Parting off query

[PlasmaParticipant @plasma]

Hi all, when parting off the 2 inch brass bar for my door knob bezel the face of the bar ended up concave with a matching convex face on the parted off disc.

Suggestions on a post card please…..

[PlasmaParticipant @plasma]

Concave face for some reason

[JasonBModerator @jasonb]

If it is an HSS blade then make sure it is ground dead square across the end. If insert then use a neutral one which will be square across the end.

In both cases make sure the tool is at perfect right angles to the lathe axis.

I'm sure someone will be along to tell you to use a rear tool post too

[SamsarandaParticipant @samsaranda]

Make sure the carriage is locked down as solid as you can get it.

Dave W

[PlasmaParticipant @plasma]

Cheers Jason, HSS blade, front tool post lathe in reverse with power feed 440rpm.

I will check the items you mentioned, got to be some thing simple as never had it before.

Plus now I come to think of it blade snapped a few weeks ago and i reversed it, may have upset something.

Mick

[Jeff DaymanParticipant @jeffdayman43397]

Brass bar can slip in the chuck jaws too. Suggest using paper strip between bar and jaws, and tighten securely. Just snug is not good enough. Suggest doing all the other good suggestions re tooling so far.

[HOWARDTParticipant @howardt]

If you snapped the blade then it may be plain HSS. I had this happen to mine and changed it to a M42 HSS-Co8, had a few snags but the blade has remained as one.

[Nigel Graham 2Participant @nigelgraham2]

I'd certainly take the above advice, but I've also noticed subtly convex ends on normal facing. Cross-slide worn / out of adjustment? That would affect parting, too.

[Mick B1Participant @mickb1]

Minimise partoff tool protrusion. If it's a thin blade there's a lot of lateral elasticity that even slight out-of-squareness on the cutting face will cause deflection, so only have the length protruding you need to reach centre.

Make sure topslide backlash is taken up toward the chuck. Make sure the blade is parallel to its direction of travel.

[Paul LousickParticipant @paullousick59116]

This is a good video on Youtube by Joe Pieczynski about parting tools.

Paul.

**LINK**

[HopperParticipant @hopper]

By far the most common cause is as Jason says: end of HSS tool not ground square and/or tool blade not set dead square to lathe axis.

[Ian S CParticipant @iansc]

The parting tool should be flat on the top for brass(bottom if going in reverse), and dead square across the end or it will pull to the leading point. An angled end seems ok in steel.

Ian S C

[blowlampParticipant @blowlamp]

Posted by Plasma on 16/05/2019 19:32:36:

Cheers Jason, HSS blade, front tool post lathe in reverse with power feed 440rpm.

I will check the items you mentioned, got to be some thing simple as never had it before.

Plus now I come to think of it blade snapped a few weeks ago and i reversed it, may have upset something.

Mick

What do you mean when you say "and I reversed it"?

HSS parting blades are usually asymmetric,viewed from the front, so the cutting clearances will go to hell if you managed to flip it over to use the back end of the blade.

Martin.

[Ian PParticipant @ianp]

If the parting blade is wedge shaped in section, then its important that its holder clamps it so the side clearance is the same angle each side. Certainly worth checking that there IS clearance on both sides. DAMHIKT.

Another thing I have wondered about with the wedge shaped blade is that the top edge is also at an angle so only go in the holder one way. If the cutting edge is ground dead square then only at one point is the cutting edge at the right height. I assume that causes some differential cutting action across the width of the cut so and therefore some blade deflection?

Ian P

[Mick B1Participant @mickb1]

Posted by Ian S C on 17/05/2019 13:51:32:

The parting tool should be flat on the top for brass(bottom if going in reverse), and dead square across the end or it will pull to the leading point. An angled end seems ok in steel.

Ian S C

I think that's true and have noticed it more in softer materials, as you suggest.

Sometimes you ain't got much choice but to got for an angled end, such as when you're parting a component with a through hole, you don't want a parting-off 'collar' around it, and you'd have difficulty holding the component to remove it in a second op. Then you just gotta see if you can get everything else on your side.

[PlasmaParticipant @plasma]

Hi all,

I reground the cutting tool to ensure it was square across and it made all the difference. Parted off nice and square.

Many thanks for the help with this one, there were some suggestions I will be trialling some of them with my parting set up for different materials.

Regards Mick

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