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Author Topic: Creeping Y position. Anyone know something else I can try?  (Read 10707 times)

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #20 on: August 07, 2008, 12:02:40 AM »
Hi Chip,

I may be misunderstanding you, but all my ratios are small pulley to large pulley, so there wouldn't be any cancelling out. I have: 10 teeth to 36 teeth  (3.6 : 1), 10 teeth  to 24 teeth  (2.4 : 1), and 10 teeth to 24 teeth  (2.4 : 1), so I have 3.6* 2.4*2.4 = 20.736 : 1, correct? What would I change around?

The error is at the cutter tip. The DRO reads just as it should (X0.00 Y0.00).

I have the pins set exactly as you described in your previous post. Tom Caudle frmo Cand CNC.com says that the Step needs to be positive (Red "X"), in order to get proper accuracy, but if I do that, the motors jsut groan and dont move any discernible amount. Putting a Green checkmark on them makes the motors sing along very smoothly. Just with this extremely frustrating creeping on Y. (It starts at X0.00 Y0.00, goes out and does some movements, and when it come back, the DRO reads X0.00 Y0.00, but the cutter bit is offset by 0.016".) It's entirely repeatable - if I run the program 100 times, the offset is 0.016" X 100, everytime.

Let me know if I'm misunderstanding your post.

Thanks

Dave
Saltspring Island
Re: Creeping Y position. Anyone know something else I can try?
« Reply #21 on: August 07, 2008, 12:05:21 AM »
Chip: BTW, I'm using Gecko 303V's. I set the Pulse Width at each option from 1 to 5.

Thanks

Dave

Offline Hood

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #22 on: August 07, 2008, 04:17:45 AM »
2000 steps for motor per rev
20.736 revs of motor to 1 output rev
41472 steps for 1 output rev

1 output rev moves 24 x 0.375 inch = 9inch

41472 steps = 9 inch so divide by 9 to get per inch =4608 steps per inch.

Think thats right  ::)

Hood
« Last Edit: August 07, 2008, 04:20:25 AM by Hood »

Offline jimpinder

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #23 on: August 07, 2008, 04:29:25 AM »
Even is the traverse is set at the right (or wrong) steps per unit, this cannot be the reason for the progressive error because the steps per could be anything - but it goes one way, and comes back so they cancel each other out.

Check you are on Absolute Stop, and bring down your speed and acceleration substantially on your motor tuning, and try again and see if the error varies.

This could be backlash, but I cannot see how if you do not touch anything. On the first move you would be out (up and back - only one change of direction) - but the next move you would have two changes of direction so this would subsequently cancel out any further backlash.

You are loosing steps somewhere and the only reason I can think of is - your gearing, although substantial, is still quite high. and the motor is struggling as it sets off, to accelerate the whole contraption up to speed. One full motor step is nearly two thousanth of an inch - so you don't need to loose many of them each way to loose your distance.
« Last Edit: August 07, 2008, 04:41:21 AM by jimpinder »
Not me driving the engine - I'm better looking.

Offline zarzul

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #24 on: August 07, 2008, 11:43:14 AM »
Dave,

The repeatablity that you describe leads me to suspect that it is a triggering error of some sort.  When your drive changes direction it will change the dir pin output.  It is almost like when you change directions it may be missing a step pulse, maybe triggering on the trailing edge instead of the leading edge?   Just a lot of guess work on my part.  If your steps are long enough you might be able to determine this by stepping one pulse in Y+, with a dial indicator to monitor the movement, then step back 1 pulse.

One other thing to consider,  One of my 4 cnc machines is a recently aquired engraving table,  it has the same problem you describe but it is on all axis.  I get a cumulative error after multiple moves.  On my machine the problem is the computer is not fast enough,  I think it is a 500mhz machine.  I can improve the error by turning off the toolpath display update, but haven't totally eliminated it. 

Hope this is helpful

Arnie
Re: Creeping Y position. Anyone know something else I can try?
« Reply #25 on: August 07, 2008, 11:45:08 AM »
Thanks Hood! That's going to help on a smaller CNC conversion I'm planning for my lathe. I appreciate that!

Thanks!

Dave
Saltspring Island

Offline jimpinder

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #26 on: August 07, 2008, 12:30:57 PM »
Just to repeat - are you on Constant Velocity or Absolute Stop.

Try running on Absolute stop.
Not me driving the engine - I'm better looking.
Re: Creeping Y position. Anyone know something else I can try?
« Reply #27 on: August 07, 2008, 12:54:09 PM »
Hi,

Thanks for that - I am using Absolute Stop. It's slower, but I found it to be far more accurate on the corners.

Thnaks

Dave
Saltspring Island

Offline Chip

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Re: Creeping Y position. Anyone know something else I can try?
« Reply #28 on: August 07, 2008, 01:27:00 PM »
Hi, Dave

Ouch !!, When your Brain, Eye's, Keyboard and Calculator aren't in gear = Bad Numbers, Bad Day in "Bed-Rock".

You stated earlier that setup's per for X = 4750 and Y = 4100.

If there geared the same then the steps per should be the Same in the End, As "Jim" stated, Your X, Y axis may have Backlash,"Belt Sage" Issues.

None of that is causing your Lost Step's though.

OK, "Direction" and "Step" "Low Actives" Are all set to the "Same State", In Config, Port's & Pin's, Motor Outputs.

I think with Gecko's it's "Checked" not sure.(Just to be different)

Also this "Checked" State may just be needed to have the BOB work It's OPTO's Properly.

Now your BOB may have a way to set the Output's State to your Gecko's
Do you have a link and part # for your BOB.?

I've also seen on some "Brake-Out-Board's",With Bad OPTO Isolator's (Slow response Time issues). Did you try the Sherline 1/2 step Mode.?

It may be time to Wire your Gecko's Direct to the printer port, Try a different computer, Get a bottle of Whiskey.......

Chip
Re: Creeping Y position. Anyone know something else I can try?
« Reply #29 on: August 07, 2008, 05:46:44 PM »
Hi Chip,

The Gecko's only run when the "Step Low Active" is set to Green checkmark. Both tom Caulde (the manufacturer - very knowledgable), amd Dan Mausch (another very knowledgable CNC guru from  way back), both say that they need to have the Red "X". When I do that though, I get a low groaning from the motors, with very, very little movement. Put them back, and the run silky smooth. I have no idea why it doesn't work as it was designed.

The BOB is CandCNC's Mini-IO combo board. They're at: http://www.candcnc.com/   The website has changed to some new products, but the The BOB is under "Quick Pick and Buy". I also have their MTA Mass Termination Card, two EZPlug Gecko dual Stepper Cards, their Mini-IO Remote Relay, and Mini-IO_Port2. The  manuals for all of these can be read from the menu on the right of the website.

I tihnk the opto-isolators must be OK - the board runs fine. In fact everything does..., : motors are silky smooth, there is no excessive heat, X and Z axes do exactly what they're told, etc. The only problem is that the router tip increments on the +Y direction for every round of my test program. (traces a 4" X 4" square hole, in Absolute mode.

You might be right - it may be time to go directly from the BoB to the Geckos.

Thanks!

Dave