Lathe question

[JPSAXNC]

Some time after I got my lathe I thought I would take out the jaws that clamp on the threading bar and lube them. Since then my pitch seems to be off slightly, I think I might have mixed up the upper and lower jaws when I put them back in. Do you think that could make a difference ? TIA

[sb327]

Are you talking about the half-nuts for the leadscrew?

If so, what do you mean by the pitch being off? Typically, the half nuts either hold or they don’t. Backlash in this area is common. I tend to hold the carriage against the direction of travel slightly so I don’t get inconsistent pitch with certain threading operations.

Dave

[WobblyHand]

Due to backlash, one needs to thread in one direction only.  Typically going towards the head stock.  That way, the backlash between the half nuts and the lead screw is taken up, and is uniform.  Once you get to the thread relief (gutter), retract the tool and position the carriage appropriately.  (If imperial thread one can open the half nuts.  If metric, keep half nuts engaged, or you will lose sync.)  Then start your next pass.  Repeat until done.

If threading to spec, use thread wires and a micrometer to measure.  Or a thread micrometer.  Else, check fit with the mating nut.  In all cases, leave the piece in the lathe while doing the measurements or fit check.  Otherwise, you will need to re sync the threads again.  Personally, I check that they are in sync if anything changed, no matter how minor I think it is.  Recently got burnt on that making an acme thread, so that lesson is still vivid in my mind!

Sometimes when fiddling with the half nuts, the engagement on your thread dial changes.  That happened to me.  Took me a while to understand what happened.  In my case the engagement point moved to the other side of the mark on the dial.  So sometimes I engaged before the mark, or after the mark.  This made bad threads.  I fixed mine by adding some shims between the thread dial and it's mounting point.  This slightly moved the engagement point relative to the dial.  I suppose a better way is to slightly adjust the position of the half nuts.  It's likely that you didn't put them back in exactly the same position as before. 

[KWK]

Don't remove the half nuts to lube them. Just clean the lead screw and put some oil on it. Some use a brass brush or a heavy cord to clean the screw while it turns. Don't get caught up in it.

[eeler1]

What they all said.  And yes, it can make a difference.  Switch them again and see if it makes things better.  Most  half nuts are cast iron and wear in even as you get wear on the lead screw.  They wear together.  Even steel half nuts wear.  You may have upset the fit.  If your lathe is new, still should switch them and see.

[JPSAXNC]

Thanks for the reply's, Yes it was the half nuts for the lead screw.