Yet another "parting off grief" thread ;)

I had a small 12mm thumbwheel, drilled and tapped to M6, sitting in a collet chuck and still attached to the stock. Mild steel, 15mm bar stock.

I trued up the parting tool, ensured it was flat to the work and checked centre height against a drill bit that was already sitting in the tailstock. I applied oil from my lathe oiler gun, a light engine grade oil. This set up usually works fine for me.

I started parting, lower speed than for turning as this is the general advice I have received. The parting was going just fine, nice smooth layers of metal coming off. This was around 300 rpm. As the diamater was getting thinner and it seemed the lathe was having to work a bit harder, I thought I would stop the cut and increase the speed slightly, to about 450rpm.

However when I went back to finish the cut, the tool only rubbed. I tried putting the speed back down. Then back up further. No luck. Applying a little more pressure and I heard a "crunk", and the final < 1mm wall remaining between the thumbwheel and its hole had deformed.

I thought maybe I had blunted the tool, but checking it on the parent stock showed it was fine.

Why might the tool have refused to cut when going back the second time?

Should parting off operations always be one smooth movement?

Thanks for reading!

Chip betwixt tip and work?

Martin

Tool slightly too high and rubbing rather than cutting?

Brian

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That may possibly be the source of the problem

[ I cannot claim more confidence than that ]

What you describe seems very much like the tool was just above centre height

.. a few thou can be enough to cause a problem.

MichaelG.

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Brian posted whilst I was composing my 'reservation' clause !

Edited By Michael Gilligan on 16/07/2019 15:07:23

I agree, a parting tool that 'stiffens up' as you get near the centre are usually too high. If you are feeding but not getting swarf, always stop as something is not right.

Drills are not always dead straight and don't have sharply pointed ends.

Neil

As Neil says, it is important that the tool is not above centre height. At best, it will not cut well; at worst it will rub rather than cut. it is taken as read that the tool is sharp.

The time spent in making a centre height gauge will be repaid by having fewer problems.

If the tool leaves a pip when facing, attempting to centre drill may well break the end off it.

A tool set correctly to centre height will cut as intended, and give a better finish.

You can make up a gauge by a variety of methods, ranging from "trial and error cuts, and set the gauge to the tool" to measurement using height gauge, finger clock and slips.

These methods compensate for any eccentricity in the chuck. Personally, I am less keen on the "half thickness bar in the chuck" types because of chuck eccentricity. (Whilst making a gauge for a friend, I found that his chuck had an eccentricity of 0.036" - 0.9mm! ).

If you want details of how I went about it, PM me.

Howard

These problems with parting off are so very hard to figure out.

You may have had too much speed for steel. I would run 100-200 rpm.

Something may have moved. If there is a choice of three or four jaw chucks, always use the four jaw chuck.

A properly set up tool height gauge is better than a drill in the tailstock and can be calibrated using a lathe tool which faces off exactly on centre.

Check your jibs on the cross slide and compound for play.

Lock the saddle when parting.

I still have problems, even when using modern industrial parting off blades, 26 and 32mm in specially made dedicated toolposts, also inverted rear toolposts fixed directly to the cross slide with rear saddle lock.

Maybe there was some work hardening going on? Are you 100% sure it was mild steel?

Problem is tool above centre height,with high speed tools cutting sped is usually half of normal cutting speed. 6 months of my apprenticehip was spent on capstan turning,(Ward 2a) collets and bar feed,mainly on small batches from 50 up to a 1000 items ,all parted off,using either front or rear toolpost ,never found any difference ,so one soon learned how to part off. Lubricant was full flood soluble oil,never used parting blades all tools were HSS bits ground with a few degrees side clearance,no carbide in those days for ordinary industrial use, nowadays at home I use hss parting blades and an insert tool,but those hand ground hss toolbits certainly cut better,and lasted longer ,on 1.375 dia brass a hand ground tool would last for a thousand cuts,similar size mild steel 200 to 300.We did use a nicely made tool height gauge to get correct centre height for front and rear toolposts.

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At risk of wandering off topic slightly, If you use carbide insert parting off blades, all the manufacturers recommend that the tip height is set 0.004" high if used for parting solid stock.

I expect the tool was too high, too. It managed to get started on the larger starting diameter and was depressed to centre line while actually cutting, but when not loaded it returned to slightly high and just too high for the decreased diameter. Admirably demonstrates the flexibility of the cutter arrangement.

Less likely to happen with a dedicated tool post fitted directly to the cross slide for gat least two good reasons. Much more rigid set up and tool height is sorted - as long as you don’t try to use one of those silly angled parting tool holders!

I will guess that you are also using a QCTP with lots of overhang? Possibly too much cutter extension, too?

If you have not altered the cutter, try facing with it. Remember that at the centre, the cutting speed is zilch, so the downwards force on the cutter is at a minimum.

This can also happen when trying to part off dry without coolant or lubricant.

Parting through a thread never can work well. One way around this is to place a piece of studding in the hole, and part off through the studding till you get to the non threaded stock. Generally leaves an ok cut across the thread.

Or else tap the part as a secondary operation, so you are just parting off a washer effectively.

Neil

Hi all

Thanks very much for all the responses and the effort that went into them!

I think the tool being slightly too high sounds like the most likely cause as I wasn't massively scientific about it. I had a little shim in there as I don't have a QCTP so maybe next time I will take a piece of it out. It certainly does seem that parting is very sensitive to setup, so its a good bit of learning.

To answer some questions, yes definitely mild steel - EN1A.

I will definitely look into a tool height gauge, thanks again!

Ross,

I dont know if I,m correct but I think you have a Warco WM250 lathe, in which case it will have a T slotted cross slide which is ideal as you can fit a rear tool post. I can only base it on my experience with a 7 x 14 mini-lathe but the rear tool post I made and fitted transformed parting off from something I dreaded to something that was a pleasure to do with no worries.

I think Warco do a rear tool post and mounting plate but it may be sized for the WM280/290 so if you wanted one you would need to talk to them, I suspect it could be modified though to fit a WM250.

Its just a thought but as I say for me the rear post transformed parting off.

Ron

Just to upset everyone. This morning I parted off some 18 mm diameter 4140 steel(pretty tough stuff), with an Arc Euro type parting tool with carbide insert, starting at 450 rpm, ending up at about 600 rpm. Went through like a hot knife through butter. This was on a Sieg C3 (7" x 14" mini lathe).

cheers

Bill

Edited By Bill Pudney on 17/07/2019 10:56:28

Rather than tailstock, put a rule flat against the face to be cut and run tool up to it, it should be obvious if the tool is high/low by tilt of rule.

Regards Ian.

Edited By Circlip on 17/07/2019 13:19:37

Or better still use a piece of plastic as suggested by Neil in an earlier thread, I cut a strip off an old credit card so I could be certain not to damage a carbide insert, worked a treat when the plastic was vertical.

Emgee