Parting off - Left over nib

JimDobson · Feb 5, 2025

Is there a way when parting off to have the left over nib (that at least I always get) stay on the piece that's in the chuck and not on the piece that's been parted off?
I've tried adjusting my parting tool every which way over the years and never managed to have that nib not end up on the parted piece.
Anyone have any hints?

RM-MN · Feb 5, 2025

When your cut gets almost to the center the piece away from the chuck will bend away from the parting tool just a tiny amount before the tool completes the cut. I don't think you can avoid the nib on that part.

Vietti · Feb 5, 2025

Try grinding a slight angle on the end of the parting tool so the cut off piece is cut deeper than the stock-kind of works

timo_gross · Feb 6, 2025

You "clearly need" a 2nd spindle

I really wonder if a spinning chuck in the tailstock would help to make the "nib" smaller. (I feel guiding small parts by hand helps a little to get smaller "nibs".

L98fiero · Feb 6, 2025

JimDobson said:
Is there a way when parting off to have the left over nib (that at least I always get) stay on the piece that's in the chuck and not on the piece that's been parted off?
I've tried adjusting my parting tool every which way over the years and never managed to have that nib not end up on the parted piece.
Anyone have any hints?

If you are using inserts, there are left, right and neutral angle inserts, the left and right angle ones determine where the 'nib' is or if you are parting a tube, where the bur is left. The same can be done with a high speed steel parting blade. It should be noted though that especially with the HSS tools, they tend to wander.

Ken I · Feb 7, 2025

I ran an automatic lathe shop (cam auto's not NC's) for many years and a pipless part-off is invariably required.

Carbide tools are always "blunt" and will in most cases (even with the angled variety) leave you with a nib.

See my post on parting off

https://www.homemodelenginemachinist.com/threads/parting-off-again.33417/

Use a large front clearance rake - I use a beveled blade and don't use any side clearance - but you must get it dead square and lubricate it.

I will sometime grind a radiussed hook rake (LH image) - this directs the chip flow toward the stock so as not to "Push" against the bit being parted off.
Also good for stainless steel.
The downside of this is the sharp corner is at the very edge - further resharpening will increase the amount of rubbing and poor finish - so life expectancy between complete regrinds is limited.

The RH image blade is simply ground down the front at an angle - resharpening is quick and simple - you only need to grind back if you have burned the edge of the blade where you have chosen to run without clearance.

The part will normally shear off at the sharp point leaving a witness mark but no noticeable nib.

If you don't want it to fly off, stop as soon as it starts to wobble and just twist it off. Same result.

Regards, Ken I