Threading problem

[jgarrett]

I was threading a short piece of steel for a new tool I am making and I had made 3 passes and all was well until I started the 4th pass. It started cutting the "hill" instead of the "valley" of the thread. Once I realized what was happening I stopped the process because I knew the tool was gone.
I have a EMCO V10 and once I engaged the half nut I did not disengage it.
I would back out the cross slide and reverse the drive to get it back to the start. Is this a no-no to reverse it to get back to the start or should I be using the threading wheel???? If so why did the first 3 passes work??? I have threaded before using this method and it worked OK. I'm stumped as to what may have happened.
Julian G. :?:

 

[Powder keg]

Sounds to me like your tool moved? It happens.

 

[Hal]

Julian

As Powder keg stated, on your third pass you probably bumped the left END of the threading grove and knocked your threading tool out of alignment. Doesn't take much.

If you haven't removed the threading tool from the lathe, check and see if the tool is still at 90 degrees to your work.

Hal

 

[jgarrett]

Thanks guys, I "may" have found the problem. Since I was threading very close to the spindle I had to move the x-slide further to the left than I normally use to prevent the saddle from being too close to the chuck. The gibs were not real tight on that end and I did have some play in the x-slide. I have retightned them and will give it another try on a test piece this PM. Since you did not mention the using reverse to get back to the start I guess it is OK to use that method??!!

Julian G.

 

[mklotz]

I must ask, "Why aren't you using the threading dial?" If you're cutting a metric thread with an Imperial lead screw or vice versa, I understand but, if you're cutting an Imperial thread with an Imperial lead screw, you should be using it.

On the (very) few occasions where I've threaded without releasing the half nuts, I've always moved the carriage to the right by (electrically) reversing the lathe motor rather than using the reversing gears in the gear train. Perhaps I'm paranoid but it seems to me that the latter technique creates the possibility of losing the sychronization between the spindle and lead screw, which would create the problem you experienced.

 

[cfellows]

On the (very) few occasions where I've threaded without releasing the half nuts, I've always moved the carriage to the right by (electrically) reversing the lathe motor rather than using the reversing gears in the gear train. Perhaps I'm paranoid but it seems to me that the latter technique creates the possibility of losing the sychronization between the spindle and lead screw, which would create the problem you experienced.

I agree that using the reversing gear could potentially cause the spindle and/or the lead screw to move in relation to each other causing the two to lose syncronization.

Chuck

 

[Hexbasher]

was this a metric thread????


with older machines, an inch lathe can't repeat a metric thread and a metric lathe cant repeat an inch thread (ie, the lead screw is a 4 TPI acme...doesn't work the same with metric gearing eh)


or, were you using the chasing dial correctly??? do you even have a chasing dial on your lathe?

as well, some crappy lathes just cant repeat worth a damn

as well, to be on the same page, its technically called a 'crest' of a thread..not hill


keeping the lock nut is always a safe bet

 

[rake60]

Julian when it comes to machining there are two processes that cause
90% of all problems encountered.

#1 is GROOVING or PARTING.
#2 is THREADING.

There are countless things that can go wrong in either of them.
In threading, the most common would be the tool moving, or insufficient
clearance on the sides of the tool

You don't have to crash the tool to cause to cause it to move in the holder.
A chip temporarily galled to one side of the tool has the potential to move
it in the holder. When threading steel a cutting oil is a necessity. It will help
to keep the chip moving. Another thing to look at would be the clearance
angles of the tool as well as the center height position of the tool to the
work piece.
As for leaving the half nut engaged, in my years I've run some very
tired old machines. Leaving the half nut engaged guaranteed repetition
of tool path on those machines.

Having not been there it is all guess work from any of us,
but I hope some of the suggestions offered give you some ideas to look at.

When it happens directly under my nose, sometimes it takes me several
more attempts to figure out exactly why….

Don’t give up on it. This is an issue you WILL WIN at! :wink:

Good Luck with it.
Rick

 

[jgarrett]

OK, It is standard American 40 tpi. My machine will not do metric.
I do have a thread chasing dial.
For some reason I did not even think of reversing the motor.
I do know it is "crest" and "root"....I do tend to be a bit crude in my descriptions sometime...I'll do better
I threaded 4 3-48 bolts to use on the horz-mill I am building but since they are so short and small I just turn the spindle by hand. No problem.
I did try another .275x40tpi using the dial and did not have a problem.
I know I did not bump anything so maybe Rick is correct in that a hung chip may have knocked the tool out of alignment.
Thanks for the tips and info.
Julian G.