I am trying to recall some 20 year old high school memories. If I am cutting a thread on a lathe, and the lathe is reversible, then I can cut the thread one way, stop/reverse the lathe, return, set the cutter deeper, and repeat, right?
I do not need to withdraw the cutter, move it back, find the proper spot to restart, etc. Right or wrong?
Unless your lathe has absolutely no backlash anywhere then you are going to need to withdraw the tool prior to reversing as it will take a different path in the reverse direction and probably ruin your piece. It is how I cut many threads especially metric as although the Harrison M300 has a universal thread cutting gearbox the leadscrew is inch.
If you are talking about using the tumbler reverse in the gear train, the answer is, "no". It disengages the gears and looses position.
If you are talking about reversing the motor, the answer is' "yes it is possible" You still have to back the cutter out of the cut, however. Reversing the motor swings all the backlash in the system the other way and the tool will drag over the threads.
Sort of right. You will need to withdraw the cutter to reverse, but that just means backing it out a set amount with the cross-slide and then setting it back when making the cut. Usually you would only use this method when cutting an odd thread, such as a metric thread on a machine with a standard leadscrew. Normally the threading dial would be used as a reference to restart the thread on each return. It's pretty tedious to reverse the lathe at the end of each pass, and then switch to forward. Much quicker to use the handwheel on the carriage to return for each pass.
I do threading like a state machine. Each starred state is uninterruptible and leaves the machine in a distinct condition, so i know where I am after measuring or answering the phone. The cross slide and compound dials are set to zero when the tip barely scratches the work.
*At the end of the cut disengage the half nuts and back out the slide exactly 0.100, ie to the next 0 at this point.
*Move the carriage back past the end of the work. Turn the slide in one turn.
*Advance the compound a few thousandths. Engage the half nuts when the proper line on the threading dial matches, or move the carriage to meet it.
The last few fine cuts to adjust the fit are done straight in with the slide, to smooth both thread flanks. Turning the dial exactly one turn preserves its setting if I'm interrupted.
I added the threading dial line and the 29 degree infeed depth to the quick-change gearbox chart.
Besides the backlash issues everyone has raised there is another problem.
If you stop the work from turning while the tool bit is cutting, either by turning off the motor or disengaging the clutch, you'll most likely break the tip of the cutter. Typically you disengage the halfnut and leave the work turning, and yes it leaves a ring at the end of your threads.
FWIW I had an old time machinist prove this to me one afternoon. We were discussing threading and he told me this and I didn't believe him so I chucke up a piece of stock, did a quick gring on a threading bit and cut a few threads, disengaged the cluths and.. nothing happend no problem.. the old guy laughed and said try it again so i did, and... snap, there went the tip of the cutter.. I re-ground it and tried again and .. snap.. same thing broke off the end.. I did it about 10 times and 8 out of the 10 snapped the tip off.. It was a great object lession :-)
IIRC his explanation was that the tool holder, tool and work piece DO deflect and when you stop the work with the tool engaged the "spring back" force drives the workpiece backwards against the tool and snaps the tip off...
Normally when thread cutting the first thing I do is cut a run out groove at the end of the thread to the minor diameter of the thread, or major in the case of internal threads. I can't consistently stop at the same place each time to avoid possibly taking a heavy cut if I stop a little to far in towards the end of the htreading, now if I ran at the lowest speed, 40 RPM, maybe but for what I do a small runout groove allows me to thread faster.
Were those high school memories here int eh US, or in Russia? If the latter, then you were probably cutting metric threads, and the machine may have been different enough to add to the confusion.
Wrong -- because there is backlash in the leadscrew/half-nuts connection, so it does not return along the same path that it took when cutting.
And stop/reverse requires a very fast reverse if you are cutting towards a shoulder.
Better to cut a runout groove to the depth of the thread, and wide enough to allow for your reaction time with the half nuts, use that to disengage the half nuts, crank the cross-slide back far enough to be
*sure* to clear the threads, and hand crank back to off the start end of the thread, then wait for the threading dial to reach the right point (what is the right point depends on the thread pitch being cut -- for some pitches, any mark on the dial will do. For others, every numbered mark. For others every *even* numbered mark, and for yet others only the same mark as was used the first pass. (Where you are doing every even mark -- if you start on an odd mark, it can be every odd mark instead.) Check the manual for the lathe -- it will tell you which selection of marks to use for a given thread. Basically, if the pitch can be evenly divided by four you can use any mark on my lathe, but yours may be different.
Oh yes -- also start with the cross slide set to 0 on the dial and the cutting tool touching the workpiece, and with the compound set to zero as well, and with an angle of 29.5 degrees for US/Metric threads. Then for each cut -- you stop in the runout groove (using the half nuts), leave the spindle running forwards, withdraw the cross slide by one full turn (or two or more if necessary for a deep coarse thread), crank back to the starting point, crank the cross-slide back in the same number of turns, stopping on zero, and then advance the compound-slide to set the depth of cut of the next pass, and re-engage the half-nuts when the threading dial reaches the right point. There are stops made for the cross-slide which allow you to simply crank back in until you hit the stop each time which can save you a bit of time and fiddly work, but you still feed the depth of cut using the compound -- at least for US practice.
Interesting. When there isn't room for an end groove I stop the lathe short and pull the belt to finish the pass, then chisel and file out the stack of chips at the end of the thread afterwards. Maybe the tip never has broken because the cut is only 2-3 thousandths, or the old South Bend 10L is -so- springy it doesn't have enough force to break the tip. I keep the belt loose enough that the cutoff bit can jam without breaking.
thats the way to do it. reversing the motor is awfully slow. on mine if I flip the switch into reverse it just keeps running in the same direction. If I stop it, then flip the switch I get reverse. the stopping takes a while.
Now from someone nearly totally clueless about threading:
How does one cut a left-hand thread?
I've somehow got the idea that if you cut from right to left, you get a right-hand thread.
If you cut from left to right with the machine turning the same direction, do you get a left-hand thread?
The only experience I have with a lathe is turning to a diameter. My kid knows more than I. He made nice signaling cannon during a summer work experience program, tapered barrel and all, but has forgotten threading procedures since it was 20 years ago.
You are "essentially" correct about cutting left to right to get a left handed thread, but what you forgot to say is with the spindle turning in the "normal" clockwise direction.
What you need to do is reverse the direction the lead screw turns in relationship to the spindle what ever direction it's turning. Most good lathes have what's called a "tumbler" that set's the direction of the lead screw relative to the spindle.
Now you get to decide which way you want the carriage to move when you cut the thread.
If you want to the carriage move TOWARDS the headstock, you have to run the spindle counterclockwise and put your cutting tool upside down on the back side.. most people don't like this since the cutting force is now LIFTING the carriage not pressing it down... but for shallow cuts this will work..
If you run the spindle it's "normal" direction and the carriage moves away from the head stock, you'll have to cut the equivelent of the "run out" groove but use that as a "starting" groove.
FYI one of the issues with the 9x20 I'm selling (and other cheap imports) is that they have no tumbler so you can only cut right hand threads.
It avoid the stack of chips, you can try doing it the way old machinists used to do it. This calls for threading mounted between centers so you can remove it and replace it to continue. Make a scratch cut only, remove the workpiece and take it to the drill press or mill and drill a hole right where you want the thread to stop and just deep enough to receive the cutter, then you can hand feed it to the hole and the chips break off cleanly. I've never yet done this, but I probably will try it sometime.