Re: Carbide - Surface Finish - 9A

Mike Poore

Ideally, I'd like to be able to use one tool to take a heavy cut followed by an acceptable finishing cut. Is that possible with carbide on a 9A?

You and Ondrej recommend 400-600 sfpm with carbide. I take it carbide cuts better at a faster sfpm? Maybe I misunderstood carbide usage. I thought it was capable of faster cutting, but faster cutting was not essential for a good cut. I know the speed effects the chipbreaking, but I did not know slower cutting will actually make the finish worse. So will cutting at a lower sfpm lead to the tearing finish that I wrote about originally?

I have about 100# of various carbide inserts, so I'd really like to find something I can use rather than buying something else.


On 7/14/2022 10:10 AM, Bill in OKC too via wrote:

I'm afraid that you need a much sturdier/more massive lathe than the lighter SB lathes. My experience is with Clausing-Colchester 13 & 15 inch school lathes. The smaller 13" lathe weighs about 2700 pounds, the 15" lathes about 3800lbs. My Heavy 10L weighs just over a third of what the smaller CC lathe weighed and about has to be a great deal less stiff. Mild steel, in our class was specified 90SFPM for HSS, and 400SFPM for carbide tooling. I could get a mirror finish on the 13" lathe. I've not gotten to use my SB yet, just the Atlas. It's about a quarter the weight of the SB. Pretty much spaghetti comparatively. Unless your SB is one of the newer lathes and a 14" or larger you're going to need slower speeds and feeds, and shallower cuts to make up for the lesser stiffness. 

You could use carbide for roughing cuts, and HSS for finish cuts if you really need or want the fine finish. You can diamond hone HSS & carbide if you need it sharper, but that can mess up your repeatability with indexable inserts. sells HSS indexable inserts which the folks who use 7x mini-lathes a lot seem to find very helpful. sells the smaller sizes appropriate for the 7" and similar machines, and in smaller quantities IIRC.


Bill in OKC 

William R. Meyers, MSgt, USAF(Ret.)

Aphorisms to live by:
Good judgement comes from experience. Experience comes from bad judgement. 
Improvise, Adapt, and Overcome.
Physics doesn't care about your schedule.
The only reason I know anything is because I've done it wrong enough times to START to know better

On Wednesday, July 13, 2022 at 09:46:54 PM CDT, Mike Poore <mpoore10@...> wrote:

I am sure this is not the first time for this topic, but I'd like to know what setup, if any, anyone has found that provides a good surface finish using carbide on 1018?

I have acquired a lot of carbide inserts and holders along with my QCTP. I have negative rake and no rake holders. I have chipbreakers and flat carbide. I turned some aluminum after rebuilding the lathe and the surface finish was as good as one could expect so far as it was nearly a mirror finish using the slowest feed. Then I tried 1018 with the same tooling and ended up with a fairly rough finish. It looked and felt like the metal was tearing rather than cutting. I have tried other combos and nothing yet is satisfactory. I have a diamond disc sharpener, but I think the carbide is sharp enough to do better than it is. There is plenty of HSS laying around that could be used, but the idea of indexable carbide would be preferred if I can make it work.

There is plenty of experimenting left to do. I was hoping someone could give me a starting point as far as insert shape, rake, speeds (I am guessing it might be a little different than what the book says for an old WWII cast iron bearing lathe.), and tool angle. Thanks for any help.

Join to automatically receive all group messages.