threads off center

[Mark Baldwin III]

I'm trying to figure out what causes a thread die to start cutting off center as it moves down the work piece. I was threading a piece of 3/4" last night for an axle, and I needed a bit over 2 inches of thread on either end. After about and inch, the die starts to move off center. On a piece that diameter, it's a lot of hard work...only to find out that it's headed for the scrap bin.
Any suggestions?

[Scott T Smith]

Usually when this has happened to me it is because I did not hold the die perfectly perpindicular to the piece being threaded.

[Bruce Page]

If you have a lathe, a concentric 30° chamfer can help get the die aligned. I have also turned the first ½” of the rod end down to the dies minor diameter and used that to align the die. After threading, cut the extra ½” off. Of course this also requires a lathe and an extra ½” for bar stock.

If you don’t have a lathe you’ll just have to be very careful to keep the die centered and perpendicular to the rod axis. It also helps to back of the die every half turn or so to clear the chips.
Clear as mud?

[Mark Baldwin III]

Thanks for the tips. I have enough material to try another one, as the first one is beyond repair. Or I may just find another way to make the axle.

[Scott Shepherd]

I've only used a die for large threads a few times in my life, and I was always taught on something large like that to loosen up the die (unless it's really cheap, it should have an adjustment screw). You can loosen it up a great deal, then it'll go on easier. Once you make the first pass, you can back the screw off, tightening the die, and take another pass on it to cut it to it's final size.

[george wilson]

It isn't a guarantee of completely concentric threads,but I often start a die on my HLVH,which has a #2 Morse taper tailstock,with a quill just the right size,by placing the back side of my diestock against the face of the empty quill. That gets the die started squarely for non critical threads,like threads for plane cap screws. The only real way to get perfectly concentric threads,though,is to single point thread them in the lathe.

You CAN get a thread real close,then run the die over them to get the threads the right diameter if you don't want to fuss a lot. If you get down to critical CLASS fits of threads,you must cut them only with the lathe,with an exactly proper threading tool, and check them with the 3 wire method,or a thread mic..