Threading issues

[TakeDeadAim]

Ok its 0600 and I have not been able to sleep due to back and leg pain. So Ill throw this into the mix just to stir the pot. It is NOT necessary to cut threads with the compound. In fact I don't even have a compound on my lathe. It was the weakest link in terms of chatter on mine and most lathes. So I took it off and could not be happier. Just square up your tool using the good old fish tail tool or in the case of most insert tools tram the shank with a dial indicator. When you cut the thread just advance the tool straight into the work slowly, use a good cutting fluid for the type of material your cutting and viola decent threads.

 

[Leadfootsmurf92]

I would like to thank everyone for their help! It has been greatly appreciated. I have made ground on my thread cutting abilities, and i must say that I'm pretty excited. I currently changed my compound angle (for those recommending to not worry about angle and just dive straight in, I'm also going to try that), and I believe I found the key to getting my cheap lathe to repeat the same thread path. I found that if I disengage the half nut after a threading pass, turn off the spindle and hand crank it back to the starting point, re engage the half nut in the appropriate position, then restart the lathe, it will pick up the thread again. I still need to work on cleaning up the thread, and getting to the right depth (I am going to purchase a pitch Mic I think). I have attached a pic of the .375-24 thread i attempted. Not great, but it's better than before!

Sent from my SM-G935P using Tapatalk

 

[TTD]

Hey, that doesn't look half bad at all! If anything, I would just run a 3/8-24 die over it now to clean up the threads a bit & call it done.

I found that if I disengage the half nut after a threading pass, turn off the spindle and hand crank it back to the starting point, re engage the half nut in the appropriate position, then restart the lathe, it will pick up the thread again.

Now, I am no single-point threading expert by a loooong shot, so I'm more "thinking out loud" here than giving advice, but I wonder if you could just leave the half nut engaged the entire time? When you're done a pass (threading tool in relief groove), shut the lathe off, flip the lead screw lever to "reverse" then turn lathe back on & let tool basically thread away from chuck until it clears the work....kinda like a spring pass but in reverse. Crank cross slide in to your desired DOC & repeat (I don't use compound when threading).

 

[Dave Paine]

Now, I am no single-point threading expert by a loooong shot, so I'm more "thinking out loud" here than giving advice, but I wonder if you could just leave the half nut engaged the entire time?

I often leave the half nut engaged but I back off the compound one full turn, or when making a deep thread like 8 tpi I will back off two full turns when I get close to the bottom of the thread. I then put the lathe in reverse to get back to the starting point.

I do not want the cutting tool threading on the reverse step since backlash in the lead screw will widen the thread.

I have used the cross slide to plunge cut a thread for fine threads, but when I am making 8 tpi threads for wood lathe tools, I use the compound. The chips get big for such a thread. I do not want to risk breaking anything by plunge cutting such a thread.

 

[dtsh]

When you're done a pass (threading tool in relief groove), shut the lathe off, flip the lead screw lever to "reverse" then turn lathe back on & let tool basically thread away from chuck until it clears the work....kinda like a spring pass but in reverse. Crank cross slide in to your desired DOC & repeat (I don't use compound when threading).

Assuming you had tight tolerances in your leadscrew, it might work, but I think for many of us, we'd just end up with mangled threads.

 

[TTD]

I often leave the half nut engaged but I back off the compound one full turn, or when making a deep thread like 8 tpi I will back off two full turns when I get close to the bottom of the thread. I then put the lathe in reverse to get back to the starting point.

I do not want the cutting tool threading on the reverse step since backlash in the lead screw will widen the thread.

Very good points, Dave. Thanks for that little tidbit!

 

[TTD]

Assuming you had tight tolerances in your leadscrew, it might work

LOL....I have a (Craftex) 7x12 Chinese mini lathe...."tight tolerances" are nowhere to be found!

 

[royesses]

Maybe I'm overthinking this but you have a 30°, a zero° and a 30° marked on your compound already. Just scribe a line at both 30.5° marks when set to zero. Then you use either line you have scribed as zero and set 30° on the new zero line resulting in 29.5° set for threading. Or am I wrong?

 

[mksj]

I was playing around with several ideas on trying to cut metric threads, disengaging the half-nut and then seeing if I could pick up the thread again on the next pass. Typicality you cannot use the threading dial with metric threads on an imperial lathe w/o some creative methods to get you back to the same starting alignment position. Bottom line, tried several methods and didn't work very well. In my opinion you are best leaving the half-nut engaged through the process when cutting metric treads. If you have a lathe that has electronic braking, it makes it very easy to stop in the relief grove, back the cutter out and reverse, then repeat your next pass without disengaging the half-nut. I tried it repeatedly at 100 RPM (not using any electronic stop), and I had no difficulty cutting a metric thread using this method. If you do not have some form of electronic braking then it can make this method more challenging because of the chuck spin down time. Whit electronic braking, I use 1 second braking when threading.

Since I use both electronic braking and an electronic stop, I cut all my threads (imperial and metric) with the half-nut engaged through the threading process. I have also reverted to using the cross slide to advance the cutter as this maintains the same stopping position of the cutter tip on every pass. I only cut a relief groove with my threading insert to slightly deeper than my expected maximum thread depth, the repeatably of the electronic stop system is 0.0002" so you can cut a thread to the relief groove/shoulder/blind cap internally easily with no drama.

I recently switched my threading holders to the lay down insert variety, both (external/internal) are made by Carmex (Iscar). I am using both Carmex 16IR AG60 BMA and Mitsubishi MMT16ER AG60 VP15TF threading inserts. The inserts have been much more durable than the TNMC/TPMC previously used and so far cut much cleaner complete threads. Example below is a M6 1.0 thread cut in O1 drill rod at 130 RPM to a shoulder and using the cross slide method. I would have cut a wider relief grove if not using an electronic stop. As far a using the compound vs. cross slide, there are a number of different factors that come into play. Size/rigidity of your lathe and the robustness of the insert/cutting system are important factors. Smaller lathes would do better using the compound. I do take shallower threading cuts using the cross slide, and for very coarse deep threads, using the compound can be easier on clearing the chip curl. When using the compound to cut threads, I often do the final spring pass by slightly advancing the cross slide to clean both side of the thread groove evenly.





Quick method of setting your tool post parallel to the chuck using a 123 block. Just loosen the tool post nut bring it up against the chuck and holder using a 123 block as shown and tighten the nut. With inserts, once the holder is parallel to the chuck, the insert alignment will be true. There is no need to use a fish tail.

 

[Tozguy]

Hey, that doesn't look half bad at all! If anything, I would just run a 3/8-24 die over it now to clean up the threads a bit & call it done.

Is that not called cheating


Now, I am no single-point threading expert by a loooong shot, so I'm more "thinking out loud" here than giving advice, but I wonder if you could just leave the half nut engaged the entire time? When you're done a pass (threading tool in relief groove), shut the lathe off, flip the lead screw lever to "reverse" then turn lathe back on & let tool basically thread away from chuck until it clears the work....kinda like a spring pass but in reverse. Crank cross slide in to your desired DOC & repeat (I don't use compound when threading).

If you change the lead screw direction like that it will cut left hand threads on top of the right hand threads. Ask me how I know