Notes on parting off

[Martin DowingParticipant @martindowing58466]

From time to time there is a need to part off a bar of larger diameter than machine is really meant for.

Lets say to part off 40 mm 316 SS bars on ML7.

Standard 3/32 blade with Dicksons toolholder as sold by RDG seems to be a good choice but in standard version it works poorly for the task.

So many people are grinding curved top rake on the blade and this works better but still there is some struggling at these diameters of stainless.

So I have modified a grinding procedure and top rake was formed by 2 cm diameter grinding stone rotated in die grinder used as a sort of form tool.

Highly polished, nice curve approximating curvature of grinding stone was produced without any trouble.

Now this blade was used for parting said 40mm 316 SS bar.

Lathe was run initially at 100 rpm and paring was proceeding with intermittent oiling with screw cutting oil.

There was no chatter and swarf was nicely driven out of job along curved top rake.

Once diameter left was 16 mm speed 200 rpm was applied and cut completed. Nice shiny surface of deapened grove was observed with no chatter marks at all.

Now drip can with standard cutting emulsion was installed. Complete part off was achieved within 7 minutes (including time to go from back gear to open speed).

There was no mishap or any troubles while parting off few details and entire operation proceeded very smoothly.

For ML7 it is IMO good performance and smoothly ground top rake was key to success.

2 part offs could be done before re-sharpening front end of blade with DMT diamond stones is needed.

I hope it is going to be of some use to others.

Martin

[Martin DowingParticipant @martindowing58466]

[Nigel Graham 2Participant @nigelgraham2]

Thank you for that, Martin.

It does seem the high-quality surface you put on the tool was as important as the rake, and of course the usual variables like speeds and setting.

[Howard LewisParticipant @howardlewis46836]

Good!

And if the parting tool can be mounted in a rear toolpost, the swarf goes downwards, reducing the risk of clogging and the problems that that brings.

Howard

[not done it yetParticipant @notdoneityet]

My rear parting tool is a straight blade. I sharpen it when needed by removal of the cutter holder and whizzing it over a sanding belt. Refitting requires no centre height checking and it just works. I don’t think my power cross feed could only cut 20mm in 7 minutes. I don’t use much stainless, but that seems very slow to me.

[Ian HewsonParticipant @ianhewson99641]

Hi Martin

Having now got a mini lathe after my Myford 254, parting off was somewhat problematic.

I tried your grinding tip, and parting a piece of 20mm mild steel with a 2mm parting tool at 250 rpm I am pleased to say that it was very smooth, nice curly chips and no chatter.

Oil was applied with a brush, nice finish on the patted off part.

Thanks for the tip.

Ian

[egaParticipant @ega]

Re-sharpening after every other part off seems likely to be too frequent for most.

I assume that the circular top rake produces a rather short-lived point.

Parting off is a personal thing; if you have found an effective MO then you are to be congratulated.

[Ian HewsonParticipant @ianhewson99641]

As an apprentice electrician at the YEB part of our training was at the ROF Barnbow.

The turning instructor had a slip stone in his pocket and used it very frequently.

The parting tool I ground to try Martin’s tip was not polished or honed prior to use and was cutting freely at the end of the cut. I would not envisage any more frequent sharpening on the grindstone than any other tool.

If it parts off without dig ins and chatter I will risk a bit of sharpening.

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