Ouch ! 2 sided machining problem

[danborel (Daniel Borel)]

Hi everybody, I’m a Shapeoko Pro XXL owner, working with Fusion 360 and Carbide Motion for about one year
Excuse my “swissmade french speaking” English !

I need to machine a guitar body but have axes issue. If anyone can help me to avoid damaging another piece of mahogany + flamed maple, any suggestion should be welcome ! I’ve a strange gap between the first and the reversed machining.

So I’ve decided to do a test in 12mm MDF. I’ve zeroed on the bottom middle, contoured the blank and bored 4 holes in order to install the stock with 10mm pins and to make it correspond to the drawing axes.

I zeroed on the front left top corner (Bitzero V2), machined 6.5mm deep pockets, slowly, with thin 1.5mm DOC.
I reinstalled the MDF upside down and machined the opposite side and noticed that the gap was 1mm in the left corner and 2mm in the right one…

The Y axe seems ok.
I don’t understand why the offset occurs and why it is different, progressive, between left and right.
My machine is squared, my index holes too…

So… ??? ???

Every suggestion is welcome !

PS: I reversed on the X axe

[WillAdams (William Adams (Carbide 3D))]

Have you calibrated the machine for belt stretch?

Calibrating the machines over long distances

The typical technique for machine calibration: https://my.carbide3d.com/faq/belt-stretch/ depends on precision measuring tools, and is limited by the reach of the tools in question. EDIT: It also depends on access to the MDI — if using Carbide Motion v6 it will be necessary to enable this in Settings | Options: [image] It should be possible to cut strips from a suitable blank of material, one along each axis — so we set up a square file which matches an available blank of material: [ima…

[danborel (Daniel Borel)]

Thank you very much for your answer! I’ll calibrate the machine as soon as possible.
One thing I find strange is that when I reverse the wood piece the pins fit in the holes… so it isn’t a parallelogram.
Mmmmmm… I’m wandering if the cables are too tight in the drag chain?

But as the problem don’t look random (each pass follow the same way) I’ll in any way check the belts tension and calibrate the machine
Thanks again, I’ll keep you informed

[Tod1d (Tod (with 1 'd'))]

I notice in your description you zero the machine twice. Once at bottom center to machine the profile & pins, and then on the lower left corner to cut the slots. This seems the most likely place for an error to occur.
Create your design for the whole test using the same zero. Including the dowel holes, profile & slots.
And of course, make double sure the slots are the exact same size & distance from zero.
I would screw the stock down where the screws won’t get hit. Machine the profile, dowel holes & top half of the slots in one program. Then flip it & cut the slots only.

[danborel (Daniel Borel)]

Thanks everybody,
Finally I’ve redone all the setup. Squaring, tramming, calibration,… and the belts were very under tightened !
Wow ! It’s impressive how it changes the life ! When I reverse the test piece and machine the opposite side, the result is perfect ! You can see a line in the middle height (due to the machining directions) but I don’t feel it with my fingers !
Thanks to all the contributors who post in this great community forum and Youtube !