threading problems

Fixnair said:
Thanks for all you r input. I have finally made something that looks like a thread! 3/4 - 10, it is a little undersize (.015") and the nut fits snugly so I'm sure it is not the correct form. My single point threading tool I made myself and all I have to make such is a bench grinder. The root and the crest of the threads come to a point so my tool needs some work. I think I'll purchase a carbide threading tool. I'll post a picture below.

I think I'll work on the grind for my tool and try the 3/8"-20 thread again, the one I tried first and wouldn't work. Stay tuned & I'll let you how that comes out.View attachment 227035
Click to expand...

Hold on there, Fixnair, You're not getting off so easily. We all want to know what the actual problem was, and how you fixed it. Thanks, JR49
 
Yea, what he said; what did you do differently to get the thread to cut correctly?

Hit the tops of the thread with a file to flatten out the tops of the "V", if your tool fits the fishtail then it's fine.
 
OK, I not trying to bail on you guys. I know we all want a resolution to this thread. I tried several thread forms and had success with most of the selections of the QC. When I used the far left row on the QC (4-8-16-32-64 and 128) the handle would jump up & down and make a knocking sound. It seems that the QC is at fault. I'm going to have to work with this some more before I actually condemn the QC. Now that I have a decent threading tool I'm going to try the 3/8-20 form again.
 
This is just a guess but it sounds like that knocking sound might be the QC gear slipping a cog or two. I would take a close look at
that area. I could be a bearing or bushing or worn gear. The handle jumping indicates to me a slipping cog...
 
Thanks Cathead! I'm getting kind of discouraged with this. I've been working on this all week now and have not made a lot of progress. I have been able to make a few fair threads, but not what I would call good threads. My 3/4-10 looks fair and a nut threads on albiet kind of loosly & the OD is .013 undersized when compared to a stock bolt.

I tried to make a 3/8 16 which again was only fair. I tried 3/8-20 which was unrecognizable as a thread. All my previous practice was done on aluminum which I was running low on stock so I switched to using steel. I tried to make another 3/4-10 and that was a mess.

When I look at the steel, under a magnifying glass, part it looked more like a buttress thread that anything else. I also tried something else that was suggested. Someone advised me to shut the lathe off during a cut and allow it to coast to a stop W/O moving any if the feed dials. Then reverse the lathe & see what happens. It should track back I'm assuming. Well my lathe is not wired to reverse so I stopped the lathe during a cut, disengaged the back gear and manually rotated the lathe backwards and observed the tool point/work piece interface. Under the glass again I could see that the piece rotated backwards an amount W/O the tool moving 'till the tool point contacted the thread in the left side of the point then dug into the work piece. This tells me there is excessive clearance in the whole drive train. Where I don't know. I've had the whole gear-train apart and I know they're worn as well as the shafting. I guess its time I learned how to make gears. Making shafts and bushings are no problem.
 
Fixnair said:
Thanks Cathead! I'm getting kind of discouraged with this. I've been working on this all week now and have not made a lot of progress. I have been able to make a few fair threads, but not what I would call good threads. My 3/4-10 looks fair and a nut threads on albiet kind of loosly & the OD is .013 undersized when compared to a stock bolt.

I tried to make a 3/8 16 which again was only fair. I tried 3/8-20 which was unrecognizable as a thread. All my previous practice was done on aluminum which I was running low on stock so I switched to using steel. I tried to make another 3/4-10 and that was a mess.

When I look at the steel, under a magnifying glass, part it looked more like a buttress thread that anything else. I also tried something else that was suggested. Someone advised me to shut the lathe off during a cut and allow it to coast to a stop W/O moving any if the feed dials. Then reverse the lathe & see what happens. It should track back I'm assuming. Well my lathe is not wired to reverse so I stopped the lathe during a cut, disengaged the back gear and manually rotated the lathe backwards and observed the tool point/work piece interface. Under the glass again I could see that the piece rotated backwards an amount W/O the tool moving 'till the tool point contacted the thread in the left side of the point then dug into the work piece. This tells me there is excessive clearance in the whole drive train. Where I don't know. I've had the whole gear-train apart and I know they're worn as well as the shafting. I guess its time I learned how to make gears. Making shafts and bushings are no problem.
Click to expand...

Hmmm. A couple things come to mind. Assuming you are cutting from right to left, the 60 degree cutter is cutting only on the left
side of the thread since you are advancing the cutter with the compound at 30 degrees. The cutter needs a little relief angle on the left to cut properly.
Also, your cutter needs the tip ground off about 5 thousandths at a slight relief angle for what you are doing. You need to advance the
compound no more than about 5 thousandths on each pass. It will take at least a half dozen passes to get to near size. If your stock is
under size, the threads either will not form completely or end up under size. If you only take off a few thousandths on each pass, it is hard
to understand why the threads could end up that much under size. It's handy to have a nut that fits the intended threads and do a test
fit or two as you get close to finished. The last pass usually will be just .001 or so for a clearance fit. Also, use some lubrication
suitable for the metal you are cutting and relube on each pass.

Also, when backing up the lathe, the cutter probably won't follow the threads due to lost motion. If you do that, I would recommend
backing out the cutter and then back up. It's pretty normal to have some slop. Once you have overcome the lost motion, the
cutter will back up at the thread rate but may not look right in line with the thread grooves. When advancing forward again, the
cutter will line up with the grooves. Even with a lot of wear on the drive train, the lost motion will catch up and cut decent threads as
long as you don't have something slipping. I hope this makes sense.
 
Thanks Cathead. I think next week I'll purchase a carbide threading tool so I can take the tool out of the equation.
 
You must log in or register to reply here.
Back
Top