Hello TrevorH and the others,

Very happy with your video, this is exactly my problem.
My cutting is as close as possible to the chuck, so this not the problem.
I did not take any compensating action for backless, bud make the clearance to be certain thad
the "working moves"are allways in the same direction.
My stepper torck is about double the value.

Rieks

TrevorH,
Just a whole bunch of basic comments which make a difference in thread cutting and final dimensions:
1.As stated by Chip, try to get the cutter on the centerline of the stock.
2.Use a cutting fluid when cutting threads and idealy it should be constantly applied.
3.When setting the axis for the cut remove the backlash.
4.The more rigid the setup the better.If poor, fix it. Use the tail for additional support.
5.Take a finish cut on the stock, so you know where to set the cutter, it provides for
   a more even cut load especialy if the first cut is deep.
6.When you set the cutter account for the tip radius. ( look at how a sharp v depth is defined )
7. Flank cutting can provide for more equal cutting / loading ( attempts to maintain equal area of
   chip / shearing of metal ). I usaully use radial ( 29 deg ) for small threading.
8.Use sharp properly ground cutters. Carbide thread inserts are great but use ones with a small
   nose radius on the smaller lathes. I have some that just don't have sharp edges and thus require more
   power to do cutting.They sit in the box. Don't have any chasing cutters but they certainly add to the quality
   of the thread form.
So be maticulous as any of the above can mean the difference between a good or bad thread.

For a 1/2-13 thread you have a "mean" tolerance range for OD of .012" and for pitch diameter .005"
if you want to fall "somewhere" within a 1,2,3A Class fit. Lets forget about uniformity of pitch over the
thread for the point i want to make.So no great shakes on the diameter if the pitch is ok. Lets say you
do a finish cut on the stock and then adjust the cutter placement to get the pitch right ( or adjust accordingly in the
gcode). Lets say you lost a thou based on the above 8 comments, that leaves you .004" range for the pitch.
You have backlash in the X and Z and .005" one or both. The chance of throwing a pitch over the plate for a
strike is...... well...........hope i made my point.
 So yes, you should use backlash compensation as a software fix for just one part of the mechanical system ills.
Even though it does work great ( SS dosn't have it yet), you will see an improvement in threading but it's a patch.
Ideally you need to get rid of it or minimize it.

Sorry for being longwinded and didn't even get to your video, but, all of the above are in my thoughts as i watched
it.
RICH

PS: Yes i will be posting additional comments but i want a friend to watch the video first.

Hi Rich,
   You do a great job explaining threading methods in general....much appreciated.
Would you mind to share your thoughts on what I mentioned in reply #60 ?
That is: Can mechanical issues, EMI or any other "Outside" influences actually cause the X DRO to change even though the program/software does not command the move ?

Thanks for your time,
RC

Thanks for all of the great comments about my video.  As for my setup I know that it leaves a lot to be desired, and I plan to work on the issues over time.  I know that I need to be cutting closer to the head stock, but the new metal way cover that were an afterthought simply doesn't allow it, so I need to move the headstock forward, and this is in the works.  My bed which can't be seen under the covers is made of a pair of 3" wide 2 inch thick Linear guides, so I unfortunately don't have a way to mount a tailstock.  As for the cutter I showed some pictures of it earlier in the thread, but it is a brand new indexable carbide insert/holder with a perfectly sharp point, no radius at all, and it is set dead center using a gauge.  My current headstock mounting arrangement isn't perfect, and does allow some chatter espeically at certian speeds.  I also know that my machine needs to be bolted to something more substantial than a wooden workbench that it is merely sitting on  and a 3" thick concrete top is in the form curing right now, this will become the new home for the lathe.  I will also play around with enabling backlash compensation and see if I can improve on my accuracy as much as possible.  I agree that all of these things need addressed, BUT although they may contribute to poor, ill fitting threads I still don't see how any of these purely mechanical items can result in the machine cutting outside of the thread in the air, and at a taper followed by a cut that is effectively too deep in a very repeatable and per dictable fashion over and over every other cut over the course 80 or even 200 cuts as is is doing?  

I will begin to sort out the mechanical "issues" that I have as I can, but I am unable to concentrate for long periods due to a problem with my eyes, so it might take a couple of weeks to get things as sorted as possible.  In the meantime I look forward to continued suggestions and I will continue to post results so that if/when the problem gets better we will all know why!

 I just had another thought, see if you agree with this logic, or not.

The X axis stepper motor is moving the bit into and out of the cut when it should not.  I don't believe this to be a mechanical issue, though I am more than willing to explore it.  So the question becomes why is the x motor moving when it shouldn't?  The simple answer that I come up with is because the computer/software/controller is telling it to, I think that this statement is supported by the fact  that the DRO is reflecting this move showing that the software is not only commanding the moves, but also keeping track of them.  If I was loosing steps, or in some other way loosing position mechanically then I wouldn't be seeing the DRO's move right?  I am pretty green to all of this but don't those little steppers just do what they are told -within reason?  I think that i have mentioned it before, but after completing the operation I can jog the bit till it touches the OD of the part, and it shows the same diameter...  (.500 in this case) as it before the operation this leads me to believe that the motors are not moving on their own since the controller wouldn't be able to keep track of position--Is this correct?

If the above ASS umptions are correct and I hope that you will let me know one way or the other, then the new question would become Why is Mach telling the motors to move like they are?

Thanks, and I think that I need to quit thinking about this so much!!

Hi Trevor,
  Good point.
IMHO,
RC

Talked to Art and he asked if you could single block through it and see if it does the same.
Hood

Hood, I would be happy trotry that it seems like a good idea.  Can I do it just by hitting the single button over and over? or will I need to change the code somehow?

Trevor

I cant remember the last time I used single block but think you have to press Start after each line has completed but not sure.
Hood

Hi All, just wanted to add, this is exactly the same issue I have, as in previous post.

I know this is brash to say but I am fairly sure this is an interference issue or a G32 bug with the parallel port. Its not G76 as this topic seems to have shown and I don't think it can be.

So, a quick poll - does everyone who has this problem have a VFD? I wonder if this can create interference at low speed and high motor slip when the cut slows down.

Next, does everyone with this problem use the parallel port?

I do wonder if there is a very small, very good at hiding bug in G32. I know that is a rash and arrogant statement and I apologise for that, but I just can't see how so many can have the same exact problem?

I've now cut 11 threads perfectly, M16.3 x 0.97 pitch (don't ask!) and with very light cuts (0.03mm - thats light!) they were all absolutely perfect, so I know the lathe and Mach3 is working well. But as soon as I move to deeper cuts and coarse threads like M1.5 or M2 and the spindle slows then the X axis starts to move after about the 3rd or 4th cut, so I have to stop and start again - eventually I can complete the thread after about 6 re-starts.

No matter what the spindle or Z axis does, the X axis should not move during a thread with no taper and it should not suddenly start moving when in the previous initial cuts on the same thread it didn't!

I'm sure we will get to the bottom of it!

Woody.