Full-profile threading inserts | Practical Machinist

I'm looking to try out full-profile carbide inserts for gunsmithing work (threading muzzles and tenons on my manual lathe). I posted here because I think general non-gunsmithing threading experience would apply.

I normally use the upside-down method and thread away from the headstock, with the barrel held in spiders, so I can spin it pretty fast (hundreds of SFM, if needed). I'm hoping to get better surface finish and more exact root/crown profiles than my HSS tooling. The profiles are a gimme, but the surface finish might be wishful thinking on a manual machine. Something that easily snugs up against a shoulder (left-hand holder) will be needed for the upside-down threading. I can run it the normal way, but I assume I won't get the best finish at the lower speeds that requires. I normally run fine threads (24-28) and/or can have a fairly wide relief (0.100+), but I'm still not happy threading into that relief at normal carbide SFM. I do run a hard stop right before the shoulder, so that if I miss the timing of the nut I shear the leadscrew pin instead of running the tool into the shoulder of my customer's expensive barrel, but I certainly don't want to thread at speeds where either of those is likely!

My primary concern about leaving HSS for this app is that I might not be able to sneak up on the final thread size - I really like being able to kiss a thou or two off the dia with a well-honed HSS tool, and get a really tight 3A fit. I ASSUME that material removal is non-linear as the insert reaches final depth and starts acting as a full form tool, so I won't be able to rely on the DOC per compound movement numbers I see on the first few passes to make the final cut. I tend to cut the exact same thread sizes in the same materials (, , 416), so hopefully I'll be able to learn the tooling and get accurate cuts with whatever DOC is needed for a good chip and finish, but any comments on how hard/easy that is would be good.

Does anyone have a specific insert/holder family they have had good luck with, and speed/DOC suggestions?

Thanks! For best results with good quality name brand carbide threading inserts you really should go by the book. The makers put a lot of effort into developing tooling that works as it should straight out of the box for all normal applications. Unfortunately for manual machinists the main market is CNC so carbide is optimised for higher speeds and deeper cuts. Which usually means it will go funny or inconsistent outside its design range. Best to talk to the makers.

Can't even rely on recommendations from folk who bought a box a while back which works just fine at lower speeds. Makers are into continuous development so although the numbers on the box maybe the same the insert geometry maybe subtly different altering or even destroying off book performance. Got caught out like that with some turning inserts and had to re-calibrate.

Suggest you look into the Zero-2-Zero / Zero-to-Zero threading technique to get round the sneaking up to final size issue. May need to make test run and adjustment a time or to to get the variables nailed down but once sorted its dead reliable and extremely repeatable to size. Basically because the lathe does all the infeed calculation stuff for you automatically for the dialled in thread depth. All you have to get right is the actual thread depth you set the (finish point) zero at for that particular combination of tool, material and thread pitch. Which may or may not be book depth.

Technique has been written up a time or two here. Albeit for right side up, front mounted, tooling. Works fine t'other way about.

Clive The full form is, well, full form so if your female thread has been cut a bit on the loose side then you would not really be taking the full advantage of the insert because you would not be kissing the OD with the insert when final depth is reached. The normal way to do this is leave your OD slightly larger and when you start touching with the insert you bring out the Mic's to start measuring the OD. Once the OD is on size in theory you have the perfect thread form for that thread if everything is aligned correctly.

If you are sure that the female thread is correct then full form inserts are a dream to use and can still be used to a shallower depth without touching the OD, if needed, to try get a tighter fit. The only thing is then the OD will not get the top of the threads chamfered with the correct rad which is not a biggie.

I think that is why rons suggested partial profile which works well but you get a sharper thread because of the profile not being 100% correct.

Buy any SER holder of any brand like the top one in the iscar link. It is better to go that way because you can source inserts from many different manufacturers. I think a SER16 would be good for most of your threads as it goes up to 3.5mm pitch from what I can remember. The 22 goes bigger and then the 27 for large threads.

Don't be scared about running it slower than recommended as said above, once you get the hang of the last couple of "polishing" cuts to get a nice finish you will be sorted.