Parting tool chatter

[goldstar31]

In Home Model Engineering Clearing house of which I have long been a subscriber, there is a series of examples of parting tool construction in'What I have done today' Runs from the 20th October

Clears throat- I have made both examples as well

 

[IanN]

Hi,

Parting is a very brutal process - if you use a standard size parting tool (rather than a thin "hacksaw blade" tool) you are probably cutting a groove about 1/8th inch wide (sorry to revert to aliquot parts but I'm of an age where the inch is my natural unit). Something to remember is the available power on a hobby size lathe

I was taught the rule of thumb that on a small machine you can reasonably remove about 1.5 cubic inches of steel per HP per minute. (This is power at the tool cutting edge, not at the motor)

So if you have a 750W / 1HP motor and ignore inefficiencies, effect of blunt tool, poor cutting fluid supply, etc, etc, and you part a 1/8 inch width with a depth of cut of 10 thou (1/100 inch), then in one minute you can expect to cut off a strip of swarf which is 1/100 thick, 1/8 wide and 1200 inches long.

Given a work piece diameter of one inch and a value of pi = 3 you have a circumference of 3 inches, so 1200 inches of swarf is cut in 400 revolutions i.e. a cutting speed of 400 RPM max. Now factor in the inefficiencies and it is reasonable to halve this speed estimate. On my 100 year old lathe I have a choice of three direct speeds via the layshaft pulleys and three backgear speeds so I can get full motor power at chuck speeds as low as 40 RPM, but "cheap" VFDs will not be able to deliver full shaft power at low speeds (whatever the makers may claim.....)

Bearing in mind the above, and the fact that early chattering is corrected by increasing the depth of cut, it is easy to push a small machine outside of its comfort zone when parting. If you do this, then you can expect to have a few interesting issues from time to time.

All the best,
Ian

 

[djh]

I am sure i have commented on this problem on this site before parting off should never be a problem even with wide parting tools but you must support it with tail stock no need to centre it first just force a running centre or a solid centre or even a flat bar held in the tailstock just keep it supported tight by maintaining pressure on the part wihlen parting release tailstock pressure when almost trough
using this method i have many times cut multiple 2 inch dia billets 6 inchs plus from chuck even when the machine has loose bearings as many of the old machines i had to use had.
David.

 

[Steamchick]

Exactly my perspective, as I have stalled my 750 watt motor - at lower speeds of the VFD. - very easily. I understand the torque is limited by the current - and with 10A fuses I found last week that was the limit! I was too slow hitting the STOP when it stalled, parting at about 3/8" dia and when my hand "twitched" and gave too much cut. Something I couldn't do on my Granddad's 1930s 3in. lathe , or on my Myford ML3, on the back gearing - both of which were smaller lathes than my current Chinese job.
To put it in perspective: 750W is drawing 3A at a spindle speed of 1250rpm... I guess to get 750 W at a spindle speed of 375rpm it will try and draw 10A - but the electronics don't work that way. (Squarish waves chopped in length to give the RMS voltage for the speed required). so I figure anything below 400rpm my lathe is drawing the "limiting current" that the VFD can deliver, but power is already down, torque is dropping off at at 100rpm spindle speed I have the equivalent of less than a 75W motor. I can't even tap a 1/4" whit in mild steel... just about manage 1/4" x 26 in Brass!
But a geared lathe (belt drive or helical) should deliver full power and torque from the motor multiplied by the gear ratio at lower speeds! = A Fantastic amount of torque that will snap taps and tools when something goes wrong.
When I have a bigger workshop I'll get a geared lathe, as VFD is just "the wrong method" of powering at lower speeds (Below 500rpm in my experience) for cutting metal.
K

 

[Noel Gordon]

Hi Ian, interesting thoughts, HSS if ground properly is very sharp and requires little HP to make it work. Carbide is never sharp and removes material by shear friction so carbide is great to use when you have unlimited HP. After 60 years I still prefer to use HSS especially if you unstable job that requires careful machining.

 

[Ken I]

An Unexpected Improvement In Part-Off Performance.

In another thread I am describing changing my single phase lathe motor (by rewinding it to three phase) and running of a VFD (variable frequency drive).

Single Phase To Three Phase Rewind - Lathe Uprate

An unanticipated (and much appreciated) outcome was an improvement if part-off performance - here's why (I think) :-

A normal squirrel cage motor responds to increased load by increasing its “slip” – it runs slower – this increases the voltage induced in the squirrel cage bars thus increasing the current in the bars and the reaction torque to the rotating field – the downside is the increased slip also increases the impedance of the squirrel cage which diminishes the current and at some “tipping point” stall occurs suddenly.

Typically slip is 5% - this is no small issue – take for instance when you are parting off – as you apply load (feed) the spindle rpm’s will slow down slightly – which in turn actually increases the force being applied to the part-off blade – so the process is made inherently unstable. Sure the inertia of the system comes to your aid – but “slip” is not your friend.

With the VFD the speed is maintained by reacting to the current draw and Hall-Effect feedback caused by the out of sync squirrel cage current reaction.

The VFD actually increases the output frequency (but not the reference frequency display) to maintain speed.

The V1000 VFD (that I used) has two features :- “slip correction” and “torque correction”, the purpose of these is to maintain the rpm’s of the motor at the “reference frequency” rpm.

Example if I dial in 50Hz. The unloaded 4-Pole motor will turn at 1500rpm but as the load increases it will slow down to 1425 rpm – what the inverter does is increase the output frequency to ±52.6Hz to keep the motor turning at the indicated 50Hz/1500rpm.

Now I hadn’t given this much thought but I found that parting off was much improved by both the increase in deliverable power, torque "smoothness" and speed stability provided by the VFD.

I'm not entirely sure how much I can ascribe to just the increased power but I suspect the rpm stability is significant.

Regards, Ken

 

[Steamchick]

Brilliant! (I think?). - But not for those of us who are not about to convert our 750W single phase motors to 2HP 3-phase... with a new VFD. - I'd sooner buy a geared lathe - to gain the torque at low speed - and with a larger stiffer bed. (Lathe bed, not the thing I sleep on for my aching back!). - But thanks Ken, I think this screws my hypothesis that the single phase small motor lathe is no good because the VFD limits the torque as speed drops.... so the job can easily stall?
No matter, I rarely have any problems parting-off (I never say never). "Slowly and carefully" has worked for 50 years for me. - And applies to my 90W motor Unimat SL just as well. (But a small hacksaw is much quicker!). But I'm glad to hear of your success and a worthwhile theory! Bravo!
K2

 

[Ken I]

Ken 2, I used to have (in fact I still do) a much smaller D.C. motored lathe - which I persevered with (But a hacksaw is much quicker!) my new geared lathe was something of a disappointment in this respect - now much better.

Regards, Ken I

 

[OrangeAlpine]

I found the problem!

Being almost 80 and loosing strength, I had decided to switch to smaller, easier to mount chucks. So I machined #4 Morse taper onto three slugs of mystery metal and a drawbar to hold them into the lathe spindle. The 5" chucks are mount (of course) on these Morse taper adapters. I decided that perhaps the #4 tapers were not making as good a contact in the lathe spindle as I thought. I was right. A little Prussian Blue and a fine file file put brought them up to where they should have been.

I have recently parted some 1" stainless steel. I have no idea the type. It was purchased simple as 1" steel. It machines fairly good, but seems to require a rather high pressure and continuous feed. Anyway, the parting was successful with nary a chatter or dig in.

BTW, I love my new "quick change" chucks!

Bill

 

[jackary]

Here is my submission to the discussion, its all about rigidity at the mounting including the topslide etc
Alan

 

[Steamchick]

Here is my submission to the discussion, its all about rigidity at the mounting including the topslide etc
Alan

Better than my back-toolpost! Must modify that now, to increase stiffness. I like your mount and blade though, that may be the best ever!
You are right about maximising stiffness. The thrust from workpiece to lathe bed simply compresses the cross slide and saddle slides - no chatter possible there. Can't improve the stiffness beyond the bed to head stiffness made in the factory though...
Or buy a better lathe.
K2
K

 

[Steamchick]

My back tool-post is a post with tool holder mounted on it, tool " upside-down", but seeing you have solid metal all the way from cutting edge to surface of slide you have eliminated the bending I have in my toolpost. This tiny bending is probably the "prime exciter" when I do get any chatter, so to modify the toolpost so I have solid metal from cutting edge to cross-slide surface should eliminate that. I.E. eliminate the overhang of my tool.
Thanks!
K2

 

[jackary]

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

 

[Steamchick]

Thanks Alan, very clear. My thoughts are to convert my rear toolpost (tool up-side down and "normal RH rotation") to make it stiffer with a rib supporting the tool right down the the surface of the cross-slide, and with "rotation reversed, LH" as you demonstrate. A bit like "sticking your hands out to the cutting edge and having a plank on-end beneath to take the thrust".
I am sure your sharp cutting edge, tool cutting angle, speed and feed rate are all "text-book" as well.
Thanks for excellent advice.
K2

 

[goldstar31]

Ken the GHT tool holder is structured into protruding exactly 1" and there will cut any bar with a maximum of 2"diameter.
You make it, and all that you need to do in a normal lifetime is 'lick' and tap the refurbished blade to dead centre again as it was before the grinding. the front cutting edge.

None of this 'standing up in a hammock', GHT does it-- end of it- simple. He did the thinking-- and got it right. Far too much blethering from the lesser mortals . My opinion of a LOT of years

Norman

 

[Steamchick]

Thanks Norman. Generally, I have no problems with my set-up and tooling, but reading this thread I have recognised some clever and better ways of killing the pig. And , of course it is nice to make things and experiment to see "why" things are done different ways. I have such a small (Chinese) lathe it poses some odd challenges, as it definitely will twist or stall at max cut rates! So "slowly and carefully works best". Been making a couple of jets this last couple of days - in gaps from other jobs. Drilling 0.25mm (0.01") holes in brass focuses the mind. Production sequence solves most problems! 0.3mm dia is much easier as drills are just stiffer and stronger. I figure my beard hair can break a 0.25mm drill. It's tougher than the swarf!
Be safe, be happy,
K2

 

[goldstar31]

Thanks Norman. Generally, I have no problems with my set-up and tooling, but reading this thread I have recognised some clever and better ways of killing the pig. And , of course it is nice to make things and experiment to see "why" things are done different ways. I have such a small (Chinese) lathe it poses some odd challenges, as it definitely will twist or stall at max cut rates! So "slowly and carefully works best". Been making a couple of jets this last couple of days - in gaps from other jobs. Drilling 0.25mm (0.01") holes in brass focuses the mind. Production sequence solves most problems! 0.3mm dia is much easier as drills are just stiffer and stronger. I figure my beard hair can break a 0.25mm drill. It's tougher than the swarf!
Be safe, be happy,
K2

thanks for comments but I thought that you were using a ML3- so silly me. So from my sieg c4, a bit bigger than yours? I made sub plates to accept my Myford accessories( and vice versa)
So today my Potts vertical slide with a 9 hole dividing head and a Myford 2MT spindle and nose has arrived.
next job is making Tee nuts( if the weather warms up but my central heating has broken down and I;m wearing down jacket, gilet and whatever.

So keep in touch

Cheers

N

 

[Steamchick]

Hi Norman. The little beast is a Chesters DB8VS. 8"Swing over bed, but not robust for that size of job, except with light cuts. You really need a gear lathe - not a 1 HP / 750 W variable speed that equates to about 7W when at lowest speed ~100 rpm. But good up to and inch or 2. Better than the ML3 it replaced, except for the back-gear on the Myford that was great for larger diameters like flywheels. But I often found that bed twisting when I tried a cut that was too heavy.
The DB8 fills my limited space.
Cheers,
K2

 

[goldstar31]

Hi Ken

Thank you for your detailed explanatio,

best Wishes

Norman

 

[Richard Hed]

Thanks Norman. Generally, I have no problems with my set-up and tooling, but reading this thread I have recognised some clever and better ways of killing the pig. And , of course it is nice to make things and experiment to see "why" things are done different ways. I have such a small (Chinese) lathe it poses some odd challenges, as it definitely will twist or stall at max cut rates! So "slowly and carefully works best". Been making a couple of jets this last couple of days - in gaps from other jobs. Drilling 0.25mm (0.01") holes in brass focuses the mind. Production sequence solves most problems! 0.3mm dia is much easier as drills are just stiffer and stronger. I figure my beard hair can break a 0.25mm drill. It's tougher than the swarf!
Be safe, be happy,
K2

How deep do you have to drill those tiny holes?