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Parting off on Myford lathes
| Alex gibson | 21/01/2011 19:49:59 |
| 35 forum posts | Hi All,
I don't have any experience of Myfords, I have a Sieg C4 and have experienced all the scary stuff when parting off and have often wondered if the problems are more to do with the surface speed of the work passing the tool tip. If for example I try to part off a workpiece and set everything up to cut sweetly at first. As the tool is fed into the workpiece the rpm and feed rate are constant however the actual speed of the metal passing the tool tip decreases as the diameter gets smaller and sooner or later I get the "submarining effect" that TerryD described before. So at some point in the cut should i increase the rpm or slow down the feed to maintain the same surface speed?. My pea brain has been contemplating this for some time now, but I can't get to a conclusion. So parting off in my shop (garage) is usually done with a hacksaw, or if it"s bigger than 10mm the bandsaw.
alex
ps sorry i if I'm being too basic for this thread. |
| John Olsen | 21/01/2011 22:46:47 |
| 1294 forum posts 108 photos 1 articles | I'm not too convinced that the poly V belts make much difference to parting in the Myford, I have done this conversion to my ML7. It is a worthwhile thing to do for many reasons, but I don't think it has improved the parting off performance. Which incidently lately has been better than it was before I realised that the spindle bearings were due for an adjustment. I think it is still not as good as my Unimat 3 acheives, but then, the Unimat does not attempt quite such ambitious sizes, and is not nearly so old too. (The Unimat is driven by toothed belts. ) Incidently the Poly v conversion was described in ME for the Super 7 and Hemingway sell the bits for it, although mine didn't come from there. It can with a bit of ingenuity can be applied to the ML7. I managed to give myself four belt speeds instead of three, with the top and bottom being much the same as the original, so the ratios are close. Probably superflous because I also have the Newton-Tesla drive on it. Somebody said something to the effect that chatter was always present. I would say rather that the potential is always there, in that you have a system, or rather a number of systems, with the potential to oscillate. Drawing on my electronic background, such a system will not oscillate unless the energy input is sufficient to overcome the losses. There is usually some sort of loss, in an electronic system it will be resistance, and in our system it will be damping due to friction. If the damping is sufficient then oscillations will not be sustained. You can sometimes see this with an interrupted cut, where there may be a little sign of chatter just where the cut starts, but the rest of the cut is fine. Alex, with a small machine like the Sieg, I would suggest trying the narrowest parting blade you can manage. I have used one just over a mm wide on the Unimat with good success. This is ground from a piece of 1/4 inch HSS, so takes a while to make, and is long enough to part off up to about 5/8 diameter. The usual commercial blades are just too wide for these small machines. You can apparently make a holder and use a piece of hacksaw blade to make a tiny parting tool, but I haven't tried that myself. regards John |
| Terryd | 21/01/2011 22:47:57 |
 1946 forum posts 179 photos | Hi Martin, If you didn't intend to compare the two why use the comparison? It seems disingenuous to me. I'm sorry but so called 'chip crowding' takes place over a relatively long period of the process (several seconds) I'm surprised that you are not aware of this so called phenomenon happening. By the way, if chips are jamming so firmly, how come they can simply 'fall out' when a rear toolpost is used, by the simple effect of gravity? As to your last point - with all due respects, 15 to 20mm depth of HSS parting tool, which is the normal size as most engineers are aware, I assumed that someone as perceptive as yourself would understand that without it having to be explained. Best regards Terry Edited By Terryd on 21/01/2011 22:49:13 |
| Terryd | 21/01/2011 23:20:09 |
1946 forum posts 179 photos | Hi Martin, By the way, you can try the chip removal with a scriber or similar tool after the parting failure, the chips should still be there if they are so badly jammed. The only chip which is difficult to remove is the one produced when the tool has finally jammed. Kind Regards Terry Edited By Terryd on 21/01/2011 23:21:36 |
| Nicholas Farr | 21/01/2011 23:25:51 |
 3988 forum posts 1799 photos | Hi Tony and Michael,
while performing your strobing experiment, I'll assume that you were strobing the motor pulleyat the same time and found that there was no oscillation in your timing mark there then, and/or suitable calculated for the two differant speed ratios.
Regards Nick. Edited By Nicholas Farr on 21/01/2011 23:28:33 |
| blowlamp | 22/01/2011 00:57:50 |
 1885 forum posts 111 photos | Terry.
To answer your last posts. "If you didn't intend to compare the two why use the comparison?"
I didn't compare drilling with parting-off. I simply used drilling as an easily verifiable example of the effect of chip crowding i.e., the drill bit stops turning when jammed with swarf. "It seems disingenuous to me."
Read the post again and you should see that it isn't. "I'm sorry but so called 'chip crowding' takes place over a relatively long period of the process (several seconds) " "I'm surprised that you are not aware of this so called phenomenon happening."
I am, I was the one that mentioned it first. "By the way, if chips are jamming so firmly, how come they can simply 'fall out' when a rear toolpost is used, by the simple effect of gravity?"
The majority of them fall out of the way because there is no parting-off blade underneath them to block their exit. When compared to a front mounted parting-off tool setup, it's obvious that swarf will rest upon the upper surface of the blade by the effect of gravity and by that same effect, along with vibration, the smaller particles can tend to collect in any gaps that are present, thus magnifying the chances of a log-jam style cram-up. "As to your last point - with all due respects, 15 to 20mm depth of HSS parting tool, which is the normal size as most engineers are aware, I assumed that someone as perceptive as yourself would understand that without it having to be explained."
Well you wrote 15 to 20 mm wide and knowing what a stickler you are in your writing, I took you at your word. You made a straightforward mistake, but instead of correcting it, you're trying to put me on the spot for asking if what you wrote, was what you really mean to say - now that's disingenuous. Martin. Edited By blowlamp on 22/01/2011 00:58:54 |
| Ian S C | 22/01/2011 10:11:06 |
 7468 forum posts 230 photos | John, you mention hacksaw blades used for parting. When I first got my lathe, we were restoring a Lanz Bulldog tractor, It required 16 new nuts for the radiators, these I made from a 2" dia stick of bronze, and they were all parted off with a piece of a broken 12" x 1" industrial hacksaw blade. Front toolpost, no jamb ups. Ian S C |
| chris stephens | 22/01/2011 22:49:44 |
| 1049 forum posts 1 photos | Hi Guys,
For all those who think that you can only part off successfully in the rear position, today at Ally-Pally kids as young as ten, perhaps even younger, were parting off on a mini-lathe in the normal front position. As many of you may know a mini-lathe has a slight reputation for giving problems when parting, but a properly set up (by Arc Euro, not a SMEE mechanical wizard) lathe with a half decent parting tool and a complete beginner youngster can do it. So as the old pre smoking ban expression went, "stick that in your pipe and smoke it"
Anybody who wants to see, for themselves, can come along tomorrow (Sunday, the last day), just head for the SMEE stand. For anybody who has never seen or tried a Diamond/Tangential tool working you can have a try with one, but you might have to wait till the kids have finished first.
chriStephens
|
| John Stevenson | 22/01/2011 22:55:34 |
 5068 forum posts 3 photos | Aahh but a decent setup minilathe is tons better than a 64 year old design Myford.............. John S. |
| chris stephens | 22/01/2011 23:45:50 |
| 1049 forum posts 1 photos | Hi John,
"Oh Yes" as a dog called Churchill would say. And soooo much cheaper, too.
I will however stand up for the Myford, there has been a enormous amount oftrulyexcellent work done on a Myford, but admittedly on machines that were significantly cheaper than today's lathes. OK OK they also took lots of small cuts instead of a few large ones, but not every body is in a hurry or even has a clock in their shop.
Regards
chriStephens
|
| Alex gibson | 23/01/2011 00:34:44 |
| 35 forum posts | Hi Chris
I'm glad to hear that ten year old kids are taking an interest in the hobby, I don't suppose they stop to compare Myford to Chinese, I'm guessing they just got on with the task and enjoyed it. (sounds like you did too). Hope you have a good Sunday.
Regards
alex |
| KWIL | 23/01/2011 08:49:47 |
| 3681 forum posts 70 photos | I do not know about being cheaper, my Myford ML7, long retired and sitting pristine on its own bench in the corner of a workshop cost me the princely sum of £50 in 1952, it could sell for around £700 now. Reversing the direction, today's chinese costing half that would equate to £25, still I would point out a few weeks wages then. Edited By KWIL on 23/01/2011 08:50:25 |
| chris stephens | 23/01/2011 10:28:39 |
| 1049 forum posts 1 photos |
Hi Alex,
To be fair, as I always try, the kids are often pushed forward by parents/grandparents or see others having a go and want to see what it all about. There is, I suspect, no great interest by the kids in making a Loco at this stage but parents, who just took the kids out for the day, realize that making things is a possibility. The whole point of the exercise is to try to encourage anybody to have a go, under supervision, and see how easy it all is.
We have had retirement age people having a go for the first time, and by trying our little set up, they will have some idea of what they can expect a lathe to do. So much better than just buying a lathe in a box and trying by themselves.
chriStephens
PS this is on topic because we part off as part of the little exercise.
Edited By chris stephens on 23/01/2011 10:30:21 |
| chris stephens | 24/01/2011 13:00:44 |
| 1049 forum posts 1 photos | Re Above "other",
I totally agree, although not on a Myford or in the rear position, when parting on my Bantam with a carbide parting tool and under power feed, just dropping a few drops of oil can really smooth out the cut. So little oil is needed that you can use either cheap neat cutting oil bought by the gallon or expensive like CT90. There is no need for messy flood cooling, but if you have it all the better.
Just to go back to the belt "stretching" issue, I seem to recall reading that a well known engineer and forum post writer of great repute and respect, but sadly no longer posting on this site, removed the dual V belts from his Chester lathe and replaced them with the join together round (and stretchy) type and he can part off!
chriStephens
|
| Stephen Leacock | 17/02/2011 22:12:42 |
| 4 forum posts | seems to me at least that the belts cannot be an issue if the lathe will part better with rear tool post or in my case running in reverse with parting blade mounted upside down .
surely in this fashion all saddle forces are reversed ie. instead of pushing down on the ways of the bed, cross slide , compound slide parting conventionally we are now pulling up on the ways trying to lift the tool off the machine as it were . perhaps this method works because in tension the slides tighten up on any tapered ways wereas conventional parting pushes these tapers apart allowing them to move freely due to any minute amounts of play ? any thoughts guys
|
| mgj | 17/02/2011 22:55:55 |
| 1017 forum posts 14 photos | Stephen - this takes a diagram or 2 to dispel all the myths. Basically any leadscrew has a clearance, whether in tension (back tool post) or compression (front toolpost.), so as you infeed there is a clearance in front of the nut - in tension or compression of the leadscrew. Now while the tool is cutting OK obviously the tool is against the feedscrew and the back lash in front of it. If for some reason the tool can cut faster than the feed it will overtake the leadscrew and move into the backlash, promoting a dig in -(more on the nature of the dig in in a sec) So what causes the movement forwards - you need something that will allow the work to grab the tool- ie to generate a forwards vector. Rake is what does it - its the only thing in the diagram of forces that has an inwards component. Remove the rake and you remove that vector or force. That vector will unload a leadscrew with an ordinary knife tool cutting towards the headstock (typically in brass but sometimes in steel. We've all felt it with steeply raked tools, and thats one reason why the tangential tools are not so fab in brass and bronze., So all you need to do is to decrease rake or increase feed and keep the feed nut back against the feedscrew. Notice that no-one has mentioned back or front toolpost yet, because the geometry is identical. About the dig in. If you have a tool above the CL (front tool post) or below the CL, rear toolpost , if that toolpost moves forwards then essentially the tip is dragged downwards into a thicker cut (and vice versa at the rear) Self evidently. If the tool is below the CL front TP, then its going to be dragged downwards into a thinner cut. Self evidently. So the answers are very obvious, and there is no great magic about a rear toolpopst position, because tools can jam at the back too, and if "They" were right about the geometry they wouldn't. All one has to do is to keep the tool a tiny bit below centre (front toolpost), apply little rake and feed such that the is a pressure required to force the tool forwards, and parting off on the front toolpost becomes pretty simple. (because from an engineering point of view you have reversed the vectors) It ain't rocket science, but for years for me parting off was an exercise in agony, and for years I ground steeper and steeper rakes to get a "sharp" tool. I narrowed the tool to reduce pressure, and fed ever so gently into ever more slowly rotating workpieces, and still managed to screw it up imperially, leaving a trail of busted blades and fouled up work. Of course, one would, because in my caution I was setting up the conditons to create a jam - either toolpost because I tried the back position too. Then about 18months ago ChrisS mentioned rake, I engaged brain, and the penny dropped. I have tipped parting tools for both lathes, but most parting is still done with ground blades because the front edge is offset by a degree or so to drop the cut bit off without a pigtail. On the Myford now I just go straight in from the front, with a good positive feed by hand to position the feednut correctly, naff all rake at about 600 RPM, or one pulley slower, same blades different grind, and the parting problems just don't exist. And that too is no surprise, since one is now playing out of the sandtrap. Its not a new lathe - about 30 years old. And the back toolpost hasn't seen the light of day for yonks, and never will again. (Bit late tonight, but I'll sketch out the forces and take a photo of one blade which used to jam and now never does - but you can see the remnants of hte old grind angle . Edited By mgj on 17/02/2011 22:59:38 |
| Ian S C | 18/02/2011 00:49:30 |
7468 forum posts 230 photos | Just a thought regarding chatter, some old motors have heavyer armatures of larger diameter than the modern light weight motors, there by having a greater flywheel effect. Would this extra weight dampen the vibration in the system. Ian S C |
| Gordon W | 18/02/2011 10:42:58 |
| 2011 forum posts | Ian SC, yes I think thats part of it. I've noticed on my lathe, it's better with the 4 jaw, and better still with the big old faceplate, this is for all turning, not just parting-off. I also use the faceplate when turning between centers, seems to improve things. |
| Ian S C | 18/02/2011 11:28:19 |
7468 forum posts 230 photos | Gordon, with the face plate on when turning between centers it gives you some thing to hang onto to turn the work, it can be difficult other wise.
I wounder if a flywheel in the system would help in some cases, I have seen it advocated in vertical milling machines. Ian S C |
| chris stephens | 18/02/2011 15:48:40 |
| 1049 forum posts 1 photos |
Hi Guys,
Graham has raised an extremely valid point about chuck jaws, and curse him he got there before me.
 I was going to write something similar last night, as I have discovered that my three jaw is suffering from the dreaded bell mouthing. I was wondering why I was having problems, then realized that I was using my three jaw, which is an unusual event as I prefer the four jaw or collets, and the work was moving. You can have the best parting tool in the world but if the work moves you are in the deep Doo-Doo. Many years ago I read about a fixture to hold the three jaws under load for grinding and made one thinking that sooner or later I would need it. You can guess that now I need it, I can't find the wretched thing. Here (hopefully) is a picture of the old one, and as we all know if I make a new one the workshop Gremlins will bring the old one back and leave it in plain sight on my bench for me.
Although it does not look like it it is made in one piece not three separate pieces. I don't remember who to credit with its design but "thanks" go to him.
 Why, I can hear you thinking does he prefer a four jaw over a three? Well, I am extremely mean and using a four jaw I can turn commercial 8mm stainless to a reasonably accurate 5/16" with the minimum of waste. Also I can turn both ends of a piece of metal to the same diameter without a step in the middle. I find that a four jaw is better than either of my collet sets and almost as quick., but then I am not working in a commercial environment where every milli-second counts against profit.
chriStephens
Edit, sorry no picture, will come back to that later.
Edit two, picture now showing
Edited By chris stephens on 18/02/2011 15:50:08 Edited By chris stephens on 18/02/2011 15:54:19 Edited By chris stephens on 18/02/2011 15:56:29 |
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All Forum Topics > Manual machine tools > Parting off on Myford lathes