Problems threading on the lathe

[ART]

Hi Chip:

 I see what you mean.. doesnt look too bad though, other than the wandering... Ill give it some thought..

ARt

[mehcjose]

I have just started to use Mach 3 lathe . When I try the threading program it runs but stops at at a line with G76 X0.4 Z-2 Q2 P0.05 J0.006 L0 H0.005 I29 C0.025 B0.0001 T0.  I understand that this is a fine boring canned cycle not supported by Mach3 .has anyone else had this problem? and what is the fi?.
  mehcjose.

[Hood]

G76 is a threading cycle not a fine boring cycle and it is supported by Mach.
 If you machine stops at that point is there a message in the ticker window?
 Do you have spindle feedback to Mach?
Hood

[mehcjose]

Hi . Hood

I think my spindle feedback may be faulty . am checking.
I did not find G76 in the Mach3 G code list so assumed it was not supported.

   mechcjose.

[Hood]

G76 should be in the Mach Turn manual, it certainly used to be anyway.
Hood

[RICH]

HI ALL,
I know there were problems with the lead changing when threading. Take a look at this thread as i just did some thread testing on the converted lathe.

http://www.machsupport.com/forum/index.php/topic,12484.msg82874.html#msg82874

The lead error and heavier cut ( somewhere during the cycle ) still exist as I have shown.
I know it is not mechanical.
Any thoughts on the matter?

RICH

BTW.........1/2 a good thread is better than none! 

[simpson36]

Rich,

Seems to me the solution is an encoder on the spindle motor and a servo drive. An encoder can be put on just about anything.

The servo drive automatically does all of the stuff MACH is thus far been unsuccessful doing, i.e. compensatng for the motor slow down under load.

I can cut any thread with my 4th axis including tapered NPT and re-cut the same thread over and over with perfect registration.

This scheme would also allow a drill, small milling head or engraving spindle to be mounted on the cross slide for additional machining capabilities.

What would really be slick would be to take an X2 head and column and mount it on a slide behind the lathe. Bada-bing! Mini-Machining center II !

Downside, of course, is cost.

[Hood]

It would be interesting to find out where the problem lies, from the way I see things it has to be something to do with either index pulse type or computer or VFD control. Reason I say that is I have never had an issue with threading and I previously put this down to my spindle  motor having no chance of slowing down during the cut, but now seems Rich has plenty power so that doesn't seem to be it.
 I now have a big servo on the spindle and use the Index pulse from the encoder through the SmoothStepper, however previous to that I had an induction motor (through gearbox for speed change) and an Opto (OPB 916B)for the index pulse and used the Parallel Port. As said previously I never had an issue in either of these setups.
 That seems to leave the index input (type) or the computer itself as the culprits, the other thing that it may be is VFD control. I am not sure if Rich is using a VFD on his new lathe or not?


Hood

[RICH]

Not using a VFD, but have a 1 hp DC motor and VFD drive for it. Figured I would just use the AC motor and see how all worked and maybe later try out VFD. Not sure on the motor though since i have no info on it.

Can change the Z over to a servo and replace the gecko dirve. My controller ( four drives in it ) is set up and used for multiple machines as it is portable along with the computer and sits on a cart. Thus that option would be a pain and really don't care to do it.

I can say that compared to other testing from the Sherline that the lead relative to length was improved some.

I also narrow it down to the index and computer. If the machanical spindle rpm is solid ( i wathced it during regular cutting and took measurements ) and it stayed within 1 rpm / the accuracy of my indicator consistently.

I know Graig can slave an encoder to the Z axis and provide for slowdown in one direction of a stepper. I was more interested at that time for both directions.
Agree that an encoder would be the ticket. We have used one for threading using DeskCNC. In fact you can turn the motor off and turn the system by hand, even go back and forth on a very small thread and it would just track whatever the encoder movement was doing. Maybe i will try and contact Graig again.

The thread testing back some months ago never came to closure. Art seemed to think it could be computer, but those tests were strickly for the PP.

One thing for sure is that the problem is not going away, it's a tuff one, and a lot of users will experience it.
RICH

[panaceabeachbum]

I havent been having any problem the past few months with either of mine, one with a 2 hp spindle motor and the other with a 30hp . The 30 hp machine uses smooth stepper and the 2hp does not. Both use VFD and a single pulse optical setup  . I have cut somewhere over 2k threads on each machine, most 1"x 20 tpi with no problems