Parting off

[Howard LewisParticipant @howardlewis46836]

There have been many explanations of why a rear tool p[ost is better for parting, but I don't necessarily understand mall the arguments.

But I made a four way rear toolpost for my rather heavier than the usual hobby lathe., and very rarely (Tha'ts asking for trouble! ) have problems parting off.

Today, I parted off a piece of 2.5" diameter steel, under power, at the lowest possible feed rate barely enough to keep the tool cutting; with a drip feed of soluble oil..

Tipped tools have a V shaped top face, so that two narrow streams of swarf curl towards the centre of the groove.

My experience is of wrecking an inserted tip parting tool ,in the front toolpost, because of dig ins.

In the rear toolpost is an old HSS blade, 3/32" wide. It has no top rake, but side clearances, and, to make matters worse, I ground the front face at angle, supposedly so that there would be no pip on the work as it parts off. Doesn't happen, but the wider swarf thankfully does not jam in the groove. I think that the zero top rake guards against dig ins.

HTH

Howard

[Anonymous]

Here are the results of some turning trials. Material was 25mm diameter EN1A and a CCMT insert. Spindle rpm was 1200rpm,and depth of cut 0.1". I did two tests, one with a feedrate of 4 thou/rev and one at 8 thou/rev. I then measured the width and thickness of the swarf:

4 thou/rev width = 0.098" thickness = 0.007"

8 thou/rev width = 0.117" thickness = 0.010"

Somewhat inconclusive on width, but we can say that the chips do not get thinner. It would seem strange if they did as it's not clear what force would be pulling the chip away faster than it is being generated.

I also looked at the swarf generated by a 3mm wide insert parting tool:

The insert curves the swarf and in particular curls the edges to make a shallow channel. The width of the swarf was about 0.117". So only a gnats wotsit less than 3mm.

As an aside all the swarf was quite brittle, which is not a quality one would associate with EN1A. May be it's a consequence of the shearing action?

I'm aware of the claimed advantages of rear toolpost parting; my question to David was about why the forces are different in parting off as opposed to turning. If we assume a square parting tool I would expect the force vector to resolve to two orthogonal forces. There should be no force parallel to the axis of the work. Depending upon material and tool shape there might be a force perpendicular to the work axis pulling, or pushing, the tool into, or away from, the work. But the main force will be downwards. It seems all wrong to react that force upwards trying to pull the cross slide off the saddle and the saddle off the bed. The machine designer has gone to a lot of trouble to make the bed stiff in order to resist downward forces so why not make use of it?

A claimed advantage of rear toolpost parting is that the swarf falls away. That may well be true for brass and cast iron but for steel where the swarf may be slightly wider than the tool the swarf is so light that the force on it due to gravity will be negligible.

Front parting works fine for me so I'll stick with it.

Andrew

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 12/06/2020 21:22:51:

Front parting works fine for me so I'll stick with it.

Ditto.

I think a rear parting tool gives confidence, and nothing improves parting like confidence.

I had some 3/4" bar to part off a couple of weeks ago and hadn't parted for a while and it started to make unpleasant noises.

So I sped up the feed and it was OK.

Next slice, I used power cross feed, and it was even better.

Neil

[Martin ConnellyParticipant @martinconnelly55370]

I was looking through some of my old videos for something that had nothing to do with parting off and found this one. After I had CNCed my mill but before I had CNCed my lathe I did the reverse of what a lot of people do and used my mill as a lathe. I had a number of special screws to make for a friend so set it up in the mill, The item is parted off at the end, not a front tool post, and not a rear tool post. The workpiece can be seen moving to the left due to screwcutting so has a number of spring passes. Y axis as a tool changer. Mach3 threading works for me.

I also get parting off swarf like the picture above shows. I think this was stainless of some sort.

Martin C

Correct video embeded now.

Edited By Martin Connelly on 14/06/2020 11:55:32

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