Stepping up to CNC

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04 Nov 2007 22:05 #6 by pacrat186
Stepping up to CNC was created by pacrat186
Hello, I\'m hoping this forum stays around for awhile.

I am planning on converting my knee mill to CNC over the next year, but after I finish building my mini stepper CNC mill.
I need to install DRO on my knee mill to have repeatable accuracy for some of my projects. For the cost of a decent quality import 3 axis setup I could purchase a dedicated computer and 4 high quality linear slides (X,Y,Z-quill,Z-Knee) and a rotary encoder for my rotary table.
My question is this, I believe it is possible by what I have read, but it is currently all a jumble in my mind right now. Can I use the EMC2 software for reading them in just a DRO mode, then as funding allows, use the DRO slide and rotary encoders as positional feedback in a closed loop stepper or (dream) servo motor CNC system.
If this is reasonable and possible then I would have around $600.00 usd available to put toward ball screw upgrades for the table, variable speed motor for the spindle,or stepper motors after building the DRO.

Dave

After writing this I thought, if I use my old laptop I could get the ball screws and rotary encoders (higher resolution I believe) for the X,Y lead screws immediately and already be partially ahead of the game.

Any feedback would be great.
Thank You
Dave

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05 Nov 2007 16:33 #9 by skunkworks
Replied by skunkworks on topic Re:Stepping up to CNC
Welcome.

In regards to reading rotory/linear encoders back to emc2.. If your going to try to use the printer port as the input device - your going to be limited by speed. The best - expecially if your going to servos would be a mesa (pci) card or maybe a pico systems card. These have dedicated encoder inputs that will count at the speeds you need. You can see the supported hardware here.
wiki.linuxcnc.org/cgi-bin/emcinfo.pl?EMC2_Supported_Hardware

Now as far as linear encoders - something to think about is backlash in the system. If there is any backlash (screw, bearings, gibbs) then you will have a hard, harder time tuning. What happens is emc says move. The servo starts to ramp up and emc doesn\'t see any movement - so it ramps up some more. Then the linear encoder starts to see movement. The movement is already too fast an it over shoots - and tries to correct. You get an nasty ossolation you have to tune out. (I am no expert at tuning servos yet).

Although someone recently converted a mill with an anilam(sp) controller. this had linear scales. He used a mesa card and I think the original servo amps. It took him a bit to get it tuned - but he was pretty happy with the results. I think he had a few thousands of backlash.

Hope this helps.. BTW - Yes - you could use emc as a dro.

sam

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