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Parting tool chatter

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goldstar31

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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

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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

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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

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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

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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

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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
 
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Steamchick

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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

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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
 

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