Help with choosing thread cutting inserts

[Stuart Cox 3Participant @stuartcox3]

Hi all,

Quick question, can anyone tell me what I should be searching for please. I am looking for a metric thread cutting insert for a13mm tool, but I need to be able to cut the thread up to a shoulder to the left if that makes sense. My current inserts I have won't allow me to do that as the tip points at the wrong angle. Is there like a LH and RH version?

Thanks Stu

[Stuart Cox 3Participant @stuartcox3]

[Stuart Cox 3Participant @stuartcox3]

I think I may have just answered my own question, it is an external thread insert I am looking for, I only have internal thread inserts! Doh!!

[JasonBModerator @jasonb]

You need a code with ER in it = external RIGHT

If your lathe will run OK in reverse then the internal tool can be used at the rear of the work, I tend to do that as it is easier to work away from a shoulder than  stop just before it

Edited By JasonB on 29/08/2021 16:32:10

[Stuart Cox 3Participant @stuartcox3]

Thanks Jason, I did actually try cutting the thread at the rear of the work using the internal tool but I ended up with a LH thread, but I realise now I should be cutting away from the shoulder as you mentioned not towards the shoulder, sometimes it needs someone to point out the obvious!

Thanks again, Stu

[John ReeseParticipant @johnreese12848]

An "on edge" insert will let you thread very close to a shoulder. If you grind your own HSS tools you can grind a tool that lets you thread right up to the shouder.

[SillyOldDufferModerator @sillyoldduffer]

I'm self-taught, which means book learning supplemented by experiment corrected by much help from forum. Great fun, but self learning leaves a multitude of gaps!

Is there a good reason for threading up to a shoulder?

My reading suggests, and what I've always done, is to start by turning down the blank (hex or round stock), to thread diameter and then cutting a slot as deep as the thread at the shoulder with a parting tool. This creates start or finish space for the threading tool and much reduces the chance of a collision.

I don't believe the slot makes any difference to bolt strength, so why not always make one?

On the subject of collisions, I always thread away from the chuck in reverse like Jason. Possibly I learned the trick from him! However, reverse threading isn't a good idea if you're unfortunate enough to own a lathe with a screw-on chuck…

Dave

[JohnFParticipant @johnf59703]

Dave aka SoD making an undercut at the shoulder is industry standard but doing it with a parting tool will potentially reduce the strength due to sharp corners creating stress points. Undercut tools as a general rule would be 1-1/2 x pitch wide with a radius on the corner adjacent to the shoulder and the "thread side" having an angle equal to half the thread angle or 45 deg.

John

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