Threading - Something Went Wrong Here...

randyc said:
Good call, my old eyes missed the cross-tracking !



No offense but the point of using the compound set at 29 degrees is NOT to cut both flanks of the thread at the same time.

The tool skims the right flank, producing a good finish, while removing most of the metal on the left flank. At least this is the way that I was taught and to the best of my memory, literature supports the method.

Here's a pretty good discussion from "Practical Machinist"

http://www.practicalmachinist.com/vb/general/setting-compound-angle-when-thread-cutting-278289/

PS: I missed the "D2" material, which changes the process a bit :)
Click to expand...


I saw an interesting video or article on the left half cutting thing, they used a half shaped tool so the right hand side dosn't rub.

Stuart
 
Thanks for all the advice guys. I do appreciate it.

I think I'm going to order some 12L14 for this little project and see how much that helps.

The reason I went with carbide was because I haven't had experience grinding tool bits yet.

I have been studying up on grinding tools this morning. I got a right cutting hss bit ground this morning. Now to do the threading one...
 
Yes, pretty obvious the last pass was on the crown of the threads caused by engaging the half-nut on the wrong mark.

Mostly through a not too clear understanding of the benefit of engaging and disengaging the half-nut, I've always just left it engaged. I make a pass with the tool, stop the motor at the relief cut, then back out the cross slide 10 thousandths and reverse the motor to back the carriage past the end of the piece. Then move the cross slide back to it's original setting and advance the compound 3-5 thousandths and make another pass through the work. Rinse and repeat until the thread is satisfactory.

My understanding of the benefit of using the half-nut is it is supposed to be a little faster operation in production work. If you are like me, fairly inexperienced (I've been using a lathe less than 10 years), and don't do lots of threading, the reversing method is less prone to disaster, not that there aren't other factors to make disasters.

I've had a similar thread result because my ways had gotten loose and my tool was a tad above center. The tool grabbed the work, rotated down causing it to gouge in too deep, and jerked the QCTP out of position.
 
Franko said:
Yes, pretty obvious the last pass was on the crown of the threads caused by engaging the half-nut on the wrong mark.

Mostly through a not too clear understanding of the benefit of engaging and disengaging the half-nut, I've always just left it engaged. I make a pass with the tool, stop the motor at the relief cut, then back out the cross slide 10 thousandths and reverse the motor to back the carriage past the end of the piece. Then move the cross slide back to it's original setting and advance the compound 3-5 thousandths and make another pass through the work. Rinse and repeat until the thread is satisfactory.

My understanding of the benefit of using the half-nut is it is supposed to be a little faster operation in production work. If you are like me, fairly inexperienced (I've been using a lathe less than 10 years), and don't do lots of threading, the reversing method is less prone to disaster, not that there aren't other factors to make disasters.

I've had a similar thread result because my ways had gotten loose and my tool was a tad above center. The tool grabbed the work, rotated down causing it to gouge in too deep, and jerked the QCTP out of position.
Click to expand...

That is a pretty cool idea that I haven't thought of. Thanks for the tip! I might just have to try that out.
 
If you can mount a indicator or stop on the cross slide so it stops on the same point every time it is reset will help too. Like they said use plenty of cutting fluid. I always had trouble threading. Production shops use threading dies for speed. I always kept a hand die handy to finish my threads. Just keep practicing, cut off the bad end and cut more threads. 1/2 -13 is a good thread to practice cutting. When you use HHS be sure there is plenty of clearance ground on the sides and front cutting edges.

Good luck
 
It looks to me that the radius on that tool was way too big. The tool you used was for turning at high rpm not threading. Having that large radius also puts more torque into the cut as it's doing too much work. This may have caused your piece to slip in the chuck which would result in a crossed thread even if you did everything right. Check the piece for jaw skidmarks. Hard to tell for sure but your DOC looks pretty aggressive. Don't take more than .005/cut. Good luck
Ed P
 
I was just playing around with the threading dial on my Grizzly G0602 in preparation of cutting a thread on an important piece. I noticed the marks on the dial did not line exactly for all marks. There are 48 teeth on the spur gear and 12 marks on the dial meaning you can engage the half nuts at three positions in between the dial marks. For some of the marks they were off by almost a half tooth so it would be difficult to judge whether engaging the leading or the trailing position. If you missed by one tooth, you would end up with a thread (no pun intended) like the thread at the beginning of this thread.

I visually lined up each mark and recorded the carriage position from the DRO. While there is variation in duplicate readings, it is clear the dial marks are not evenly spaced. The gear is running true and there is no noticeable runout in the dial wheel which leads me to believe that the dial markings are not concentric with the wheel or that they are just plain sloppy. The attached graph shows the variation between marks for my dial. Some of the observed variation was due to an inability to visually align the dial marks with the witness mark. However the four sets demonstrate a clear trend.

Four separate sets of measurements were made the last set was made after disengaging the dial and moving it by 2 inches on the lead screw before reengaging. This was done to establish that the lead screw was not at fault. As can be seen, the shift did not have a corresponding shift in the curve, showing that the problem lay solely in the dial itself. The variation of 30+ thousandths is almost half the 83 thousandth of the lead screw pitch which could easily lead to engagement one thread off with the half nut.

I have not had a problem in cutting threads because the few that I have cut on this lathe were either metric or I always engaged on the same number on the dial. My next thread was to be a 3/8-24 which should have allowed me to engage on any dial mark and could have been a disaster.
One of the next projects will be to make a new dial for this lathe.
G0602 Threading Dial.JPG
 
Ah ha - an Excel spread sheet guy, my man ! (as Denzel Washington said) :)
 
RJSakowski, Really useful info there. thanks for doing the work and posting.
 
You must log in or register to reply here.
Back
Top