The setup:
I recently rebuilt/modified a Tree Master Journeyman 425. I replaced the old servos with stepper motors (with encoder feedback to the drives). I had to setup the new Centroid ACORN controller I installed, among the settings I had to enter was the "steps per revolution" and "turns per inch". The steps per revolution I got off the stepper drives (in this case 3200). For the turns per inch I used a dial indicator with a 1 inch travel. I setup on the x axis and took a few measurements and tuned the turns per inch vaule before arriving at 10.113 revolutions per inch. I did the other two axis Y and Z the same way. Made sure it was repeatable and moved on.
The Problem
I was drilling some holes on a larger project 25 inches apart and noticed things were not lining up. I do not own a precision measuring device that will span 25 inches, but a simple tape measure was enough to show about 1/16th inch of error.
I made sure the vice was trued up correctly then I got out some gauge blocks, squared it in the soft jaws (yes, I know you shouldn't use them for this) and using the probe took a measurement on the 1 inch gauge block. It came out to 0.9997 inches and checking the turns per inch they were still 10.113 repeated this a few times and got similar results, sounds good, right?
Next I put the 4 inch gauge block in and preformed the same test to get 4.0128 inches. I repeated it several times with results varying only 0.0003 different.
Finely I recomputed based on this information for the 4 inch gauge block to read accurate, the turns per inch value would need to be set at 10.144 turns per inch. I entered the updated value and was getting measurements of 4.0001.
I have no idea what would be causing this type of error. I've never see something like this in half dozen or so machines I refitted, built, or worked on. How can for 1 inch the turns per inch need to be 10.113 and at 4 inches it need to be 10.144? The only thing I can think of is the turns per rev are incorrect, but I am unable to think a good way to check this?
My apologies this is a long post.
(I didn't post this in the Tree CNC Machine section of the forum as I do not think it is relevant to the machine)
Machine frame = Tree 425
Controller = Acorn by Centroid
Steppers = 34HS59-6004D-E1000
Drives = CL86Y (shielded control lines between controller and drives)
Probe = DrewTronics