Centroid Community CNC Support Forum

Marty, you're the man !!
You had it right, needed backlash values in the motor config table, well done.

I went to the effort of adding all the values to the screw comp table for each axis.
Making the assumption, that would be the only thing needing to be done to account for machine motion mechanical variability.

I guess I'm not sure I understand the difference between backlash comp and screw comp.
Is backlash the slop between the screw and ?carrier? where screw comp is for variability of the screw profile?

Anyway, it's working GREAT !!!
Dimensions are all within 0.0005" (measuring with calipers)

I'm going to start bumping up the speeds and feeds, then width of cut.
Curious how much this little machine can eat.

Thanks again!!

Screw comp is a different animal. Generally you need specialized instruments and you are checking the tolerance of the ballscrew at incremental intervals.

See this tech bulletin:
http://www.centroidcnc.com/dealersuppor ... ads/58.pdf

Video Alan did on screw mapping you might find interesting:
https://youtu.be/yMRqL8b0Orw


Sounds like you are on your way....

Marty

If you do screw compensation, you should enter 0 for your backlash values. The "moving from home" and "moving towards home" comp values include the backlash.

tblough wrote: ↑Fri Jan 25, 2019 3:24 pm If you do screw compensation, you should enter 0 for your backlash values. The "moving from home" and "moving towards home" comp values include the backlash.

martyscncgarage wrote: ↑Fri Jan 25, 2019 3:21 pm Screw comp is a different animal. Generally you need specialized instruments and you are checking the tolerance of the ballscrew at incremental intervals.
See this tech bulletin:
http://www.centroidcnc.com/dealersuppor ... ads/58.pdf
Video Alan did on screw mapping you might find interesting:
https://youtu.be/yMRqL8b0Orw
Sounds like you are on your way....
Marty

I actually completed everything for screw comp and had the same understanding as Tom, as how that would work.
But things weren't right until I added the backlash values.

I'll share a screen shot of my config screens a bit later.

Here's a video I did of the screw comp results. This is WITHOUT backlash comp values.
The program homes the axis before executing the incremental movements.

https://www.youtube.com/watch?v=UcxK_5M5PLU

Set your backlash to 0. Record your DRO reading every 0.5" starting from home. From last point recorded, do not change anything, but record the DRO reading every 0.5" moving towards home. If you do this right, every 'from home position' - 'towards home position' should be pretty close to the backlash reading you previously had, but there will be slight differences because this also takes into consideration screw errors, i.e. if the ball track is a little wider in some places, the backlash will be greater at those locations.

I was just unclear on screw comp, thinking it was the silver bullet.
But it's only making up for motion variability when the nut maintains contact with one side of the screw.
It's not doing anything when the nut is jumping from one leading edge to the other on the screw, that's the backlash comp.

I just didn't have it clear in my mind, now I can see it, newbie misunderstanding.
Part dimension tolerances are exceeding my expectations.

Getting feeds and speeds right is the next step.
Running the machine at less than 10 IPM feed rates is no problem, but I'm loading the EM as I attempt to increase rates.
Dry cutting.

Backlash comp is built in to screw comp. The control knows what direction it was last moving. If you move to 5.12" going away from home, it looks at the going away values and interpolates the position using the two values that bracket the requested point. In this case 5" and 5.5". If the next move is to 4.7", then it looks at 5" and 4.5" from the going towards home table. That accounts for the backlash.

If you want a magic bullet, switch to the oak or allin1DC and add scale feedback. No need to worry about backlash or screw comp.

First project with the CNC mill done. Wrenches for collet tool holders, ER16 & ER20.

Thread milling next, then some steel.

Very nice!

I'm curious if Acorn has the capability for "spindle speed offsets".
My commanded speed versus actual is a bit off.

I dialed it in as best I can with the POTs on the drive/controller, but it's not quite right at some places in the range.
I've considered an encoder, but it's going to be a pretty big project to figure out mounting.
I'd be nice if there were some adjust-ability on the software side, that would alter the 0-10v signal to the drive.

I'm considering a tapping head to be used for smaller thread sizes. (not rigid tapping)
From what I understand, accurate feed & speed is important.

Or is this something I'll need to figure out in the post?
I'm considering the potential of writing a program that would apply the needed adjustment to the hard coded value in the program used specifically on my machine.

Thx
Chris

Tapping head --> https://www.tormach.com/store/index.php ... ortrelay=1