Fun with Internal Threading

[Just for fun]

Guys, Thanks for all the pointers.

I gotta hit the ground running this morning. I'll catch up later.

 

[maspann]

Several have mentioned threading away from the chuck. Here is Joe Pie's video on the subject.

 

[Jim F]

that does not work with threaded spindles.

 

[Parlo]

To prevent the possibility of a crash when internal threading in a blind bore, turn the tool upside down, and run the lathe backwards. That way you are cutting from left to right, out of the bore rather than into it. I do this also when threading an external thread up to a shoulder.

Turning a R/H threading tool upside down is still a R/H tool that now cuts on the rear of the bore. The spindle still needs to turn in the same direction so feeding away from the chuck will produce a left hand thread.
To cut R/H threads away from the chuck, the spindle needs to run in reverse and a L/H tool is required.

 

[WobblyHand]

Turning a R/H threading tool upside down is still a R/H tool that now cuts on the rear of the bore. The spindle still needs to turn in the same direction so feeding away from the chuck will produce a left hand thread.
To cut R/H threads away from the chuck, the spindle needs to run in reverse and a L/H tool is required.

If your lathe does not have reversing tumblers to change the rotation of the lead screw relative to the spindle, then no matter what you do, the thread will be right handed. My G0602/G0752Z cannot make LH threads without modifying the lathe. My mini-lathe can cut LH threads, since it can reverse the lead screw relative to the spindle. My Grizzly cannot change the relative rotation between the spindle and lead screw.

I can and have cut internal RH threads using a LH tool cutting away from the chuck on my G0752Z. The pictures below are of the beginning stages of machining my ER40 set true collet chuck backplate. And no, my chuck did not unscrew from the spindle. To be fair, I do have locking clamps on the spindle to prevent such a calamity. The clamps will prevent disaster, but not loosening. To date, I have not seen any evidence of any chuck loosening.

 

[Parlo]

" If your lathe does not have reversing tumblers to change the rotation of the lead screw relative to the spindle, then no matter what you do, the thread will be right handed. "
True, but you will always need a left hand tool, which most of us don't have, if you want to cut internal threads away from the chuck.
The point I was making was an internal threading tool can't be simply turned upside down to make it left handed, which is what is needed to thread away from the chuck.

An external threading tool can be turned upside down or cut from the back to make R/H threads with the spindle in reverse away from the chuck.

I would not bother with the compound slide at an angle, nor would I plunge straight in. With the compound slide set parallel to the bed. Just advance the tool 1/2 the depth of cut on every pass to keep the cutting on the leading edge of the tool. These X & Z moves simulate a 26.5 degree compound move, which keeps a small amount of cutting on the secondary edge.

My old CNC only threads towards the chuck. To produce left handed external threads I use a R/H boring bar upside down cutting on the outside. With the bar held in a bush the toolholder it stays on centre. I find this easier than cutting from the back using negative figures in the program or resetting the centre height on an inverted external tool.

 

[WobblyHand]

Have to agree with you that turning over a RH tool doesn't make it a LH tool. In my case, I borrowed an internal LH tool to do the job. Doing that was more time efficient than modifying my lathe.

For external threads it would seem that turning the tool upside down does work, however it will be difficult or impossible to thread to a shoulder. The tool holder body is perpendicular to the work piece and that gives one additional degree of freedom. Internal thread tools by necessity have to be held outside the hole on the tailstock side.

Of course, if you really need an internal LH threading tool they are not hard to make. Just drill a hole perpendicular to a steel rod near the end. Put a piece of HSS through the hole and put in a set screw through the end to retain the bit. What's nice about this is you can flip the cutting tip and make it either LH or RH. You don't have to broach the hole, it works ok with a round hole.

 

[T Bredehoft]

I suppose I'm old fashioned, but I found a totally stress-free way of threading internal threads. Right or left, front or back upside down or normal.
I put a crank on the left end of the lathe, in the spindle, and turn it by hand. Run it in or out, it doesn't matter, you won't crash. Sure, it takes time, but if you're selling your services (and time is important,) you're good enough not to have to worry about threading.
If you are patient (slow and old) you can get a perfect thread every time, just leave the feed engaged and remember which way to turn the spindle, cut or withdraw.
Yes, you have to back the tool out of the cut between cuts.

 

[WobblyHand]

I suppose I'm old fashioned, but I found a totally stress-free way of threading internal threads. Right or left, front or back upside down or normal.
I put a crank on the left end of the lathe, in the spindle, and turn it by hand. Run it in or out, it doesn't matter, you won't crash. Sure, it takes time, but if you're selling your services (and time is important,) you're good enough not to have to worry about threading.
If you are patient (slow and old) you can get a perfect thread every time, just leave the feed engaged and remember which way to turn the spindle, cut or withdraw.
Yes, you have to back the tool out of the cut between cuts.

I made a hand crank for my mini-lathe. It did take the stress out of threading. Don't use it much these days, but it really did help with learning the threading process. Still use it if I need to thread to a tight shoulder, or there's no thread relief. It goes as slow as it needs to.

 

[Parlo]

I made a hand crank for my mini-lathe. It did take the stress out of threading. Don't use it much these days, but it really did help with learning the threading process. Still use it if I need to thread to a tight shoulder, or there's no thread relief. It goes as slow as it needs to.

Is your crank on the spindle or the end of the leadscrew?

Have to agree with you that turning over a RH tool doesn't make it a LH tool. In my case, I borrowed an internal LH tool to do the job. Doing that was more time efficient than modifying my lathe.

For external threads it would seem that turning the tool upside down does work, however it will be difficult or impossible to thread to a shoulder. The tool holder body is perpendicular to the work piece and that gives one additional degree of freedom. Internal thread tools by necessity have to be held outside the hole on the tailstock side.

Of course, if you really need an internal LH threading tool they are not hard to make. Just drill a hole perpendicular to a steel rod near the end. Put a piece of HSS through the hole and put in a set screw through the end to retain the bit. What's nice about this is you can flip the cutting tip and make it either LH or RH. You don't have to broach the hole, it works ok with a round hole.

I use full form flat inserts so a R/H internal insert used on the O/D becomes a L/H external insert.