# Cutting LH internal threads question ??...



## 56type (Feb 8, 2017)

OK, I finally managed to round up some tool bit holders & a set of change gears for threading on my recently acquired Atlas 10100 6" lathe. Was looking to make make some muzzle nuts that have an internal metric thread of 14-1 LH and was reading thru the manual on the lathe as well as some of the info. in the  Atlas Manual of Lathe Operations & Threading Info. for Six Inch Lathe.

 Which brings me to the questions I can't seem to has out for myself...On pg.41 of the Threading Info. for Six Inch Lathe in table III it states metric threads are possible but on pg.27 under the "cutting metric threads" section it states..."the motor must be reversed after each cut. This procedure is necessary because metric threads have no definite relation to the threading dial". It also goes on to state that "after the half-nut lever on the carriage is engaged for the first cut, it should not be moved until the thread has been completed. As the tool reaches the end of each cut, back out the cross feed, stop the lathe and  reverse the motor until the tool has been returned to the starting position". 

 Since the Atlas 10100 I have doesn't have a reverse switch mechanism for the motor and additional reading advises against moving the reverse tumbler on pgs. 6-7..."After the tumbler gear lever has been shifted to the proper position, it should not be moved until the thread is completed. This is especially important because a shift in the lever position destroys the relation between the threading dial and the lathe spindle and causes splitting of the thread"....My question is how do I cut an internal 14-1 MM LH thread on the Atlas 10100 ?? Also, does the info. on NOT moving the reverse tumbler even apply since you don't use the threading dial when cutting metric threads but instead lock the half-nut lever until threading is completed ?? 

If you can actually move the reverse tumbler lever without screwing up the tool relationship to the initial cut, can this be done instead of having to reverse the motor since I don't have a reversing switch ?? This is my first lathe and I'm trying REALLY hard not to screw anything up...Thanks.


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## Rob (Feb 8, 2017)

Here is a good link on metric threading.  http://conradhoffman.com/metricthreading.htm

My understanding is you do not want to disrupt the relationship with the half nuts, lead screw and the work.  To that end you do not want to disengage the forward reverse lever.  Your best bet would be to add a reverse switch to your motor.  I have added one to my 12" but I kept the original on/off switch.  I turn the lathe off and then switch it to reverse to move the tool bit to the starting position and then do the next cut.  I was cutting to a shoulder but you are doing an internal left hand thread so you will not be cutting to a shoulder so it should be easier for you.


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## wa5cab (Feb 8, 2017)

56type (please set up your Signature),

As Rob wrote, you do NOT want to disengage the tumbler to back the cutter up or in your case run it back in.  You will have to be able to reverse the motor.

However, you CAN disengage the half nuts so long as you re-engage them in exactly the same position on the lead screw as they were in when you disengaged them.  The file that Rob referred to (Metric Threading on Lathe by Conrad Hoffman.pdf) is in DOWNLOADS on this site.  First, read the general instructions for using DOWNLOADS in the Sticky area at the top of this Forum.  Then go to DOWNLOADS, click where it says Click Here, scroll down and click on the next Click Here, Atlas/Craftsman/AA, A/C Lathes, and finally A/C Lathe Charts, Tables, Metric & Misc Docs.  The file is currently at the top of the list.

After you have downloaded the PDF file, open it and go to File Pages 6 and 7 and read the instructions.  You are of course threading backwards to how he describes it so you will be stopping the motor, disengaging the half nuts, running the cutter in (not backing it out), reversing and re-starting the motor, and re-engaging the half nuts all with the cutter out of the nut.  And if this is a simple nut with the threads running all of the way through, there is no bottom of hole to worry about running the cutter into.

And don't forget that threading is normally done at a much slower motor speed than you would use for routine turning.

The other metric threading related file in the same folder applies only to QCGB equipped 10" and 12" lathes.


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## GarageGuy (Feb 8, 2017)

I made a hand crank for my old Logan lathe for difficult threading where coasting of the motor could cause a crash.  It was basically a milling vise crank handle with a long bolt, washers, and a couple rubber plugs that expanded inside the head shaft.  It allowed me to keep everything engaged (lead screw, tumbler, QCGB, etc) and reverse without fear of crashing into the chuck.  With left hand threads you'll be turning away from the chuck instead of toward it, but it will give you the added low speed control you need for tricky operations.

GG


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## 56type (Feb 8, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## T Bredehoft (Feb 9, 2017)

Yes, it's reversable. you will need a double pole, double throw switch, a drum switch is ideal, but small toggle switches will handle 15 amps, enough for your motor. That's what I did, but put the reverse switch out of easy reach so I couldn't hit it inadvertently. 
Threaded chuck. The initial jerk of the motor is liable to break the threads loose in reverse, the solution GarageGuy above gave is Ideal. I have a similar hand wheel on mine I use for tapping, could easily use for threading.


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## wa5cab (Feb 9, 2017)

Yes, it is wired for reversing.

As far as there being any great danger of the chuck coming loose just from reversing the motor, so long as you properly seated the chuck with a "clunk" whenever you install it, don't worry about it.  I have a 25#+ Pratt-Bernerd 3-jaw chuck that over the past 37 odd years has spent most of its time on the spindle of my 3996.  Over that same period, I have probably started the motor in reverse 500 times or more, for any of a multitude of reasons.  Not once has it ever come loose.  So don't worry about it.  However, don't ever try to cut with the motor in reverse.


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