Building for Increased Machine Accuracy

[CanAm1]

Hello Again,

I posted this on the Zen Toolworks site and thought maybe I should go ahead and add it here also.

I thought it might be prudent to start a new thread and explain my humble findings on building and “tweaking in” my machine. This post is a little long and may not cover everything, but should help out those that are in the building phase and others. I apologize for not getting back very swiftly to others on some prior threads, but things here should cover prior questions.

In building the machine:

Build it on flat surface
Loosely assemble major assemblies then place them on your flat surface, get everything square and then tighten.

Surfaces of parts are flat and clean
Make sure to clean any manufacturing residue from all surfaces. Also check the pre-drilled holes and make sure there is no “puckering”. I found quite a few holes on various pieces that required a little filing or trimming with a razor blade to make the hole flush with the surrounding area.

Make sure your threaded drive screws are straight
Mine required a little finessing to get straight.

Square the gantry to the table
For no better words, perform gantry alignment with spacers as explained in prior thread.

Lube the drive screws and run each axis back and forth several times
My drive screws had burrs on them which made things bind up at different points. After lubing with some Teflon grease and several passes back and forth everything smoothed out nicely. Wipe off excess grease.

Ensure any mechanical play is at a minimum
This is directed primarily at the little drive screw ends that you use to lock the screws to the bearings. I found that I had to play with these a little to get the play out.

Mounting stepper motors
This may be overkill, but when I bolted on the steppers I noticed that there was enough play in the bolt holes which allowed the motor to be off center slightly. So, what I did was prop the entire unit on its side and end to mount the X and Y motors. This allowed the motor to find its center prior to me tightening down the hardware.

Backlash Nuts
In industrial machines there are generally optical encoders which provide very accurate feedback as to how far an axis travels. Our little machines do not have this feedback back which means that we are constantly running in “I hope it’s there mode”.
For example, when we tell an axis to move 2 inches, our controllers convert this into lets just say 200 steps. It then happily drives the stepper motor these 200 steps and presumes it is at our 2 inches. If however there is any mechanical slop or worse binding causing a missed step or two we will not be where the controller thinks 2 inches is.
In measuring my machine I found that my X axis backlash nut had some play that was causing a constant error. While watching the nut I manually turned the shaft back and forth. I noticed that there was a lag from when the flange side turned and the free end caught up with it and started turning. I added a .006 shim and my error went away.
I would check this on your machine on each axis only after running it some to “bed” things in. Also if shimming is necessary, leave a couple of thou of play. Do not lock things solid.
I have included a photo of where to watch and shim.

Hopefully this helps some of you out there.

Mark

http://www.cnczone.com/forums/attach...1&d=1295112938