internal metric thread

[savarin]

I'm just getting ready to cut the internal 39x4mm thread on my new backplate and I'm having a seniors moment.
Do I set the compound the opposite to external threading, ie. the handle of the compound is now on the back side of the lathe but still driving towards the headstock.
Same process as external but pull cross slide to contact, advance tool, push cross slide, reverse lathe, pull cross slide to 0, advance compound, take cut etc etc.
Hope this is understandable

 

[darkzero]

Yes. To try & give it a visual, say for external threading you have the compound with the handwheel pointing in the 5 o'clockish position. For internal threading, you can set it in the 1 o'clock position & advance the handwheel to advance tool feed. Or you can set it in the 7 o'clock position & backout the handwheel to advance tool feed.

What I do is mount the threading bar upside down & I cut the threads on the backside of the bore with the compound in the same 5 o'clock position as for external threading. Nothing changes, tool feed, carriage feed, & spindle direction is the same as for external threading. Much easier that way for me & I can see what I'm threading.

 

[savarin]

I like the idea of going backwards and using the same settings.
Thanks

 

[GarageGuy]

For internal threading I was taught to have the compound at 29 1/2 degrees (on my lathe that's about 7 o'clock), backing out the compound to increase the depth of cut. For external threading the compound is also 29 1/2 degrees, but at about the 5 o'clock position feeding in. I don't know if it's right or wrong, but that's what I was taught.

I know there are other ways, but running in reverse seems more complicated. I could be wrong. My compound is very easy to adjust, so it has never been an issue for me.

I have the same carbide insert thread cutting tools as Darkzero, and I really like them.

GG

 

[12bolts]

.....I know there are other ways, but running in reverse seems more complicated. ...

GG, Will isnt running in reverse, He is just threading on the opposite side of the work but using the tool bit upside down, so the work cuts as it travels up (in its rotation) on the face away from the operator. Every thing is running in forward, CCW rotation, leadscrew direction, etc. Advantage to this method is that you could cut an external thread and then without changing any settings, (except for reversing the tool orientation) cut an internal thread.

Cheers Phil

 

[GarageGuy]

So, the only advantage is not having to change the angle of the compound? Just curious. Thanks!

GG

 

[epanzella]

When I cut any thread I set the compound on the bottom shelf of my workbench and feed with the cross slide.

 

[ki4byz]

Tool upside down to cut on the back side is the safest way to cut internal threads.
It keeps the compound and your hands away from the spindle. You can also better see what you are doing.
Travel outside to inside for RH, reverse feed to travel inside to outside for LH

 

[Hotswarf]

I was taught to set the compound slide to half the inclusive angle of the thread being cut, 60 deg for metric 55 for imperial usually. Set your tool square using grinding gauge and tighten tool post. Then when your tool tip is just touching on work, zero all your hand dials. Now feeding in on the compound only allows the tool to cut on only one face which has advantages. A bit of trig( Hypotenuse) needs to be done to know final depth needed on compound or if your lucky you might have a root diameter to touch on where you can zero compound and work from first cut back to zero. Only use cross slide to clear work and put back to zero. Sorry if its long winded

 

[12bolts]

Hotswarf, something less than 1/2 the angle is desirable for threading with the compound.

I was taught to set the compound slide to half the inclusive angle of the thread being cut, ........

If the tool travels in at just a little less than 1/2 the included angle it will take a small amount from the right thread flank and clean it up as you go. If you go in at 1/2 or more then there is a (very) good chance you will leave a staircase effect on the thread flank, which is not desireable.

Cheers Phil