Threading Problems (cont)

As if there weren't enough suggestions already..... This comes from The Machinist's Third Bedside Reader by Guy Lautard, and is a suggestion for cutting metric threads on an English lathe (may be the opposite of what you have, but might be worth a try.)

The problem inherent in the transposing gears is that you can pick up the lead screw in several different places with the thread dial on the same number, unless you pick it up at exactly the same spot in carriage travel each time. This appears to be what's happening here. So it's imperative to pick up at the same spot for every cut.

This method uses a micrometer stop for the carriage, but you can lock down the tailstock and stop the carriage against it for the same effect.

1) Set up the lathe to cut the desired thread.

2) Position the tool ahead of the starting point of the thread (back the cross slide out first.) Jog the motor until number 1 comes up on the threading dial and close the half nuts. Might have to jiggle the saddle a bit. Position the micrometer stop (or tail stock) against the right side of the saddle (carriage) and tighten.

3) Open the half nuts and zero the cross slide and compound to the work as per usual practice. Move the carriage to the right against the stop, start the lathe, and close the half nuts when number 1 comes up. Cut the thread length and open the half nuts as per usual, and move the carriage to the right against the stop (same starting point as before.) Infeed the compound, etc., and close the half nuts when the dial is on number 1 again. Repeat until the thread is finished.

This should start the thread at the same point in the lead screw / gear train each time and avoid the doubling / tripling you're experiencing. I've been meaning to try this for metric threads myself but haven't gotten around to it, so afaik it's untested data, but sounds like it should work. Hope it's of some help..
 
It looks to me like a problem with how the thread dial/leade screw is set up on your lathe. I had a Jet belt drive lathe that would do that. You could hit any line on the thread dial when cutting a 16 thread or any thread that was a multiple of 8. If you tried to cut a 18 thread, you had to hit the same number or opposite number on the thread dial or the threading tool would hit on the crest of the thread instead of in the v. My current lathe will cut any even thread on any line of the thread dial. I'm not sure what it does on odd threads as I haven't cut many odd threads with it.

I missed seeing that you had engaged the half nuts on the same number each time when I originally posted this. You might check to make sure that your lathe is set up for standard threading and not metric. On most lathes that will also cut metric, the lead screw has to be left engaged from start to finish on the thread as you are cutting the thread. If someone has put in the gearing to cut metric threads, it may be that you are trying to cut metric instead of standard english threads. You might also check to see that you are cutting the pitch that your lathe shows it is with a thread gage.
 
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Threading Problem

Jerry,
Here is a test for you, place a new piece in the chuck (it doesn’t need to be the same diameter as what you cut the first time) and apply your Dykem. Do a very light cut (16 TPI). Cut about an inch or two. Leave the haft nut engaged, stop your lathe move your cutting tool back from your work with your cross feed, reverse you lathe until the tool is just right of your starting point. Stop your lathe turn the chuck by hand until you return to the start point, now move you cross feed back were it was on your first cut, feed about .003 on your compound and start your lathe. It should cut in the same thread as the first cut. If not you have a big problem.
john
Mims, Fl.
 
This may be the DUH moment

Went at it again today. The not disengaging seemed to work to a point. Wasn't getting the overlapping cuts but after 5 or 6 passes it started looking ugly again. Went back to the manual and tried to see if I had change gears to set it up for metric in case I had a metric lead screw. Didn't have the gears necessary for that so I took the side panel off to verify the gears I had installed. Just kept looking at them and the manual trying to see where I might have made a mistake and something popped into my head that someone had said about the double gear in the center. It was gordone in post #33 in the first thread. He said that I might have to turn that double gear around. The diagram in the manual is pretty simple and it's not real explicit depicting the gear engagement. However I think this may be the answer. I tried to turn the double gear around but was unable to get full engagement with the gear above it. Inspection showed that the adjustment bracket that holds the double gear was hitting the gear above it. The bracket had relief ground into it but was not quite enough with this gear arrangement. I'll remove the bracket tomorrow and take some metal off with an angle grinder to get enough clearance. Hopefully this could be the answer. :D
 
Jerry,
It doesn’t matter what gears you have in your lathe (standard or metric), if you keep the same gears in from start to completion of your thread turning. You do need to have the right gears in when you start to cut you threads (18 TPI or what ever). When you do your test you make a very light cut on your work, then you count the number of threads in one inch. If you have the 18 TPI gears in correctly you will measure 18 threads on your work. Your lathe should repeat the same cut each time you make a deeper cut. Now if you have play in your lead screw it will not be the same. What your pictures show is that you may have engaged your half nut at the wrong time. If you leave the half nut engaged then you will eliminate this problem. Do this test, move your carriage back until your thread dial is exactly on any number, now set your tail stock against your carriage and lock it in place. Now move your carriage toward the chuck an inch or two then return it to the tail stock. If your thread dial lines up with the same number that you had when you started then your problem is not in your thread dial. Now see if you can move the lead screw any at all, if you can this will give you problems.
john
Mims, Fl
 
After all this, I still think the problem is with the compound set at the wrong angle and he is feeding in the compound at 60 1/2 degrees, the reciprocal of 29 1/2. The compound should be set at something less that 30 deg perpendicular to the lathe horizontal axis.
 
I agree. I brought it up earlier on this thread on some of the Chinese lathes you have to set the compound at 59.5 degrees to do threading. A simple test would be try a thread using the cross feed only and not moving the compound. If the thread looks OK the compound is set at the wrong angle.
 
Jerry,
If you shoot a picture of your setup, looking straight down at the lathe, your compound angle will be shown in a way that can be roughly measured from a print of the picture. If you post the picture I can print it and draw a line through the center of the work and another through the CL of the compound, and measure the angle with my trusty plastic protractor. I could also draw a line through the CL of the cross slide and measure the angle between it and the CL of the compound. If you will post the picture, I will do the measuring, scan the result and post it.
 
Threading Dial Pinion Gear

On my Grizzly G4003G lathe, with its 8 TPI (threads per inch) lead screw, the pinion gear on shaft of the threading dial has 16 teeth. Among the supplied accessories I find two other pinions of 14-teeth and 15-teeth respectively (with no instructions as to their use :) I suggest you tip your threading dial housing back to where you can count the teeth on its pinion.

See post #144 here for lengthy enlightenment on the subject.
http://bbs.homeshopmachinist.net/showthread.php?t=39314&page=15

And post #21, bullet 14 here and for other things to check.
http://bbs.homeshopmachinist.net/showthread.php?t=39823&page=3

I'll bet the original manufacturer or the previous owner of your lathe assembled a less commonly used (or simply incorrect) threading dial pinion on your lathe. If so, and the correct one is not among your accessories, order a replacement pinion from HF.

David Merrill
 
Boyd Allen said:
Jerry,
If you shoot a picture of your setup, looking straight down at the lathe, your compound angle will be shown in a way that can be roughly measured from a print of the picture. If you post the picture I can print it and draw a line through the center of the work and another through the CL of the compound, and measure the angle with my trusty plastic protractor. I could also draw a line through the CL of the cross slide and measure the angle between it and the CL of the compound. If you will post the picture, I will do the measuring, scan the result and post it.
Click to expand...

Will do. I'm just about to leave for my shop. I'll try to put something long in the chuck to give a better baseline to measure against.
 
You don't need a photo or lines drawn on same. Simply look down on the compound. Is it at the same angle as the right side of the cutting tool (which should be 30 degrees). Even if it's off a degree or two either way it ain't goint to produce the thread that you've shown.

JMHO

Ray
 
Cheechako said:
You don't need a photo or lines drawn on same. Simply look down on the compound. Is it at the same angle as the right side of the cutting tool (which should be 30 degrees). Even if it's off a degree or two either way it ain't goint to produce the thread that you've shown.

JMHO

Ray
Click to expand...

Even if it's off 30degrees it won't produce the thread shown ;)

al
 
do two things before you go anyfurther. Back the tool away from the metal your cutting, and engage the half nut while the spindle is running. Now stop the spindle with the half nut still engaged. (in the area your normally cutting metal of course). Now take a dial indicator and see how much back lash you have in the half nut and lead screw (a new lathe will have a little bit). Now release the half nut and find yourself a good mirror and flash light. Check the engagement of the gear at the botton with the lead screw. I've seen more than one that became partially disengaged. Get somebody to crank the apron from side to side while your down there watching to see how much slop is between the gear and the lead screw. Loosen the thread dial and see if it will go in any further (most of them pivot in an arc). Do not force the gear into the lead screw, but just push it in till it bottoms out, and then let it relax before tightening the lock screw.

Now if you find a lot of back lash in the half nut, you can plan on some fun. Some are harder to change out than others (why I hate LeBlonds), and most good machines are adjustable in the throw to engage the half nut. Bad thing is most of the time you have to at least pull the top of the apron, and in many cases the whole carriage. The gear on the thread dial is easy to change, and is pretty much self explanitory. One other thing I failed to mention is the drive key on the lead screw getting loose as well as the locking device that holds it in place. You can also check that with the half nut engaged to see if it moved in and out of the housing. The drive key is another story, and a little harder to check. Rarely have I ever seen a bad gear in the quick change box, but alas I have a couple times.

Seeing that you later said that you were always using the same number on the dial (really the best way); I'd start out with a check on the half nut (they are supposed to wear out). Then do the gear engagement with the thread dial. My guess is that this is the problem. If it's in the lead screw drive, you will have to take the drive box apart. If the keyway is bad on the gear that drives it, you can broach a new keyway 180 degrees away from it. If the slot is bad on the lead screw (also common) you can cut another keyway 180 degrees from the old one. Also check to see if the gear is loose on the lead screw. It should slip on with a couple thousandths clearence. I have sleeved many gears and rebroached new keyways to save the drive gear. I've also stubbed the lead screw and recut a new keyway (normally a woodruff key). Most better lathes have a couple set screws setup to controll the engagement of the half nut throw. Sadly there's no easy way to do this adjustment, and if the nut is worn that bad it's time for a new one anyway
gary
 
As most know, I have been a machinist my entire life, and own a pretty good sized Marine Machine Shop Facility. I have cut every type of thread amaginable, and have seen just about every thing that can go wrong.

On older lathes, with plain bearing spindle thrust, if the thrust gets too loose, you will have threads "walk" back and forth if not done on with a center. But it looks like you are literally splitting the thread, which leads me to believe you have the wrong gear set in the train.

If I was there, I bet I could find the problem in about 3 minutes. But, it is difficult to diagnose certain problems over a keyboard.

For what it is worth, we cut hundreds, and hundreds of threads, from the smallest all the way up to as large as you could amagine, say 12 inch diameter 2 pitch, OD and ID. We cut everything straight in, no compound kicked off...........jackie
 
My money'd be on Jackie using tools that are ground properly, and or in the proper y axis orientation.

Which is why I thought I'd post again.

To the Op:

With the Carmex indexable threading tool you have, do you have any documentation that shows the proper Y axis orientation for that tool? If it's anything like the Sandvik OD threading tool I have here in the turret lathe, you cannot go by eye and say "this looks about right". What IS right looks about as wrong as can be. This one looks like it's about a mile below center when it is proper. Certain inserts are for particular thread pitches and part diameters, and there is only so much range in that tolerance. It might pay to look at what your tool is intended to cut. Maybe you'll find the insert is the wrong one for that part size, or pitch. Then again, maybe it's right and your Y axis (tool height) is off by a mile. And, it might still be a lathe problem and you're not doing anything wrong. I can't tell from here.

Odds are, the tool holder you have was never intended to be used in the lathe you have.
 
Compound angle photo

Boyd here is a photo of the compound in relation to the cross slide. I tried to leave as many straight lines as possible for measurment.

P1000223.jpg
 
.

Jerry,
The carriage angle looks correct to me. How about the tool angle with regards to the shaft you are turning?
It has to be at 90° to the shaft.
I am sure you know that, but I am just trying to help.

I have been following this thread, and other than having the incorrect gear set up I cannot think of any other thing it could be.
And it is not like you have never cut threads before because you have.
I am really interested in seeing what caused the problem so I shall stay tuned.
Hey, I am still using that handle you sold me on my Bridgeport!
Blake
 
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Check This

Set your quick change box to cut a 16 tpi. Turn the chuck enough so you can kick the half nut in. Then, mount a long travel indicator on the bed and place the stylus against the carriage. Move the chuck clockwise so all slack is out, zero indicator. Rotate the chuck exactly one turn. (you might have to mount a pointer), The carriage should move exactly .0625 inch.

Change to 18 tpi, and it should read exactly .0555. If it does not, then something is amiss in the gear train. .........jackie
 
This may be a bit ugly. The angle shown figures out to approx. 28.6. (My protractors are hiding so I did a little trig.)
Compound%20Angle.jpg
 
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