Parting off on a mini lathe

[Andy BrocklehurstParticipant @andybrocklehurst85292]

I’m learning turning and parting off on my mini lathe just using some 6mm brass bar, turning and facing is ok but when I part off I get left with a stalk.
The part seems to snap off before it’s cut, I’ve got the insert centralised (the best I can by shimming) running at around 350rmp with cutting oil using the insert shown.
Any advice appreciated.

[roy entwistleParticipant @royentwistle24699]

Don’t use cutting oil on brass. Is your tool too high? and slow down a bit.

Roy

[roy entwistle]

On roy entwistle Said:

Don’t use cutting oil on brass. Is your tool too high? and slow down a bit.

Roy

Ok thanks, I’ll try and get the tool a bit lower.

[NealebParticipant @nealeb]

The parting inserts I have bought always have a tip that is parallel with the lathe axis so that they produce a flat-bottomed groove. Inevitably, this means that the work will break off when the remaining material is too weak to support it and almost always leave more or less of a pip. In the days of HSS tools, the parting tool would often be ground at a slight angle on the tip so that the RH corner cut slightly more deeply than the LH corner. Get it right and the work would drop off leaving any pip on the material left in the chuck. However, my own experience was that this slight angle could also cause the tool to go in to the work and progressively move to the right, leaving a concave face on the parted-off work. These days, I use standard inserts and accept that I need a second operation to clean up the face. I believe that inserts with angled tips are available, although I have never gone in search of them.

I cut brass dry, and probably faster than you are, although my own lathe is a bit heavier than a mini-lathe. Main thing is to keep a steady feed but at least the chips tend to fly off and not jam, as they can do with steel.

[DiogenesParticipant @diogenes]

Yes, I find that can also happen because some of the cheaper tips seem to need a bit of pressure to keep them cutting – I try and ease-off a bit when I know it’s very close to the middle, and let it just ‘shave’ the last bit for a few turns, any residue left should be foil-thin and can be scraped off, much as Nealeb says.

‘Sharper’ inserts can help, also, as said, making sure that c/height and alignment are as good as you can get them.

[Peter Cook 6Participant @petercook6]

Looking up those inserts suggests they are primarily designed for grooving, so the cutting edge is square to the lathe axis. ARCEurotrade’s website shows parting off with those inserts with what appears to be a slight angle to make the RH corner of the insert do the main work.

On my micro lathe, the parting tool is ground with an angled tip as Nealeb suggests.

[Andy BrocklehurstParticipant @andybrocklehurst85292]

Thank you for all your replies folks, I’ll try all your suggestions.

[Bo’sunParticipant @bosun58570]

I realise it will be difficult to do, but I’ve often wondered if an inverted V profile on the face of the parting blade would work.  It would balance the lateral cutting forces caused by a single angled face, and help remove the pip.

[JasonBModerator @jasonb]

The ARC ones are Neutral inserts hence the N in the code, just the image shows the “ideal”. Right and left bias are available but can have the cut wandering off if the setup is not rigid.

MGGM200 inserts are available and sharper so will  put less pressure on the material and not cause the unsuported part and bit at the bottom of the cut to break off so soon but you are still likely to get a foil thin piece attached but flick it off with a finger nail or pull with pliers and then deburr the hole.

You could happily double or treble your speed with brass of that sort of diameter

[cogdobblerParticipant @cogdobbler]

At 6mm diameter,  brass with carbide tooling, rpm ideally should be about 3,000. Knock that in half because parting can be more difficult than plain turning and you are still at 1500 rpm.

Running at 350rpm on such a small diameter in brass with carbide tooling is such a low surface speed the carbide will struggle to cut, as you have found.

 

 

[cogdobbler]

On cogdobbler Said:

At 6mm diameter,  brass with carbide tooling, rpm ideally should be about 3,000. Knock that in half because parting can be more difficult than plain turning and you are still at 1500 rpm.

Running at 350rpm on such a small diameter in brass with carbide tooling is such a low surface speed the carbide will struggle to cut, as you have found.

 

 

Ok, thanks for that I’ll try speeding up.

[old martParticipant @oldmart]

I like that type of parting off insert, and have them in 2 and 3mm widths. Having larger lathes, I have found a source of 26 and 32mm blades which suit me better than the style that smaller lathes require. Inserts are available in steel and aluminium grades, and for brass, I recommend the aluminium HO1 grade. I get a small stalk on the part cut off, you can minimise it, but probably not totally eliminate them. Only experimenting with height and speed as advised will you find the best settings which may vary with different metals. Parting is better if there is a hole in the workpiece. Slow down the feed rate as the diameter left gets smaller or you get near the hole diameter.

[VicParticipant @vic]

Nealeb’s comment is what I’ve found. The effect can be somewhat reduced by grinding only a slight angle. I very rarely use inserts for parting since I tried the T shape HSS blades. They work great in all but the hardest materials and can be used on plastics and softer materials as well. They work fine on brass.

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