Havin Trouble Cutting Threads Correctly

" no matter what angle the compound is set to. "

If I set the compound to 45º and use it (the compound) to feet the tool it will NOT make a good thread. If you lock the feed on the compound and feed with the cross feed screw, then the angle of the compound is of no consequence. The two statements should be made together.
 
With all due respect, if the Top slide is perpendicular to the work piece and in line with the cross slide and the 60 degree cutting tool is set correctly you can advance the top Slide to make the cut. It is not common practice because of the work load on the tool but yes it can be done. The only problem that could arise is that of heat build up at the tool point. Any angle other than 90 degrees will require the cross slide to make the cut. I should have added that to my post. Nice catch John.

"Billy G"
 
"I cut threads from whatever angle the compound is at the time"

If you feed with the cross slide then yes, the thread form will be good regardless of the compound angle (provided the tool is set correctly, perpendicular to the work) because you will be cutting equally with both sides of the tool. The problem is a heavier work load and, depending on the rigidity of the machine and setup, potential for chatter. If the threads cut nicely though, then this method works.

If you feed with the compound slide and the compound is set to anything less than 30 degrees from perpendicular to the ways then your thread form will still be good as you will still be cutting with both sides of the tool. If your compound is set to anything over 30 degrees, however, you will not have a good thread form, as seen in the OP's photo. You end up cutting with one side of the tool only and leave steps on the other side of the thread form.

29.5 degrees is recommended because it cuts primarily with only one edge, reducing load, but still cleans up the back side of the thread to leave a good finish. Like anything else in the trade/hobby though, there are many ways to accomplish a task and if the end result is good, then use whatever method works for you.
 
Last edited:
I have been thread cutting on a lathe for about 50 years. And about 40 years of that has been cutting threads straight in with the cross slide. For me, I have no problems at all doing that way. I’m reading too much work load, chatter and whatever. Sorry, I have never had any problems like that and I would recommend anybody trying to thread on a lathe at least give the method a try. Where are the Mythbusters when you need them…Dave.
 
Thank you Dave. You will find that most answering something like this have never cut threads any way other than the norm. They read of the problems caused when they stray and let it go at that. Like you I have never had a problem. For you and people like you the Myth was broken some time ago. I was in a hurry one day many years ago and just cut the threads from where ever the top slide was set. From then on I did it from where ever the top slide was.

"Billy G"
 
"most answering something like this have never cut threads any way other than the norm."

I did not mean to imply that either way was right or wrong. It's good to know and understand different methods of any operation.

I was taught 29.5 degrees and feed with the compound during my apprenticeship, but I actually do feed with the cross slide more often than not. I have never had any problems feeding with the cross slide on a full sized lathe, but my little 10" Atlas does not seem to like it. I have really had to go back and re-learn the basics with this lightweight machine to get good results.

Definitely try both methods and do whatever works for you and your equipment.
 
One of the reasons for feeding with the compound when set at 29.5 deg. is to sneak up on the final dimensions. Using the compound at this angle allows finer cuts than when using the cross slide.
 
Last edited:
You should add a stop or a 0-1" travel indicator on end the cross-feed ways so it will always repeat to the zero point on the micro-dial. I have done it both ways. If in doubt use a die on the last pass, been there too, on thread diameters under a 3/4". Bigger threads are easier to see how the cuts are looking.
 
Late reply. Looks like the op got the problem figured out. As to using the compound or the cross slide, I almost never use the compound. But as said above, equipment makes a huge difference. When I thread at work on a 'big' lathe, using the compound is rarely needed. At home I have an old 16" South Bend that prefers I use the compound. Also have a 7x14 mini lathe that I rarely thread on, but compound used as well. It does take less tool pressure. The pitch of the thread makes a difference as well. An 8 pitch is a pretty deep thread and best done with the compound on smaller machines.
 
You must log in or register to reply here.
Back
Top