I am using 11mm plywood and when I measure it accurately I find an average of around 11.1mm. I set this in Carbide Create.
I use a bit zero.
When I cut (using contour to stock bottom) it doesnt cut through (half expected) so I tried again changing the contour to 11.5 and it still left a papery thin layer which was a hassle to remove.
The question is
A) what is the right amount to add to the contour bottom to make sure I get a clean cut through? Is this a percentage or a fixed amount?
B) can this issue be simply removed by zeroing to my base and then lowering the Z by a further 0.25mm, for example?
I’m wondering why Shapeoko isn’t more automatic with bottoms. Surely if it knows the Z for the base and for the stock top, then it should be able to remove guesswork, including needing to measure the stock at all in Carbide Create.
Are you setting Z Zero to the top of stock? If so, try switching in Carbide Create so that the Z Zero is bottom-of-stock, and take your Z Zero from the bed of the machine.
Now if your material thickness is not exactly right, it only affects how deep the first cut is, and that much difference won’t be a problem.
It all comes down to what the machine knows, and what is calculated. When your Z Zero is top-of-stock, then you know where that is, and the stock bottom is calculated. Whether or not that actually matches the real world depends on how accurate your material thickness is, at the spot where you measured Z Zero.
Conversely, Z Zero set to bottom-of-stock means the machine knows where the bottom is, and the top is calculated.
Since you are trying to cut through the stock, it is more useful to know where the bottom is.
On the other hand, A VCarve can be very sensitive to how deep it is going, so in that case, it is more useful to know where the top-of-stock is.
If you set Z zero to the top of stock, the machine does not know where the table is. It knows where the bottom of the Z travel is, but at that point your tool could be well into the table.
I would try this… Set your Z zero using your BitZero. Remove the BitZero & touch off to the top of stock. How close is it to 0.000? Now touch off to the table (Or sacrificial wasteboard) and note that value. Now YOU know where the table is, and can make an adjustment to the stock thickness in CC to accommodate a through cut without violating the table.
So I zero’d Z to the top of the workpiece, being pretty precise
I then moved the bit down to the bottom (ie my board) and it read -18.6.
I then measured this real distance and got 19.1, again checking carefully and feeling the same amount of friction on the gripped piece of paper.
Hence my Z axis must be out of calibration?
Therefore I will always be cutting slightly deeper than I think. If I set the contour bottom cut to 19.1 (my measured thickness), my machine will cut into the wasteboard by 0.5mm.