Getting good finish on carbide inserts with shallow cuts

[taiwanluthiers]

Most carbide inserts seems to have a depth of cuts at which you will get a good finish, and for most inserts it's always more than 0.5mm. That means if I want to take 0.1mm off I'm out of luck and will get crappy finish. Are there inserts that allows me to take shaving passes and still get good finish?

I'm machining annealed 4140. The surface finish gets better if it's hardened, but in the annealed state it seems the only way to get a good finish is to basically take large passes.

 

[benmychree]

I use TPG inserts, 300 and 400 series; they have a sharp edge and will take light finish cuts.

 

[taiwanluthiers]

I have WNMG tool bits, no idea what TPG is.

Any good WNMG inserts that can take a light finish cut?

I notice smaller inserts can take lighter cuts, like the one I use for a 10mm boring bar can achieve a good finish with a 0.2mm depth of cut, but it's still a lot...

 

[hman]

"TPG" is usually an abbreviation for a toolpost grinder.

As far as inserts are concerned, the leading "T" indicates a triangular shape.
I suppose you could find an insert "decoder/ dictionary" to figure out the P and G. Somebody here should be able to supply a link to such a reference.

 

[benmychree]

T is triangular, P is for positive rake, G is for ground finish, hence with a sharp cutting edge. This is a very common insert. They also come in a unground insert, abbreviated as TPU, the U I was told stands for "utility"or unground if you wish, they come in coated as well as uncoated.

 

[taiwanluthiers]

When I google "TPG inserts" I'm taken to triangular milling inserts with no chipbreaker or anything of that sort, that is clamped onto a face mill.

 

[Ischgl99]

You’ll need to find a finishing insert for light cuts like that. All the insert manufacturers have tables of their insert styles and show the minimum depth of cut each type can do. Kennametal has an insert for your style insert with the -ff geometry that will do down to 0.1mm.

This is what a TPG insert looks like in a turning holder. The insert cutting edge is angled upwards to give it a positive rake geometry.

 

[benmychree]

TPG or TPU inserts do not have chipbreakers, generally the holders have adjustable chipbreakers for lathe use.

 

[taiwanluthiers]

Ok I get it.

So what makes an insert/tool bit give a good finish cut even with a low depth?

I tried grinding HSS bits and I could never get a good finish off of a HSS bit, no matter what I do, hone it, etc. it would always give a scratchy finish. Is it the chipbreaker or just how sharp the edge is?

 

[Jimsehr]

I have WNMG tool bits, no idea what TPG is.

Any good WNMG inserts that can take a light finish cut?

I notice smaller inserts can take lighter cuts, like the one I use for a 10mm boring bar can achieve a good finish with a 0.2mm depth of cut, but it's still a lot...

I think TPG stands for triangular positive ground. And TPU is about the same but cost less because not ground to as close a tolerance. You load insert then put a separate chip breaker on top and clamp both into tool holder. You can control chip by using different chip breakers.

google McMaster Carr TPG carbide inserts