Parting off

This topic has 12 replies, 10 voices, and was last updated 5 April 2014 at 23:40 by Tom Gullan.

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2 April 2014 at 22:46 #148762
Tom Gullan
Participant
@tomgullan59234
I'm in need of some help. I'm using a HSS parting off tool which is set at centre height with the centre tip of my revolving centre. I've tried parting using both the front and rear tool posts but get the same effect. I've also tried using various speeds with a slow feed rate but still have had no joy. As you can see from the photos I'm left with a small point which is slightly larger that the point of a tack. Can anyone point out the error of my ways?
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2 April 2014 at 22:46 #7105
Tom Gullan
Participant
@tomgullan59234
2 April 2014 at 23:00 #148763
John Haine
Participant
@johnhaine32865
Edge of tool not square to the shank?
2 April 2014 at 23:07 #148766
jason udall
Participant
@jasonudall57142
just a hought..part snaps off before partoff reaches center..but partoff will face nicely?

..try sharpening part off blade like

/ rather than — or if you see what I mean…

you will have a pip somewhere but you can choose which has it ..part or stock
2 April 2014 at 23:38 #148770
Clive Foster
Participant
@clivefoster55965
Expanding on Jasons advice the parted off piece snaps off when wobble, touching tool, rotation or whatever forces become too great for the tiny piece at the centre to support. Grinding the tool at a slight angle defines which part the main pip is left on. Generally, unless turning far too slowly the small end of the pip snaps near enough flush with the face.

Make sure you are grinding off all the bevel provided on the blade for clamping purposes when sharpening the "top" (for a front mount tool) cutting face. Its easy to get into a minimal grind mindset and slip into not going down quite far enough. Went through a phase of that error a couple of years back and it took me ages to twig why parting off had got finickity. I grind mine flat on top but there is a school of thought advocating a touch of side rake. Back rake to my mind just wastes blade length and promotes tool pull in with front mounting. Not aproblem with the inverted rear mounting you use. Theoretically back rake provides side clearance behind the cutting area but I dont consider the actual clearance enough to make a difference. Especially not if using coolant or cutting oil.

Its a common error, especially for those using smaller lathes, to run too slowly. Hand feeding at slow rpm to keep a nice thin chip which won't bind up is hard.

Clive
2 April 2014 at 23:47 #148771
Nobby
Participant
@nobby
Hi Tom
Grind an angle on the front of the parting tool say about 15 degrees so that the disk / job parts off nice an clean first then carry on and face the pip off . Are parting from the front or back as you said it must be on the centre line Nobby
3 April 2014 at 07:21 #148778
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Any of above but also possibly :

Parting tool is unstable because of too much overhang from tool holder or inadequate fixation .

You can see from pictures of parted off components that parting blade is deflecting both up and down and sideways .

There is evidence of simple deflection and of vibration in both cases .

Regards ,

Michael Williams .
3 April 2014 at 09:29 #148783
Russell Eberhardt
Participant
@russelleberhardt48058
As usual it is horses for courses. There are as many opinions on parting as there are model engineers but here are some thoughts:-

1. You are parting off brass. It is best to use a back rake of about 7 deg. for steel but zero for brass. Your pictured tool has back rake.

2. Yes an angled tip can help decide which side the pip is left BUT it introduces a sideways force. Also the swarf is wider than the slot and can jam. Make sure the saddle and topslide are locked and that there is minimum overhang on the tool to make everything as rigid as possible.

3. G H Thomas always recommended grinding a slight "v" (convex) on the front face of the tool and a slight concave "v" on the top so that the swarf tends to curl in and become narrower than the cut. I have used that method for years. I still get a small pip on the work but not like yours.

Just my two euro cent's worth.

Russell.
3 April 2014 at 11:40 #148792
Ian S C
Participant
@iansc
I find that if I set the tool just a few thou above centre hight, under cutting load the tool is pulled down to centre, and as long as fed is kept up right to the end, a clean cut is the usual. Ian S C
3 April 2014 at 17:08 #148827
Tom Gullan
Participant
@tomgullan59234
Gentlemen,

Thank you very much for your words of wisdom. I'm just home from work and I'm now off to give your ideas a go.

Regards

Tom
5 April 2014 at 14:07 #148988
John McNamara
Participant
@johnmcnamara74883
I am with Ian SC

A thou or possibly two over is centre height helps achieve a clean cut.

However maybe in this case the material has broken before the tool got to the centre some materials fatigue and break off this way.

Regards
John
5 April 2014 at 14:57 #148993
Neil Wyatt
Moderator
@neilwyatt
Looking at the photos the broken 'spigots' are more or less parallel. I think John is right and they are fatigue fractures.

Slightly less speed and a sharper tool may help, together with less aggressive feed towards the end, but I would be surprised to eliminate the wee spigot unless an angled tool (which I don't like as it bends) is used.

Clipping off with side cutters and filling the stump with a riffler file won't leave any worse than the parting tool has left on the rest of the disc.

The discs could also be temporarily superglued onto a short end and faced to a good finish if necessary.

Neil
5 April 2014 at 23:40 #149039
Tom Gullan
Participant
@tomgullan59234
Gentlemen,

I've used a combination of less speed (a really slow feed rate) and an angle on the left side of the tool to overcome my problem. I am still left with a slight pip but it's nothing compared to what was getting.

Thank you.

Tom
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