Running a threaded spindle lathe in reverse to cut heavy threads in a blind hole.

How are you engaging the half nuts, are you leaving them engaged or dropping them and using the thread dial.

If you are leaving them engaged you will have the same possibility of problems when you return to the start of the thread.

If you are using the threading dial there will not be a problem.

How long is the spindle thread on the lathe, you would only need 4 to 5 threads in your nut to engage on your lathe spindle.

I make collet nuts for Hercus lathe that are 1 1/2"x8 tpi.

The overall length of the spindle thread is about 1.130", is undercut the rear of the nut to a length of .500",this provides plenty of run off clearance for the threading tool and sufficient amount of threads for the spindle.

If you thread at a low rpm you will be able to easily hear when the tool has stopped cutting and cleared the end of the job.
 
Why don't you find someone on the forum close to you who has done internal threads that can help you. Seeing and hands-on doing always helped me more. Check the member map, there might be someone next door.
 
Why not turn the tool upside down and cut the thread on the back side, still turning the chuck in the normal direction. With that huge hole you can actually see the work surface and the tool all the way to the back. Makes it a lot easier than working blind.
Pierre
 
Here is how I made mine. Bore tha hole 1/2 deaper than your spindle lingth. then cut a 1/16" under cut in the back. Run your threading tool in by hand with spindle off till it bottoms out then back it out 1.8". Mount a dial indicator on a mighty mag and set it on the ways so it reads .300 watch the indicator and stop at .300 and you will be fine. You can set up that way with nothing in thr chuck and no tool and get used to watching the gauge and stoping at .300. Hope that helps.
 
FWIW, this is from a post by "daveycrocket" on the <another> forum about using a shaft collar to help ensure a chuck will not spin loose upon reversing the spindle:

"At the time I mentioned that in a Horology course workshop we had 3 Schaublin 102 lathes . On all chucks, there was a steel split collar round the protruding collar on the backside of the chuck backplate . There was a socket head screw through the split which when tightened against the protruding collar on the backside of the backplate, locked the backplate securely on the spindle nose thread/register. Whilst it might not work on lesser wellfitting backplates /registers, on our Schaublins, the fit of backplate to threaded nose/register was so close that when that Socket headed screw was tightened, there was no way that backplate was going to screw off when the spindle was in reverse, nor even when the lathe was in forward and the footbrake stopped the spindle pretty dam quick. In 8 years in the workshop, I never once saw a chuck come off when it was not supposed to. ! If someone had just told me it worked and I had not experienced it myself I would have doubted that those thick projections on the backside of the backplate would crush enough to grip the spindle thread/register ."

This is what he's referring to:

collar.jpg
 
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One thing that I have not seen in this series of replies is the question of how the thread if cut is to be gaged for fit. On all the threaded nose spindles that I have owned and have had to fit new chucks to, I have first made a thread gage; a short threaded piece of steel that duplicates the spindle nose thread accurately, with a knurled handle on one end that is smaller than the root of the thread. Remember I used the word ACCURATELY; one needs to measure the pitch diameter of the spindle threads, likely with thread wires,or a thread micrometer if one can be had, and cut a nice smooth thread on the gage to the same measurement with the wires measured by a micrometer, then use the gage to assure that the new backplate or other accessory will indeed fit the spindle after threading; it should not fit tightly, nor loosely, but more or less fit as the chucks that one has do.
So far as the question of running backwards, I would do it on a large lathe, where the chuck can be run up hard, but not on a small lathewhere it is perhaps not so wise to run it up hard due to the likelyhood of damage; the locknut is not so good of an idea either, I doubt that it would help, and likely it would cause runout in the chuck. The best plan is to allow a generous relief at the inner end of the thread and thread the conventional way (towards the chuck) and use a visual stop to show where to disengage the half nuts; a solid stop clamped to the ways is the worst thing to do, as if it actually makes contact, it will be impossible to disengage the half nuts,
 
Good point about making a thread gauge. I will need to be sure the thread size is correct before I remove the work from my chuck to finish it on the spindle.
 
I've never had any trouble cutting internal threads toward the spindle. First, I have a dial indicator set up as suggested, with the critical stop point after a known number of turns. It also includes a solid stop rod that will cause the feed clutch to slip if I don't shut it down soon enough. This won't help much for threading, since the half-nuts, not the clutch, are being used. I really rely on that dial indication to tell me where the tool is. Just keep in mind that changes made on the compound feed won't be seen on the dial.

Here's another suggestion. When your threaded length isn't all that long, why not turn the chuck by hand? I've used it to good effect for short threaded collars. The lead screw will pick up the gearing, regardless of the power source.
 
Hawkeye said:
I've never had any trouble cutting internal threads toward the spindle. First, I have a dial indicator set up as suggested, with the critical stop point after a known number of turns. It also includes a solid stop rod that will cause the feed clutch to slip if I don't shut it down soon enough. This won't help much for threading, since the half-nuts, not the clutch, are being used. I really rely on that dial indication to tell me where the tool is. Just keep in mind that changes made on the compound feed won't be seen on the dial.

Here's another suggestion. When your threaded length isn't all that long, why not turn the chuck by hand? I've used it to good effect for short threaded collars. The lead screw will pick up the gearing, regardless of the power source.
Click to expand...

That's a great idea. If I have the belt tension off, I should just be able to turn the chuck without worrying about it coming loose from the spindle, right?
 
There has been a lot of great advice in response to your question, most of which I agree with. What hasn't been mentioned is another way to lock on a threaded spindle, which is used on my Grizzly G0602. The spindle has a groove near the edge of the (unthreaded) shoulder. There are two little blocks of steel that have a matching v protrusion, and extend out over the chuck, with a counterbored hole for a shcs that threads radially into the chuck. There are two of these 180 degrees apart. When screwed down tight they eliminate the possibility of the chuck coming off.

I made a ER40 collet holder that threads onto my spindle, which has an unusual 1 3/4-8 thread. I made a .250 landing zone for the internal thread cutting, which I did with a home-made cutter, upside-down on the back side for visibility. Threading in was still hairy, but I waited until I had added a VFD so I could dial down the speed from the stock 150 rpm gearing. (At 150 rpm, cutting 8 tpi threads has the cutter moving 5/16" per second!


chuck with collet (vga).JPGspindle end (vga).JPGYou can see the threaded holes in the collet chuck in the second picture. In the picture of the chuck mounted on the lathe you can see the groove in the spindle. Unfortunately the keepers are not installed in this photo. The larger holes are for tommy bars to turn the spindle and chuck for tightening and loosening.

Incidentally, you can buy a beautiful and relatively cheap boring/threading tool that will do both L and R internal threads from Mesa Tools in Oregon.

Let me know if you are interested and I will take a picture of the little clamping blocks. They are conceptually simple but appear to work well and could be added to an existing spindle, assuming it has enough meat to cut the groove.
Craig

chuck with collet (vga).JPG spindle end (vga).JPG
 
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