your two experiments illustrate. The fix for the rotary oscillation problem is a stiffer drive system and/or more "flywheel" mass at the spindle end of the drive train and/or more damping. Or you simply follow the old maxims..."reduce speed, increase feed" etc...to shift the system away from its resonant frequencies. Incidentally, conducting your second experiment with different sizes of chuck but nothing else in the system changed would show a change in resonant frequency; the smaller the chuck, the higher the frequency of that second resonance peak. Similarly, using a stiffer belt would increase the resonant frequency. Regards, Tony Edited By Tony Jeffree on 21/01/2011 11:07:24
blowlamp	21/01/2011 12:26:58
1885 forum posts 111 photos	Right, second bite... So what we've established so far is that chatter is always present, but with care in the design of the machine, tooling and by using appropriate speeds etc, we can tune it out of harms way and so produce good work. This is my take on what happens and is what I believe causes parting-off problems with the commonly available HSS blades when used in the front toolpost: 1/ The cut commences and is initially fine. My View Is That:- Depite the tool being quite wide and so removing more material than in normal turning, the basic Machine/Tool/Speed setup is adequate for the job. 2/ Once a certain depth is reached, sensations can be felt through the lathe and the sound of the cut can change. At this point I would say a lot of people instinctively withdraw the tool, clean the groove and carry on. My View Is That:- This is the point that the system is beginning to become unstable. 3/ A short while after the cut is recommenced, the cram-up happens - but why? My View Is That:- The parting-off blade is now working in a confined space with very, very little clearance. As various sized chips are generatedby the cutting action, most of the larger ones are ejected by other swarf as it is being produced. However, small, gritty size pieces can remain and some of these will become lodged between the tool and the workpiece and might even 'pick-up' or 'gaul' the job. From there, it can be seen that this unstable system is working in a situation where a log-jam can now occur as other swarf is produced. When the log-jam does happen, the system experiences large forces and from these forces deflections can be seen about the tool and it's mounting, as well as the visible signs of the work moving too. So to work around the problem:- Some folks will widen the groove, before carrying on with the first cut as it gives more room for the swarf to clear. Some will part-off from the rear, which allows gravity to help keep the groove clear of swarf. Others will use a specially designed 'tipped' tool which curls the chip to make it narrower and also has plenty of clearance at the sides. Etc. Martin. Edited By blowlamp on 21/01/2011 12:30:41 Edited By blowlamp on 21/01/2011 12:31:35
blowlamp	21/01/2011 13:58:56
1885 forum posts 111 photos	If anyone needs further evidence of the effect that chip crowding has, you only have to drill deep holes in certain materials such as aluminium to see how easily it can jam the bit. Another example is if you try to deepen an already threaded hole at core size. In this instance the swarf is forced back down the hole as it rotates with the bit and is grabbed by the thread section of the hole. Both are easy to verify for oneself. Martin. Edited By blowlamp on 21/01/2011 14:00:54
KWIL	21/01/2011 14:48:47
3681 forum posts 70 photos	Martin and Micheal's conclusions are the same, use the correctly designed tip tool which curls the chip narrower and incidently have radiused corners of between 0.10 & 0.30mm (Sandvik), which is what I use.
Terryd	21/01/2011 17:15:06
1946 forum posts 179 photos	Hi Blowlamp, You can't really make an analogy between a twist drill and a parting tool. Any jamming with a drill is due to completely different processes. The swarf from a drill will curl freely in the small space of the flute in a helical form, especially when cutting aluminium. This will tend to block the free passage of the swarf as the tool gets deeper. This effect is compounded by the fact that the flute is deepest at the cutting point and gets shallower towards the shank, therefore the room for the chip to exit is is getting smaller, hence crowding. The answer is peck or intermittent drilling which breaks the swarf into shorter lengths and prevents the initial curling of the swarf. A properly sharpened and set parting tool produces a spiral not a helix due to a different cutting angle compared to a drill point and it's flute, the chip is also the same width as the cutting tool. If you see chip crowding in a parting groove it is very easy to remove with a scriber. It is unlikely that this will cause a 15 0r 20mm wide hss tool to break. if properly set. If you inspect a parting groove following a tool breakage you will see the relatively deep gouge where the tool has dug in. Best Regards Terry
blowlamp	21/01/2011 17:49:49
1885 forum posts 111 photos	Hi Terry. Just to clarify the position. My last post was only intended to reinforce my point about the effect chip crowding can have with regard to it's catastrophic consequences, rather than be a direct comparision with parting-off in the lathe. I don't quite follow yourpoint about seeing and removing chip crowding from the groove, because my experience (in the dim and distant past of course) is it's pretty well over by then WOW!! Is that a 15-20mm wide parting-off tool you mention towards the end Martin.

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