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Author Topic: Noob needing help. Z axis dropping as it cuts  (Read 3910 times)

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Noob needing help. Z axis dropping as it cuts
« on: December 07, 2012, 06:03:30 PM »
Hey everyone. New to all this and I tried a couple searches and nothing seemed to pinpoint my issue. Additionally I am unsure exactly what is causing the issue and therefor don't have concrete terms to search. Hoping someone might be able to point me in the right direction.

First I'll start with the issue. I have a 9.875 x 9.875 pattern that I am attempting to cut. At different points, on different attempts, it seems like the z axis is dropping and creates a downhill slope.





I have gone through a couple guides and believe I have everything setup correctly and that nothing is mechanically loose or slipping. When testing the z axis with a dial indicator it repeatedly returns to zero without an issue cycling between .75 and 0. At first I thought perhaps the bit was slipping but I have since marked the bit before cutting and can verify that it is not coming out of the collet while cutting. I have done a few test cuts on smaller patterns and don't have the same issue.

After posting this I'm planning on swapping the z and x axis driver to see if perhaps the problem follows the driver but would rather not chase this issue all over the place if anyone might know the cause right off hand.

Thanks for any help.

Offline RICH

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Re: Noob needing help. Z axis dropping as it cuts
« Reply #1 on: December 07, 2012, 06:39:56 PM »
What happens if you lower the axis velocities and feedrate?
RICH
Re: Noob needing help. Z axis dropping as it cuts
« Reply #2 on: December 07, 2012, 10:42:57 PM »
Thanks for the suggestion Rich. Since this is all a new experience I just wanna make sure I'm following correctly. I will try decreasing the motor velocity and acceleration by half in the motor tuning parameters of mach and then regenerate the g-code with a feed rate of half at the rate at which the current one is. I won't be able to run this till tomorrow since it will be over a six hour run with the lower feed rate, but will get back to you as soon as I have completed the test.
James
Re: Noob needing help. Z axis dropping as it cuts
« Reply #3 on: December 08, 2012, 11:25:36 PM »
Well lowering the axis velocities and feedrate seemed to rectify the issue. I have cut one complete pattern without any problems and am half way through a second. My follow up questions would be what is the specific cause? Is it a combination of limitations of software and mechanics or by upgrading the z axis motor and driver will it respond better to a higher feedrate and velocity? I am currently using 425 oz-in stepping motor and a 3.0Amp 24-40 Volts driver.

Offline Hood

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Re: Noob needing help. Z axis dropping as it cuts
« Reply #4 on: December 09, 2012, 05:13:30 AM »
The likely cause is that your motor was tuned too aggressively and thus was on the edge and loosing steps on rapid moves. This happens more often on the Z positive direction as you also have gravity to overcome. So because the Z is losing steps on the way up it is not as high as Mach thinks it is and when Mach commands it to go down by the set amount it will travel further.
 Larger motor may help but maybe just limiting the Velocity and acceleration will be suitable for you?
Regarding you saying you were to rewrite the code with lower feeds then unless your feeds in your previous code were at or close  to previous rapid speeds  it is unlikely you needed to reduce them. Mach will not exceed the velocity in motor tuning anyway.
Hood

Offline RICH

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Re: Noob needing help. Z axis dropping as it cuts
« Reply #5 on: December 09, 2012, 07:01:27 AM »
jmo4134,
The problem with steppers is that a motor curve is not available for a lot of them that folks purchase.
You could get a stronger motor and it's preformance could actualy be worse than what you have.
Supplied voltage, amperage, wiring comfiguration, axis load & makeup and drive used all affect the motors preformance.

You have what you have and reliability in use at some velocity and accel should be of highest priority and if it dosen't satisfy
the need then you need to upgrade whatever is necessary to get what you want.

RICH 

Offline BR549

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Re: Noob needing help. Z axis dropping as it cuts
« Reply #6 on: December 09, 2012, 02:10:57 PM »
In a lot of cases it is caused by running too fine a pitch on the Z screw. In order to get SPEED you have to take the motor into the rpm range where it has little torque to do the function.

Most time it is as simple as replacing the feed screw with a courser pitch to move the motor rpm down into the high torque band.

Just a thought, (;-) TP

Re: Noob needing help. Z axis dropping as it cuts
« Reply #7 on: December 10, 2012, 05:48:08 AM »
Thanks for all the info everyone. I would like to be able to run at a little higher speed so it sounds like I've got some homework to do. Gonna compare the ups and downs (cost) of a more course lead screw and or a motor that supplies a speed-torque curve in its documentation and see if its feasible for me. Thanks again.

James