Triple Threads - Requesting Help

[ahorton]

My machinist has a headache right now because I'm trying to get him to make a triple-start thread for me. That is, a thread with lead of 3 x the pitch. So one rotation advances the screw 3 x the pitch.


Normally I just specify a nominal diameter and pitch when doing non-standard threads and he's happy with that. What do I need to tell him to make life easier for him?

He's asking about thread inclination and what it needs to be reduced to but I'm not sure.


Thanks in advance!

 

[precisionworks]

As long as his CNC lathe has an angular position sensor at the chuck it should be easy to program the three starts at 120° angular separation.

Mac does a nice job on these & maybe he'll have suggestions.

 

[ahorton]

As far as I can see, it should just be a matter of starting each pass 1 pitch back from the previous. I can't see that the angle or type of the tool should change at all. Of course this does assume that the lathe has the angular position sensor as you mentioned.

 

[gadget_lover]

Assuming a pitch of 12 with a lead of 4... would it make sense for him to program it as if it was 3 separate 4 tpi threads with the depth used by 12tpi started 120 degrees apart? That is the end result.

Obviously, I have no idea what the "thread inclination" means. Sounds like a term used to describe the geometry for CNC thread milling.

Daniel

 

[precisionworks]

I have no idea what the "thread inclination" means. Sounds like a term used to describe the geometry for CNC thread milling.

Daniel

" Thread inclination" describes tipping the insert with a shim seat so that the insert doesn't rub while cutting the thread(s).

Sandvik says it best:

The clearance necessary for a threading insert is related to the helix angle (λ) of the thread in that these should be similar. When the inclination angle (ρ) of the insert is different to the helix angle, the clearance angles of the insert will be different. The angle of insert inclination is calculated using the adjoining formula.

Source: Sandvik Metalworking Products, Thread Turning

 

[gadget_lover]

That's what I figured/guessed. 🙂

In my example, then inclination would be for the 4tpi lead, and not the 12 tpi pitch, right?

Daniel

 

[ahorton]

Thanks for all that. The Sandvik pdf looks like a good resource to hang onto for next time. It's times like these that I realise just how involved machining really is and that it becomes a hobby in itself.

I drew him some more pictures and I think it's all ok now. I'll just need to wait and see what he produces.

Thanks again!

 

[gadget_lover]

I love the way these threads lead to sudden insights.

I'd heard of the need to tilt the insert to avoid rubbing of the cutter's flanks. I'd not noticed a need for in in any of my work. Seeing that formula made me suddenly realize that the slant of the thread is directly related to tpi AND diameter. A 4 tpi thread on a 12 inch diameter rod will be almost vertical. On a 1/2 inch piece it will be at quite a slant. By the same token a 24 tpi thread on a 1 inch piece will be close enough to vertical that the inclination is not much of an issue.

Daniel

 

[StrikerDown]

This splains a lot! :rock:

 

[wquiles]

I love the way these threads lead to sudden insights.

+1

I am always learning in this forum :bow: