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inconsistency in Z axis
« on: February 05, 2017, 02:16:56 PM »
Home built Wood CNC

I just installed a 8mm acme rod on my Z axis and I am trying to set it up.
I don't have an anti backlash nut, not sure if this is my issue.
Nema 23 motor.

I have mounted a digital caliper to the carriage to measure my movement for set up.

I am measuring a 50mm distance using Z50 and Z0 for the movement and then measuring the distance and resetting each time.

Here are my measurements in mm:

up   -.2
down   +.36
up   +.18
down    -.01
up   +.32
down   +.24
up   +.07
down   +.35
 
I've done more cycles, but this illustrates my perceived problem

up off by ave.1925 range of +.32 to -.2
down off by ave.24 range of +.24 to -.01

both averages are less than ten thousandths.

Is this too great and inconsistency? What are acceptable tolerances for the different axis' in wood and hopefully aluminum at some point?

thanks, Mike

Online ger21

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Re: inconsistency in Z axis
« Reply #1 on: February 05, 2017, 03:15:03 PM »
Acceptable tolerances are whatever you find acceptable.
Having said that, you should be getting more consistent results.
Is everything tight mechanically?
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: inconsistency in Z axis
« Reply #2 on: February 05, 2017, 03:19:22 PM »
I ground a flat on the acme rod and added another set screw to contact the flat, so I think so. What is the standard repeated precision that I shoot strive for?

Online ger21

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Re: inconsistency in Z axis
« Reply #3 on: February 05, 2017, 11:14:08 PM »
What's keeping the acme screw in position? If it's supported by the motor, then you won't get any better, as the motor shaft will have some end play from the bearings.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: inconsistency in Z axis
« Reply #4 on: February 06, 2017, 12:46:20 AM »
I used a pillow block. See if anything i the attached picture jumps out at you. I do have another motor I can try
Re: inconsistency in Z axis
« Reply #5 on: February 12, 2017, 11:46:29 AM »
It looks like the only thing controlling the end play of the screw is the motor bearings and the flex coupler unless there is something on the other end that we can't see in the photos. You might want to look into a more rigid way of controlling the end play.


Mike
We never have the time or money to do it right the first time, but we somehow manage to do it twice and then spend the money to get it right.
Re: inconsistency in Z axis
« Reply #6 on: February 12, 2017, 11:57:46 AM »
There is a pillow block below the coupler. Maybe I need to add another pillow block at the other end

Online ger21

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Re: inconsistency in Z axis
« Reply #7 on: February 12, 2017, 12:01:46 PM »
What's holding the screw in place, just a set screw? Or the coupler?
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: inconsistency in Z axis
« Reply #8 on: February 12, 2017, 12:13:02 PM »
I installed a set screw on each end of the coupler and ground a flat on the acme thread

Online ger21

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Re: inconsistency in Z axis
« Reply #9 on: February 12, 2017, 12:40:23 PM »
So the coupler is carrying the weight of the spindle?
The screw should be fully supported, so that it can't move up and down.

I think what your seeing is the play in the motor shaft, in combination with the friction of the screw sliding through the pillow block
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html