A couple more thoughts on threading bolts from the penut gallery.....
If the bolt is bent, that transition will be hard to match for pitch diameter, which can force you to sacrifice full thread strength.
Speaking of strength......I wouldn't consider a rethreaded Grade 8 bolt to be still full strength - you've likely cut through the hard outter skin and as a result sacrificed strength. The intended application will dicatate if this is a real concern or not.
Only advance the compound about .001-.002 per pass.
Hey Doug you are far more patient than I am - I usually will be taking cuts in the .005" to .010" range and don't start with the .001" passes until I get CLOSE. I also calculate (or look up) the total thread depth (that I am cutting) so I know when I am close - it has proven to save my butt on parts that HAD to pass outside inspection more than once. Not that THAT would apply to rethreading bolts [img]/ubb/images/graemlins/shocked.gif"%20alt="[/img]
Taking more than it can handle will snap it off easily.
Assuming I am understanding your point - which is start gently - which realy is good advice, but my experience there has been slightly different. The beginning part of the thread provides little tool load since you are not taking a "full swath" so to speak - as such I find myself taking fairly aggressive DOC's in the beginning (.020" or more all depending) then as I go and hear the machine labor slightly I take progressively lighter cuts - more along the .005" to .010" range stated earlier. Untill the nitty gritty when I get into the .001" range like you stated.
Matching the flank is a challenge and each pass should be verified and compensated for tool lead at the flank the compound it set to follow.
Yep that's always fun!!! My biggest gripe there is that it can be confusing when you are wanting to set you dial handles in a certain position - too much fiddling around sometimes. Rechecking each pass sounds like a lot of work - I usually set it pretty close on the first pass - I stop the lathe mid cut (of course it's a dry run so I am not cutting yet as the tool is .100" or so OUT) so what I get is the slop out of the geartrain - then It's a matter of getting out the loop and dialing it in - little here a little there- aside from tool load/spring it should be VERY CLOSE now. Rest your dials and away you go!!!
I have to wonder since our methods are fairly different what type of cutting tool are you using?? I am using a "momax" cutter if memory serves me. Much more durable than HSS but not as brittle as Carbide.
Not long ago I had a BUNCH of studs to thread for my HOTROD [img]/ubb/images/graemlins/grin.gif"%20alt="[/img] (ok a "bunch" for me was 20 pcs) I needed to put fine threads on one side and coarse on the other. I found some TGP shaft laying around so I commandeered it and started to form a plan. Initially I thought I would just single point them, but the quantity steered me away from that. In the end I wimped out [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img] and used threading dies. Since I lack a die stock that I can use in the lathe I chucked the die into the lathe chuck and the part into the tailstock chuck. Hookey maybe, but it worked great. Like you I used LOTS of sulfur based oil and marked each piece for a thread length (magic marker) - zing zing zing - it was done in a fraction of the time I could have single pointed them.
FWIW my buddy made his studs out of rethreaded bolts - and I remade a few of them for him when they snapped. Granted this is no high stress application, but it was enough to break his - mine are still going strong. Stronger is not always better [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]
Which leads me to another point - in another post in this thread there was mention made of using set screws for studs. Most industrial grade set screws FAR EXCEED grade 8 and as such are substantially harder and of course substantially MORE BRITTLE. We learned this the HARD WAY where I work when we had them start to fail in the field. Can you say "retrofit" [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img] and you say "expensive" [img]/ubb/images/graemlins/shocked.gif"%20alt="[/img] Yikes.
Well here's the problem - set screws are designed to be used in compression NOT tension - when we use them as studs and we tighten/ tension them -they don't like it- [img]/ubb/images/graemlins/frown.gif"%20alt="[/img] as a result set screws are HARDER and more BRITTLE than Grade 8 bolts -this can lead to failure depending on the application. [img]/ubb/images/graemlins/mad.gif"%20alt="[/img]
Oh well I've drooned on ad on long enough. OOPS!!!! [img]/ubb/images/graemlins/crazy.gif"%20alt="[/img]
As always YOUR mileage may vary!!!!!
Time to go work on HOTRODS!!!!!!![img]/ubb/images/graemlins/grin.gif"%20alt="[/img] [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]