Thank you all, William of course the most, for your help designing joints. I learned so much about manipulation of toolpaths, layers etc.
However: I designed this joint:
jointtest.c2d (84 KB)
all fine.
cut it, and this is how it looks:
huge gaps, not useable.
Ideas?
Which machine are you using?
How does a calibration test cut work out?
Shapeoko pro XXL, and calibrated some months ago. Will check tonight, and report back.
The file you provided doesn’t have both parts. It has two sockets only.
Why cut the contours first?
Wouldn’t it be better to offset first and cut as a pocket for two operations? (three if you add a contour to avail yourself of tabs)
turn one upside down
sure, this one is just a proof of accuracy - and failed 
final project will have the contour cut as pocket / contour for tabs.
probably also for speed will use a 0.25" bit instead 0.125".
Endmill cutting oversize so your offsets end up wrong? cut a simple drill operation 3/16 deep in the material you’re cutting then measure (ideally pin guages but you don’t have them so calipers) whatever size you measure above the nominal size of the endmill used is your offset on the outside right, inside left.
Cutting deep slots is not a good approach to achieve accuracy, which is why the usual test has one removing pockets:
If the dimensions of that test don’t work out well with endmills which are verified as to dimension on a machine in good mechanical condition, then see:
Understood! So far I almost never did precision work, toys etc. always just need to fit in relation to each other.
So the bits’ cutting edges pull away the bit from the designated path, obviously. And that depends on the kind of bit, every bit is different I am afraid?
Some experimenting to be done.
William: did a brief calibration test, X- and Y axis are precisely calibrated, did not Z yet, but was not the problem here.
Will test geometrical shape precision soon.
But I rather think it is all about specifics of bits deviating from the designated path depending on their cutting edges, so I will try next cutting the edges as pockets and not straight through.
Will take a little, probably next WE. But I will keep you posted.
You likely have a mechanical problem:
So, either something needs adjustment, or the cuts are too aggressive and are deflecting.
I took your file and made a few changes, but the outline of the objects is exactly as in the original (although 1 is flipped).
jointtest.c2d (96 KB)
I created new toolpaths to avoid slotting. There’s a pocket for most of the depth, then a contour to leave tabs.
My (admittedly small) experience with trying to create mating edges is that it will work a bit better if you offset inwards by a small amount, say 0.1mm.
Also, some square ended mills actually have a tiny radius at the outer edge, or have worn a bit at that corner - if you don’t cut deep enough, the bottom edge will stick out very slightly, and on a tight fit it can be enough to mess things up.
Michael: like day and night!
Thanks!
I never would have imagined that the pocket-contour “trick” would be that important!
Mission accomplished:
Wouldn’t have been possible without you, you all.
This joint was inspired by that site:
but I designed it from scratch.
So please feel free to use it.
jointtest3.c2d (164 KB)
I think with that large surface for glue that joint should be sturdy.
I’m fascinated by this: 50 Digital Wood Joints by Jochen Gros – WINTERDIENST
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