Messages

11

General Mach Discussion / Re: Problems threading on the lathe

« on: March 01, 2009, 02:40:03 PM »

Art, I am glad that you are working on this with us.  I have mentioned before, but I think it may be improtant to answer the question that you posed earlier.  At least in my case no matter ow "wrong" things seem to go the controller never seems to loose track of the actual position of the cutting bit.  That is to say that I can jog the tip of the bit back to the outer corner of the workpiece after a cycle is completed and it will still indicate that it is at 0,0 just like before the cycle began, as a novice this would seem like a pretty simple indication that the software/controller is keeping track of position correctly.  I hape that my way of thinking is correct.

Trevor

12

General Mach Discussion / Re: Problems threading on the lathe

« on: March 01, 2009, 12:49:26 AM »

Art, Thanks for the reply I am using the latest stable release.020, and the parallel port,  Thanks.

Rich, yes I did read your reply, and your suggestion of lots of lighter cuts is probably the best solution for now.  Thanks

Trevor

13

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 09:16:38 PM »

OK so i did some more testing today, and I took video.

First here is a video of my lathe cutting a ball radius on the end of a .500 Ally rod.  I intentionally set it to cut the .500 sphere with a z center of -.500 so that it would not only round over the end of the rod, but also have to cut an extra .250 off of the end of the rod.  This isn't realted to threading at all, but it does serve to demonstrate that if nothing elsel I think that I do have pretty good repeatability of the X axis, check out the .010 nub that remains until the final pass.

http://www.youtube.com/watch?v=uuz8B3oVLNs

Next is The video of the lathe threading while cutting air as Hood had suggested.  As you will see the movements are normal at first, but then when I alter the spindle speed the funny x axis moves begin.  I also noticed that these errors can be induced with an decrease, or increase in spindle speed which makes sense though not many of us have a problem with our spindles speeding up during a cut

http://www.youtube.com/watch?v=x-rHNH8VVnI

I do also want to clarify one other thing based on my observations so far.  If a thread is to be cut in whatever number of passes, lets say 80, then it will be cut in that number of passes no matter what happens.  In other words if the problems begin and the x starts to cut a taper in the air it is in place of a programmed move.  It does not appear to me that mach is inserting some kind of extra move in order to compensate, rather it is inserting a funny move in place of a programmed move, and replacing it.  This causes the next normal move to remove more material than intended since the prior pass didn't do any cutting.  So If I have programmed 80 moves I will get 80 moves, some of them OK, and some of them goofy, but I do not end up with 160 passes of which half would be normal, and half goofy.  Let me know what you think of the videos.  Thanks

Trevor

Now if i can just come up with a way to convince the wife that I need a 2hp VFD/motor combo that can maintain +- 5 RPM I will be all set 

14

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 04:06:29 PM »

Rich,  I may be confused but I don't think that there is any possible way that I can be missing steps, because when the program is done running I can jog the tip of the tool to the end of the workpiece and it is at 0,0 just like before the program started.  If it is reading 0,0 then doesn't it mean that the motors haven't lost any steps.

Hood,  Keith, very good idea about cutting air and varying the spindle speed to see what happens.  I could do it by loading up the spindle with a board, or rac, or whatever, but i think I will just turn the dial on the speed indicator to see if I can create the problem.  I will go do that right now and report back, seems like a simple, but very telling idea.  Thanks

15

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 01:48:18 PM »

No VFD here, I wish!  I just have a 1hp dc motor controlled manually with the knob on the Dayton motor control box.  Mac has no control over my motor I have to turn it on with the switch, then dial up the desired speed.  I will try to run the code line by line later as suggested  and report the results, hopefully later today.

Trevor


PS, I am using the parallel port.

16

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 01:28:27 PM »

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

17

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 11:18:52 AM »

 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!!

18

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 10:49:08 AM »

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!

19

General Mach Discussion / Re: Problems threading on the lathe

« on: February 27, 2009, 09:35:26 PM »

Rich,  No backlash compensation enabled, maybee I should play around with that?  The steppers are 191oz/in increased by the belts pulleys 1.6 to 1, they are driving 5/8 rolled ball screws with a pitch of .200, or 5 Turns per inch.  I am using Linisteppers to drive the steppers in half step mode currently at 12v and 1.5A per phase.


The thread that you cut and posted a pic of looks really nice, I can only hope for those kind of results at this point.

20

General Mach Discussion / Re: Problems threading on the lathe

« on: February 27, 2009, 07:41:38 PM »

At least you can see it so that you don't think that I (we) are crazy.  Thanks for putting the video in for me, how did you do that?