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Internal thread fit problem

Ken the mariner

Plastic
Joined
Aug 15, 2007
Location
California
I'm making six thread jacks to raise my lathe. Each female nut is approx. 3 1/2" OD. Threads are 2"- 10 tpi 60 degree right hand. The first five units fit from either end. The sixth will fit from one end only. Male part enters approx 3/4" on second side. #6 male fits other nuts; #6 female is a partial/ tight fit in other male parts.No attempt was made to make the parts interchangeable. When cutting the internal threads I cut until the male part fit to my satisfaction (screwed in by hand with very very little play). I need the male thread to fit from the second side/end. Any suggestions on the possible cause, and correction?
 
You're likely working with a tighter than necessary fit. A little bit of tool tip damage and you've got an assymetric thread which hangs up. I'd just chase #6 female with a resharpened tool. I'm assuming you had these all set up accurately and bored accurately for the minor diameter.
 
Coat male threads with bluing and reassemble to see where interference is. Maybe you will get lucky and only have to bore female a bit or file a bit from male threads. Time to check male with measurement over wires. Otherwise pick up the female thread and take a bit more off.
 
I'm making six thread jacks to raise my lathe. Each female nut is approx. 3 1/2" OD. Threads are 2"- 10 tpi 60 degree right hand. The first five units fit from either end. The sixth will fit from one end only. Male part enters approx 3/4" on second side. #6 male fits other nuts; #6 female is a partial/ tight fit in other male parts.No attempt was made to make the parts interchangeable. When cutting the internal threads I cut until the male part fit to my satisfaction (screwed in by hand with very very little play). I need the male thread to fit from the second side/end. Any suggestions on the possible cause, and correction?

putting back on lathe and picking up existing threads and cut deeper. hard to get exact and threads might be loose. even advancing .001 at a time. most likely some eccentricity error. bigger threads having a extra .002" clearance normally not a problem. loose thread its often better than not getting parts together without forcing them
.
sometimes a thread file works if its just a burr or rough spot. they come in different pitches. not often needed but good to have a set of thread files just in case in the shop
 
Thanks for all your suggestions. I have examined the thread tool with 5X loupe, and no sign of damage or wear.Coated threads with Dykem, nothing visible at point of resistance (will try bluing). Accurately picking up the internal threads isn't possible, with my knowledge. Bore is approx 2", and with the boring bar inserted I can only see one edge of the tool, the following edge.Any suggestions on how to pick up the internal threads for another pass would be welcome. I have successfully chased male threads after removing the part from the lathe.I have lightly filed the male threads, and dressed the female threads with emery, got a very slight improvement.I have tried #6 male in other female parts and fit is OK. Tried #6 female with male parts and they get stuck.
Ken
 
Any suggestions on how to pick up the internal threads for another pass would be welcome.

Chuck the female part in the lathe.
Screw in male portion.
Use male portion to pick up existing female threads.
If there is any axial backlash, measure how much.
Find the inner and outer limit of the backlash.
Position cutter in the center of those measurements.
 
To pick up the internal thread, mount up and blue the part, set everything to what ever angle you used before and the same thread settings. Mess with the compound until you get the cutter into the thread use the compound and bed feed to move the tool into position in a thread. Turn lathe to threading settings locking lead screw. You may need to turn the spindle by hand. With the tool set into the thread with a tiny scrape turn the spindle by hand and watch it scrape. If it scrapes both sides of the thread you are ready to go under power. If it scrapes on one side only adjust with compound as needed. When it looks right, reset your dials to zero and go.

That may or may not suit how your lathe operates but outlines the basic concept. For a fairly big thread where you are only trying to get a fit to work its not too hard.

Depending on what tool holder you have you can also move the tool in the holder to get the cut right on again. Sometimes this might help if it suits your tooling.
 
It is not uncommon to get a slight;y tapered internal thread when cutting the tread with a long internal threading tool and not allowing enough finishing passes.
As far as correcting thigh threads - best is to pickup and re-cut, but sometimes forcing the parts a few times back and forth with an abrasive slurry (say, 80 grit aluminum oxide and oil) can fix it.
 
Pretty much as above, but close the halfnut and use the carriage handwheel to load the halfnut in the direction it will be loaded during the cut. Helps to have a flashlight to get it set. You need all the light you can get in the hole like that. I used to do a LOT of this when making and repairing hydraulic parts like pistons and tubes with damaged internal threads.

I would set up the lathe for internal threading, then move the cutter to a position just one or two threads past the start of the thread, lock and load the halfnut (by using the long travel handwheel to roll the carriage back towards the tailstock and take all slack out of the leadscrew), then juggle compound and crossfeed until I got it all centered up, then zero compound and crossfeed dials. Back out a ways, fire up and close the locknut to chase, but with the crossfeed a turn in to prevent engagement of the cutter. Let it run in a few threads, then stop the spindle and crank the crossfeed out until it zeros and touches. If it all works out right, you are set to chase ...if not, start over with locking and loading the half nut and juggling compound and crossfeed until it works right.

Reason for repetition is that you can get slack in the half nut and leadscrew, but also in the gear train between the spindle and QC box, as well as in the QC box itself that can change the position of the cutter once you start trying to actually run the lathe to cut the thread. Your fit problem could have been caused by no keeping the machine loaded when threading, too. Again, when cutting a thread with any lathe, I drag my hand on the carriage handwheel to keep everything tight. If it wanders on the first cut or two, the tool will tend to follow the imperfect thread if left to float. Same for light last passes, in that it may cut more on one side or the other. Keeping that steady load on the handwheel prevents the cutter wandering.
 
Also check that the crests of the threads are not the cause of the interference. I don't generally wing custom threads, I thread to standards so the correct OD of the male thread is known. I always file off any burr, or anything that allows the male to measure full nominal diameter,which means it is a little over on the diameter and it will hang up on next to nothing.
 
Based on the intended use as lift legs for a lathe. I would take my 600 grit Clover lapping compound, smear it in first with a rod that passes through, then with the part that doesn't go. Turn it tight then back off, Do Not jam it hard, 5-10 minutes and done. If you don't have any go to the auto parts store for valve grinding compound. its coarser than 600 but will work.
 
All very good info. I was aware of the hand loading of the long feed wheel, but didn't use it. The thread file tip worked. Five passed under power with the thread file, on the male part and a near perfect fit!!! I think I'll touch up the other parts too. Again, thanks too all for your experienced help.
Ken
 








 
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