Hello Guest it is October 16, 2019, 11:44:39 PM

Author Topic: z axis moving up  (Read 9753 times)

0 Members and 1 Guest are viewing this topic.

Offline RICH

*
  • *
  •  7,367 7,367
    • View Profile
Re: z axis moving up
« Reply #10 on: May 30, 2012, 07:14:30 PM »
Your steps per unit for the Z are calculated correctly.
Now test the Z axis movement accurately using a dial indicator. Do this at vel setting of 10 and accel of 2 and over a larger movement
of 1". The movement should be in the Z downward direction. Bring the Z all the way up and then move it down say .5". Now move the Z in the downward direction via MDI. Record the distance for the move as shown on the indicator. Do another downward movement of 1"
and again record the indicator ( you may need to reset the indicator). If the two moves were 1" each your steps per are ok. If not
adjust the steps per until the moves are correct as indicated by the indicator.  

You may want to also try the reverse of the above and see if screw has a different pitch ( small maybe but it can add up) when the Z goes up.

With the steps correct, you should now check for backlash. So just raise the Z all the way up and down again but for 1.5".
Set the indicator and jog the Z up in .001"  movements. The number of jogs required to just move up .001" up is your backlash.
How much is there? You can use backlash comp to deal with, not ideal, but may serve you well.

----------------------------------------------------------------------
Do you have specific info on your Z motor?
Are your ther steppers the same?

Your power supply is 31V @ 2.42 A. That is is not adequate for your three steppers. If all the steppers are the same then the min required
amps maybe around 3 x 2.8 amps per stepper x .7 = 5.88 amps.

Don't know the basis of  your motor torque rating but will say that the microstepping, reduced amperage available will reduce the the available torque and speed of the steppers.

---------------------------
Can't see how your motor is attached to the screw and how the screw is supported in the pic you posted.
Can you post another pic showing that?

Post back,
RICH

Offline dray

*
  •  19 19
    • View Profile
Re: z axis moving up
« Reply #11 on: May 30, 2012, 08:51:58 PM »
Hi Rich
Thanks for your responses. The z motor is direct coupled to the lead screw with out a bearing. Before changing the lead screw it had a bearing on the motor end and had the same problem. The z motor is a 290 in/oz motor which replaced a 90 in/oz motor which had the same problem. It has not improved with a stronger motor but I have 1 more 435 in/oz motor which I will install on the z tomorrow. I will check backlash as you said but have varies values with no effect. The ps is only running z during the loop test, I would like a better one but that may have to wait. Thanks again.

dray

Offline RICH

*
  • *
  •  7,367 7,367
    • View Profile
Re: z axis moving up
« Reply #12 on: May 31, 2012, 06:31:30 AM »
DRAY,
You should properly support the lead screw with a bearing. The stepper motor has axial play and you'll have backlash along with putting
unnecessary loads into the motor. The bearing should be preloaded to minimise backlash and also needs to aligned properly.

Consider that when microstepping you reduce the motor torque probably in half, then the torque required to raise / lower the Z axis, headroom for cutting, reduced power requirements ......thus skipping is just waiting for the right condition.

Your going to have varied values when you measure because the screw is floating.

Now on to the coupling between the motor and screw.........What kind are you using?

I didn't see any recomended step or direction pulse durations for your drive and you may want to ask Mechatronics what they recommend.

Just a quick test you can do:
Mount  a ring ( can be made out of paper) about 3 to 6 inches on the motor shaft along with a stationary index. With the motor energized make a fine mark. Do an up and down movement for say 1". The index should be in alignment with the mark after the moves.  Doing this will isolate the motor from the screw and thus eliminate the mechanical influence of the axis. You can also calculate how many steps for a given movement and thus measure steps within reason.
 
So the poor mans step counter works like this:
- 6" dia x 3.14= 18.85" circumference
- 18.85" /200 ( steps per rev of motor ) x 8 ( micro steps per )= .012" of circumferential  movement per sent step
So a fine mark could be around .005" and you can see or even measure steps to see if there is an electronic issue.
(BTW, one of the reasons I like motors with dual shafts)

RICH





 
« Last Edit: May 31, 2012, 06:33:36 AM by RICH »

Offline stirling

*
  • *
  •  2,188 2,188
  • UK
    • View Profile
    • www.razordance.co.uk
Re: z axis moving up
« Reply #13 on: May 31, 2012, 08:04:25 AM »
Consider that when microstepping you reduce the motor torque probably in half
Have you got a citation for this RICH?

I've read this: http://www.micromo.com/microstepping-myths-and-realities.aspx but on the other hand I've read this: http://www.cnczone.com/forums/127081-post7.html

Personally I like Mariss's but then he's not always right - or is he? (I got burned over his advice re: caps on switching supplies...)

Ian
Re: z axis moving up
« Reply #14 on: May 31, 2012, 08:05:59 AM »
Hi Dray,

Simple way to isolate positioning errors is to mark the motor shaft and the faceplate of the motor. With the marks lined up, note the position of the DRO (or set a zero) now run it all you want -minutes, hours, days etc... when done, command a move back to the DRO position that you noted at the beginning.

If the marks line up - EVERYTHING on the control side is good. -step rates, power supplies etc... etc.. and your problem is something mechanical from the motor shaft out.

If the marks don't line up, it's something in the control side.  

By doing this you don't have to worry about lash or anything else.  Your steps per unit can be completely wrong -it doesn't matter.  Running this test will immediately and unequivocally isolate the issue as either a mechanical issue on the other side of the motor shaft or something in the control/electrical side.

Offline RICH

*
  • *
  •  7,367 7,367
    • View Profile
Re: z axis moving up
« Reply #15 on: May 31, 2012, 04:16:10 PM »
Quote
Have you got a citation for this RICH?
Ian,
Nope. Just a ball park number to make a point.
RICH

Offline dray

*
  •  19 19
    • View Profile
Re: z axis moving up
« Reply #16 on: June 03, 2012, 06:38:00 PM »
Hi Rich
I ran the test that Automation suggested. With the stepper removed from the z axis and a pointer on the shaft and a mark on the housing. The stepper is set to drive a 16 thread acme shaft. If I have it run a 50 cycle test to move up .0625" (1 full rev. of the motor) it always has a .004" error in the up direction. So I tried the test in a down direction and it still had an error of .004" in the UP direction!! This happens at 10"/min or 40"/min. Accel. doesn't seem to effect it either. If I double the cycles of course the error doubles. Any ideas?

dray
Re: z axis moving up
« Reply #17 on: June 03, 2012, 10:00:06 PM »
Hello,
I'm new to the site and to CNC. I am using Mach 3 demo just getting my machine to run so I can start learning. I have a question that kind of relates to this topic. I need to know how to changes the Z movement + -. Just got the machine running this afternoon for the first time and the Z axis moves down when I jog Z+. The roadrunner demo Gcode has the Z set up just the opposite so when it starts the code the bit comes up when it should be going down. Do I have to changes all the Gcode lines for Z or is there a + - reversal somewhere. I can't find it.

Offline Hood

*
  •  25,855 25,855
  • Carnoustie, Scotland
    • View Profile
Re: z axis moving up
« Reply #18 on: June 04, 2012, 03:09:16 AM »
Config menu then Ports and Pins then Motor Setup, change the active state of the Z axis Dir setting.
Hood

Offline dray

*
  •  19 19
    • View Profile
Re: z axis moving up
« Reply #19 on: June 05, 2012, 02:44:00 PM »
Hi Rich
well I ran the test with a pointer on the shaft of all the steppers and they all show an error in the direction which would be up. I also tried a different power supply, no change. Finally deleted the xml file and reconfigured every thing. No help. Would reloading mach3 be of any help?

dray