Threading Problems (cont)

adamsgt

adamsgt

Jerry Adams
The original thread was getting long so I thought I'd start afresh at a further point in the exploration of the problem. Went to my shop today to check some things on the lathe. Pulling on the tool post revealed more than a little movement. Adjusted the gib and got it out. The compound was just fine. So I proceeded to try to cut some threads. This time I put some dykem on the metal to view each cut. Now before I go into that I'm going to post a picture of the bolt jeweling fixture adjustment screw that I made in gunsmithing class. I have cut decent threads in the past.

P1000209.jpg

The next picture is the first pass for 16 tpi

P1000211.jpg

The next two pictures are after two more passes

P1000212.jpg

P1000214.jpg

This last picture cleaned up show three distinct tracks for the three passes. I engaged the half nuts on the same number. I'm stumped as to what is causing this. One way to eliminate the problem is to blow 5 grand on a new lathe. Or I can take my barrels, reamers and gages to someone with a working lathe. Stick comes to mind except I lost the url to his website. Lew chime in if you read this. That would be cheaper than buying a new lathe. I hope somebody out there has some idea of what I might try next.
pencil.png
 
Gear selection has got to be wrong. It is hard to see your gear choice in the manual. It looks as though it is a metric lathe.
 
Could someone please explain the "gear selection wrong" thing to me? I can see "wrong" being not 16tpi but dude's cutting three tracks!! He's not resetting somehow.

Seems to me that whatever gear he's in it should be locked in...... dunno what it might CUT, but it should cut a thread.

Sounding more and more like Jerry has some serious lash problems because he seems to understand conceptually very well. So, while I'm still hoping it's a "DUHH!!" of some sort, I'm losing hope here.

al
 
I think the end gears are not in the right combination which MIGHT be a timing issue with the lead screw and thread dial.
 
According to his thread chart it shows Inch thread gear setup, that has to be correct for the desired pitch.

Are you sure your controls on the headstock are set for threading?

Are you engaging the correct feed lever?
 
alinwa said:
Could someone please explain the "gear selection wrong" thing to me? I can see "wrong" being not 16tpi but dude's cutting three tracks!! He's not resetting somehow.

Seems to me that whatever gear he's in it should be locked in...... dunno what it might CUT, but it should cut a thread.

Sounding more and more like Jerry has some serious lash problems because he seems to understand conceptually very well. So, while I'm still hoping it's a "DUHH!!" of some sort, I'm losing hope here.

al
Click to expand...
adamsgt, Stop disengaging the half nut. Stop and reverse the lathe in order to take the carriage back to the beginning of the tenon.

Al,
He is disengaging the half nut. There are probably 4 possible numbers on which to re-engage that but with the lathe most likely in a metric mode, all 4 of them are wrong. There may be a need to actually open it up and change gears in order to run SAE threads. Some require that, some do not. Under any circumstance, not disengaging the half nut solves that trouble. Simply back the lathe up and go again. I do this by always setting the cross slide back to 0 and do all feeds with the 29.5 deg compound.

adamsgt, it also looks (though it is hard to see in the pics) that you have an improper relief angle on the tool. It sorta looks like you're burnishing the part some. That might also explain some of the rough surface on the threads. It's hard to say from here.
 
Jerry,
Is the work secure in the chuck? I've made some ugly threads only to find the part was slipping slightly in the chuck.
Pete
 
4Mesh said:
Al,
He is disengaging the half nut. There are probably 4 possible numbers on which to re-engage that but with the lathe most likely in a metric mode, all 4 of them are wrong. There may be a need to actually open it up and change gears in order to run SAE threads. Some require that, some do not. Under any circumstance, not disengaging the half nut solves that trouble. Simply back the lathe up and go again. I do this by always setting the cross slide back to 0 and do all feeds with the 29.5 deg compound.

QUOTE]

Ok, I'm diggin' it now....

AND..... the lathe does seem to be cutting nice parallel tracks. Now I'm feeling that the fuzz and the tracking are separate issues as some a' y'all's have been pointing out.

There's hope yet!

al
Click to expand...
 
Adam...find a machinist/friend in your city and get him to take a look at your lathe...this is like calling your doctor and having him instruct you on self surgery...


Eddie in Texas
 
Just to clarify

I have installed the three gears that are shown in the chart for inch threading. As far as I can tell, the lead screw is not metric. I set the crosslide at zero with no backlash and the compound at 29 1/2 degrees. The compound has no backlash or slack. I run the spindle at 70 rpm. I set the thread controls at T1F which the chart says is for 16 tpi. I engage the half nut on the same number for each pass. I have tried odd/even numbers with no difference. I used the carmex threading inserts for this last attempt. Looking over the scrap pieces I've generated the last few days the problem seems to be the same. The cut does not start in the same place each time I engage the half nut. I only did three passes this time so the problem was readily visible. I tried the method on not disengaging the half nut yesterday and that didn't work too well but I hadn't taken the slack out of the cross slide yet. I'll give that a try tomorrow.

Could it just be that the half nut is worn and needs replacing? I've run out of barrel stubs to cut so I'll have to get some crs to play with.
 
It's entirely possible. Half nuts are made of a soft material, usually bronze, and if not maintained properly will wear out rather quickly.
Common practice before threading is to clean the feed screw thoroughly and lubricate if, if ignored the half nuts wont last.
 
hi all

I have installed the three gears that are shown in the chart for inch threading. As far as I can tell, the lead screw is not metric.
adam
how did you come to this conclusion did you measure it.??? simple test change the gears to a i mil (metric) pitch then cut a thread by disengaging the half nuts if this works then you have a metric lead screw.
and as stated by others you can not disengage while cutting a imperial thread when you have a metric lead screw. it will not track as you put it i would bet my left nut it is a metric lead screw.

regards tasy_ted
 
Just occurred to me, some leadscrew/gearbox setups use a shear pin to couple the devices. Check to see if you have a "bad connection" between the leadscrew and gearbox.
 
Check the thread dial. I had one lathe that relied on friction between the dial and the gear shaft, it was slipping and giving similar results. This can be proven out by trying the non disingaged half nut technique. It dose not matter whether the lead screw is inch or metric, or the gears are set right, as long as there is no slop you will cut a thread. The only thing you change with those is the pitch.
 
I always engage the half nuts before starting the machine. This way I am CERTAIN that I am on the right number. Also, I always use the same number. Even if the book says a certain thread can use any number I always use the same number for each pass.

Another thing I was taught by an experienced machinist is to always disengage the thread dial when you are not using it.
 
Jerry H said:
I think the end gears are not in the right combination which MIGHT be a timing issue with the lead screw and thread dial.
Click to expand...

I was about to post something like this. Look at the screw to the thread dial to find out whether it's skipping or severely worn, or the thread dial being loose and getting jogged around.
 
tasy_ted said:
hi all

I have installed the three gears that are shown in the chart for inch threading. As far as I can tell, the lead screw is not metric.
adam
how did you come to this conclusion did you measure it.???
Click to expand...
Actually, yes. I have a 12" metal rule with inch on one side and metric on the other. I set the rule against the lead screw with zero on the back of one thread and then looked down the length of the rule and every inch mark was on the back of a thread. I turned the rule over to the metric side and did not get any such alignment. Now I guess that doesn't PROVE that the lead screw is imperial but it is a strong hint.

I've stated that the lathe manual is skimpy, a few pages of text and the rest exploded parts diagrams. I've also assumed that it is correct. That may be a dangerous assumption. I noticed that the layout of the controls on the headstock and gearbox match the manual, the layout of the controls on the apron do not. Same controls just different layout. One odd thing is that the threading dial is on the left side of the saddle.

I'll keep plugging away at this for a while and then I'll have to decide if I dump this lathe and buy a new one or just keep it for turning tasks not involving threading and have someone else do my barrels. :confused:

And whichever way this goes, I think you ought to keep your left nut. ;)
 
adamsgt

May be a meaningless question so late in the game, but when you do a straight turning job, does the surface finish show uneven results? A lathe with lots a backlash and/or a worn lead screw and/or half nuts will do this, especially when using a fast feed.

Ray
 
"adamsgt, Stop disengaging the half nut. Stop and reverse the lathe in order to take the carriage back to the beginning of the tenon."

I agree with 4Mesh on this, and I think it'd work. May not fix your problem with the lathe but it should allow you to cut the threads and do a good job of it. 0 your cross slide and tool bit against the work where you are going to start the cut and 0 your compound. Make a scratch cut and then back up your cross slide one turn and reverse the lathe back to your starting point. Put your cross slide back to 0 each time and feed in your compound and make another cut, keep doing the same until threads are to the depth you want.
If you have the time, it'd be interesting to see, but I think it will allow you to thread it correctly without using disengaging the lead screw.
 
Ray, earlier this year I made some shotgun choke removal tools for some friends out of brass round stock and got a real nice finish on them.

Kent, I tried that once yesterday and got so-so results. But that was before I adjusted the cross slide gib to take out the slack. I'll try it again today. I'll use the same method of coating the stock with dykem as I did on this last try. It'll only take 2 or 3 passes to show any problem.

How come we don't have a smilie for crossed fingers?
 
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