Parting off on Myford lathes

[Ian S CParticipant @iansc]

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 stephensParticipant @chrisstephens63393]

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 1Participant @johnstevenson1]

Aahh but a decent setup minilathe is tons better than a 64 year old design Myford…………..

John S.

[chris stephensParticipant @chrisstephens63393]

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 gibsonParticipant @alexgibson50133]

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

[KWILParticipant @kwil]

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 stephensParticipant @chrisstephens63393]

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

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Nicholas Farr : Yes – I took into account motor response in the tests described .

 

When motors are simple one and three phase induction types these prove to be almost dead in their response to chatter type load feedback and motor spindle speed /input current just show very small ripple variations .

 

Three phase induction motors in particular are known from other work to be so stable that they can be taken as effectively dynamic earths when doing response calculations .

 

Other : Cutting oils have not been much discussed in this topic . Those of you who have not tried parting off with cutting oil should try it and see what a big difference it can make when tool clogging is a problem .

[chris stephensParticipant @chrisstephens63393]

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 LeacockParticipant @stephenleacock91334]

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

[mgjParticipant @mgj]

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Edited By mgj on 17/02/2011 22:59:38

[Ian S CParticipant @iansc]

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 WParticipant @gordonw]

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 CParticipant @iansc]

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 stephensParticipant @chrisstephens63393]

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

[NJHParticipant @njh]

Hi Chris

 

How does this work then?

 

I had always imagined ( never having – yet!- been in a position of needing to grind the chuck jaws) that the way to proceed would be to cut a narrow ring and open the jaws such that the outer steps on the jaws tightened on the inside of the ring thus holding them firm. Please tell me why this wouldn’t work. ( I hope that I don’t need this information for some time!)

 

Regards

 

Norman

 

 

[Stephen LeacockParticipant @stephenleacock91334]

Again i still don`t understand guys If as you say the chuck jaws are bell mouthed surely i would get the same chatter in both forwards and reverse setup ( this is certainly not the case as there is a difference of night and day in my forward and reverse setups for parting )

[David Clark 13Participant @davidclark13]

Hi NJH

If you tighten the jaws up outwards onto a ring, you are pushing the bell mouthing the wrong way.You would pull the jaws in at the front rather than push them out. This means they would be like / rather than like / .

regards David

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by NJH on 18/02/2011 16:11:22:

Hi Chris

 

How does this work then?

 

I had always imagined ( never having – yet!- been in a position of needing to grind the chuck jaws) that the way to proceed would be to cut a narrow ring and open the jaws such that the outer steps on the jaws tightened on the inside of the ring thus holding them firm. Please tell me why this wouldn’t work. ( I hope that I don’t need this information for some time!)

 

Norman –

 

The problem with constraining the jaws from the outside as you describe is that you want the jaws to be forced inwards as they would be when clamping a piece of bar. So, if there is any play in the jaws, your method will cause the resultant ground jaws not to be parallel when clamping a bar. Which kinda defeats the object.

 

On my Taig lathe, which has soft Aluminium jaws, they recommend using a small washer, clamped as far into the chuck as it will go, and then using a boring bar to skim the inner faces of the jaws. You can then take off the small pip left where the washer was with a file. The nice thing about that method is that if you chose a washer the size of the stock you want to machine, the chuck will grip the stock with minimal runout.

 

Regards,

Tony

[NJHParticipant @njh]

David and Tony –

 

Thanks – that makes perfect sense. I will file the information away in my memory banks until required. (Although if that’s longer than a couple of weeks I’m not so sure I will be able to retrieve it!)

 

Regards

 

Norman

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Not much point in me saying anything now, you lot beat me to it, again.

chriStephens

[chris stephensParticipant @chrisstephens63393]

Hi Stephen,

I quite understand you question. What I think this shows is that there is more than one reason for problems with parting. A sloppy chuck is just one of the causes, there are many others that work singularly or in tandem to cause you to have a bad day. As an instance, there is very common fault on mini-lathes in that the top slide gib dovetails are badly machined and by parting from the rear it might put the load on the good one. This however is not the answer to the problem just a way around it, the answer here would be to file off the sharp corner of the dovetails thus allowing the whole of the jib to come into contact, not just a line contact.

If a lathe is in good order (and of a reputable make) and a decent quality parting tool is used, parting from the front should not be a problem. If parting from the rear does make life easier and you don’t notice any other problems I am not suggesting you go looking for some, but if say your gibs are loose, you might find that you will get a better finish when turning by sorting them.

chriStephens

[mgjParticipant @mgj]

Graham – I don’t know your tips, obviously, but your saying that your tips had about 7 deg of rake surprised me.

 

So I went out and took a look at mine, whch are Sandvik 3mm and Zenit GTN2 clones (same as the Seco 2mm ones for the Glanze parting tools)

 

Yes, they do have a steeply dropped front, which could easily be 7 deg, BUT the actual cutting edge is more or less at zero rake. (The ramp may be there for chip clearance?, but it is not at the same angle as the edge, by no means, and its the edge which defines the rake angle and the forces and vectors generated.)

 

Then, if you install that set up at centre height and walk into a hunk of metal and apply a download at the tip, of course all will give a bit, as it must, and one is actually cutting below centre and with negative rake, or more or less zero rake. Which is one reason why that design of tip is so chatter and jam free.

 

In a rear toolpost the same thing happens exept the tool lifts a smidgen, so again one is using the same “below” centre/shallow rake geometry.

 

Your tips may not be precisely like that, but most parting indexable tips are. (There are also very specific negative rake parting tips too of course)

—————–

Its also one of these great falsehoods about parting on centre that many come up with. Being set on centre is not the same as cutting on centre. Setting on centre with the tool unloaded means cutting below and with a reduced effective rake angle. Obviously, since nothing is 100% rigid. If one were using an Oil Country sized lathe and tooling to part a small bar that statement might be marginal, but its true enough for our ME purposes

 

 

 

 

[John CoatesParticipant @johncoates48577]

Posted by Stephen Leacock on 17/02/2011 22:12:42:

the lathe will part better with rear tool post or in my case running in reverse with parting blade mounted upside down .

Bloody hell Stephen – an ephifinal moment there for me! Never thought of that and it’s so flippin’ obvious. Next parting off I do I’m going to try that

 

D’oh!

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