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Author Topic: Movement of Z axis is great in down direction but is incorrect in Up direction  (Read 1686 times)

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Offline CES

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Sorry, if you can't post what was asked or take some simple advice  won't be able to help you.

Have  Happy Holiday and good luck,

RICH


Hi Rich,

  I have posted all requested information except a video of the movement (I don't know how to post VERY large files).

Have a great holiday season yourself.
CES

Offline ZASto

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To check backlash, here is a macro by CNCNutz
Attached metric and imperial. Edit the macro as it suits your machine (https://www.youtube.com/watch?v=CS2rosB_ce0)
Make no mistake between my personality and my attitude.
My personality is who I am.
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Offline RICH

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CES,
Don't need a video. Just need to see how the stepper is mounted, attached to the ball screw, how the ball screw is mounted.

Also info on the steppers in oz rating. If you have info fine, a link to info on them.

RICH
« Last Edit: December 05, 2018, 08:14:34 PM by RICH »

Offline CES

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Hi Rich,

  I have Nema 24, 3.1Nm Steppers on all three axis connected to spider couplings and 5 terns per inch ball screws. I have attached pictures of the Coupling on the Z axis but all axis are the same. I've also attached a picture of my Mill, a Grizzly G8689 CNC controlled using a GECKO G540 4 axis controller and Mach3 licensed.

The flood is under construction... also the home switches for X and Y are under construction.

I've searched for the original manufacturer of the parts but can't find them. Out-of-Business or my migraines are getting in the way.

BTW - I have all axis calibrated to less then 0.0005 in all directions except Z in Up direction (I am now thinking the Z is under a heavy load due to the weight of the head). The direction of Z that makes a difference for me is Down which is good. Up is not nearly as good, something like 10X the down accuracy.
« Last Edit: December 09, 2018, 01:42:33 AM by Tweakie.CNC »

Offline CES

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Not sure why the picture was blown up to such a degree. Select it and you will see a smaller view.

Offline TPS

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BTW - I have all axis calibrated to less then 0.0005 in all directions except Z in Up direction

that is one of the bad sides if you are living in the imperial world, in the metric world it is more than you want in the Hobby world.

sorry that i have sayed this
anything is possible, just try to do it.
if you find some mistakes, in my bad bavarian english,they are yours.

Offline RICH

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CES,
How exactly is the stepper mounted to the ball screw.
ie;

Stepper flange bolted / anchored to fixed bracket - inside the bracket is the coupling  which connects the stepper shaft to the ball screw.

Are  there any bearings  in the above description?  I can't tell from the pictures.

If there are bearings are they mounted and  or how were they preloaded?

What make is the coupling?  

It would helpful to know more about the motors. ie; 3.1 nm ( 438 in oz ) motors but at what amperage and voltage. If memory serves me
the G540 is limited to 3 amps for an axis. How many wires do the motors have.?

Your steps per unit of 10000 may need to be tweaked some. More on that later .........
Believe your accel is too high. We'll get into refining your velocity and accel later.......

For now just post the info on how the  Z stepper is connected.

RICH





« Last Edit: December 08, 2018, 06:03:05 PM by RICH »

Offline CES

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RICH,
  The 24V, 4 wire motors are connected to the ball screw with a spider coupler ($15 from Amazon) but mine came in a kit. The ball screw is a Thomson and the spider coupling is red or orange but I cant find them on the web and I can't read the text on the mounted units but a picture is attached.

When I calibrated the axis, the 10000 came down to 9999 and change. They are as accurate as I can get them. Much less than 0.0005 inch over 1 inch movement. The Z axis is the worst at 0.0004 in Up direction and 0.0002 in down direction over 1 inch. I think the up direction is pulling a lot of weight as the ball screw in on the side of the column.

The X and Y axis both calibrated to 0.0001 in both directions.

Calibrations of X, Y and Z were done using Mach3 calibrate function over 1.000 inch. The Backlash is set to very low values to give the correct response.

Offline CES

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Oh, I forgot, 3.1Nm motors.

Offline RICH

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CES,
You have 0.010" of Z backlash according to your posting.
I suggest you try to minimise the backlash. You will have backlash no matter what you do it's just
a matter of how much! Can't see how the screw is attached, so in the picture, I point to what I think is
a support bearing.

Now consider doing the following:
You should have a bearing / bearings ( depends on how you want to do it) which anchors the upper end of the screw. You preload the bearing support such that the torque required to turn the screw is minimized. With the anchor fixed you can use a solid coupling and get rid of the spider coupling. Consider increasing the torque ( don't think you actual  have 438 in oz with 24 V) and also consider providing for a 2:1 ratio or so for increased resolution and motor torque.

If done properly you "MAY" only have a 1/2 to 1 thou free play due to the bearing. The ball screw is rolled so don't be suprised if the there is a slight diference in the up or down accuracy of the screw. In the end, the major contributor to  backlash will be in the nut.

Total backlash is the sum of each axis component non-movement.

RICH
« Last Edit: December 13, 2018, 07:24:38 AM by RICH »