Turning acme screw thread past limits of lathe

[crossthread]

I have an acme threaded screw to make that is used in the steering gear of a boat. Neither one of my lathes will allow me to cut a thread less then four per inch. One of the lathes will allow me to shift between the half nuts and the lead screw to more or less put the saddle in a "neutral" position. The bronze nut on this screw is still tight and doesn't have much play at all. I was thinking of connecting the nut to the saddle and rig up a way to turn the shaft and feed the saddle in neutral to make the cut. I would use a form tool to cut the acme thread. I would need to play with the VFD on the spindle to synchronize the feed. Does this sound reasonable or is this really stupid? I could farm it out but I hate to admit that I can't do something. You know how it is.....we can fix anything but a broken heart.

 

[donie]

I have cut many acme threads, the last one was a double lead 8tpi, the actual cutting was set at 4tpi. Things move fast cutting 4tpi. I think it may be better to mill really coarse threads. The classic set up might be a universal mill with a geared index head.
It seems possible to rig a lathe to function similar to the attachments used on Hardinge, where there is a master thread attached to the spindle.

 

[crossthread]

I thought of milling the threads but couldn't figure out a good way to synchronize everything such as the x axis feed to the rotation of the screw.

 

[Bobw]

I thought of milling the threads but couldn't figure out a good way to synchronize everything such as the x axis feed to the rotation of the screw.

You've got a screw and you have a nut.. Couple your new rod to your old screw, and just feed it into the cutter.
This is totally micky mouse, and no need to move the X axis, and no need for a 4th axis.

 

[Milland]

With many lathes there's the option of changing out a gear on layshaft between the spindle and the feedscrew, you should check your lathe carefully for this feature (remove whatever covers you need to).

If you can gain access, you can probably come up with one or two gears that if swapped or supplemented with the correct tooth count will allow a halving of the nominal feedscrew output. Surely you can cut 8TPI? Or 6, or whatever is the double of the pitch you need.

I don't think what you originally proposed would work (at least in the sense of giving a usable thread when done).

 

[Philabuster]

Does this sound reasonable or is this really stupid? I could farm it out but I hate to admit that I can't do something.

This sounds ridiculous. Nobody can do everything.

Sub the job out to another shop who can do the work and tack on some $$$ for yourself and make profit off the job.

Worst case, turn down the job. You need to accept the fact that not every job out there is suited to your shop's capabilities and not every job will be a winner.

 

[crossthread]

Thanks Bob. Mickey and I have not been on speaking terms since the accident.

 

[Bobw]

Thanks Bob. Mickey and I have not been on speaking terms since the accident.

The more I thought about it, its not very mickey mouse at all.. As long as your existing screw and
nut are in decent shape..

Your existing nut way over there ----> 2 bushings, one on each side of the milling cutter.. And then
attach your bar stock to the end of your existing screw.. And crank your screw.. The bar stock will
have the right pitch and rotation..

This sounds ridiculous. Nobody can do everything.

Back where I used to work, we had an older japanese lathe, little guy, I think it was a knock off copy of
one of the hardinge tool room lathe.. It was actually a really handy little machine..

The way it threaded.. You had a bunch of male threaded bits, one
for each pitch.. And a bunch of female nuts, but they were cut in half. I never actually threaded with
it because it was missing some pieces, but basically, put the male thread on the back of the spindle, and
then there were some lever arm things that attached to the saddle. The half nut went on the lever thingy, and
then you engaged the half nut with the male thread and it drug the carriage along and made your thread.

 

[crossthread]

I think I will give that a shot Bob. Sounds pretty straightforward to me. Mickey and I will have a freaking field day. I see threads....I think lathe. The nut and threads on the original shaft are definitely in good enough shape to do this. The business end of the shaft is broken and distorted for about 12 inches. There is not enough left to repair the end and I wouldn't trust it anyway. I may have to regrind an end mill to fit the profile but it is basically a square thread. I may have to reposition the bar stock to get the original length but that should not be an issue.

 

[Bobw]

I think I will give that a shot Bob. Sounds pretty straightforward to me. Mickey and I will have a freaking field day. I see threads....I think lathe. The nut and threads on the original shaft are definitely in good enough shape to do this. The business end of the shaft is broken and distorted for about 12 inches. There is not enough left to repair the end and I wouldn't trust it anyway. I may have to regrind an end mill to fit the profile but it is basically a square thread. I may have to reposition the bar stock to get the original length but that should not be an issue.

I was thinking about this some more, because it seems like it would be a lot more fun than any of the crap I have to make...

You could single point it.. Say if you clamp your threading tool to the head of your bridgeport. Use a drill to drive the screw, move the head up, or the saddle down, retract, move tool back to work piece and advance a bit and do it again. Heck, you could even run it off of your lathe.. If you had some kind of telescoping poles like off of those awning tent things.

Here is mickey mouse for ya.. (I didn't capitalize it because Disney scares me).. Part of a giant spinning die, just the nose piece. The bulk of it was made from wood (and it wasn't made here).. 39" in diameter on a 40x20. Cool rotary table, has the normal crank, and then a powered input which is geared down a lot and has a forward and reverse on it..

So. Mill providing the positioning (radially).. Craftsman Atlas lathe to drive the rotary table.. Grabbed in the chuck is a 3:1 reduction box I had made years ago to try and make a tractor steer better.. Then to a U-joint that you would normally use with a socket.. Then to a tent/awning/shelter pole, telescoping, to another u-joint to the rotary table. Took longer to cobble all that crap together than it did to machine the part...

 

[woodfarmer]

It might be more work than the job is worth... but if the end gear on the leadscrew has an even number of teeth, is it possible to make a replacement gear with half the teeth? If so and if you can reach that smaller gear with your drive chain you could cut your thread as the tpi (or satanic) would now be 1/2 that selected.

 

[crossthread]

Thanks for the input guys. Yes I could probably make a gear to cut 1.5 TPI but I really don't want to do that. I would much rather use the existing screw and nut as a lead screw or like Bob was saying, mill it using the nut and screw as a lead screw to advance the stock. The sweet thing about doing it with the mill is that there are no synchronizing issues.

 

[MushCreek]

Just curious- Have you looked to see if you can simple buy a piece of Acme threaded rod the right size? McMaster-Carr sells a variety of different sizes and pitches. I bought a piece to make a new cross slide lead screw for an old lathe, and it was surprisingly accurate.

Way back in my career, we did a job like that in a B'Port with an attachment that was geared to the table feed. You took the handle off, put on a gear, and then set a series of change gears to get the correct pitch, and milled the thread. I think it was an actual B'Port branded attachment, but I could be wrong. It wouldn't be too hard to make something up with a rotary fixture and a few gears, but probably way too much work for a one-off.