I saw a video a while back how one maker always creates his work with bottom zeroing. (Basically, Zeroing from the top of your waste board.) He stated by doing this he saves from cutting into the waste board.
Is anyone actually doing this?
Would this be a good practice?
Is there anything to watch out for doing this?
I typically zero the Z-axis off the waste board for all toolpaths that cut entirely through the stock. If I have features of critical depth on the top surface I export those as separate tool paths and zero those off the top. All that said, if my stock is uniform/flat AND I plan to cut all the way through (e.g. cut a part out) I can typically get away with making an accurate measurement of thickness and zero off the waste board. I do this specifically to avoid cutting into the waste board. (I know it’s a WASTE board, but I have grid lines and other marks on it and prefer to keep it as clean as possible.)
As I understand it, if you zero from the bottom of your stock, then the sizing of your stock becomes hyper-critical.
Do you mean that size doesn’t matter ( 
) while zeroing from the top of your material?
I don’t see that there’s a difference, especially if you take the time to flatten the surface as the first operation. Since you have to design with a certain material thickness anyway, the only extra material left on the top (after zeroing on the bottom) is the material that you don’t cut.
With Vectric software there is little difference in the design process. I suspect that CC would be the same.
I use spoilboard zero for pretty much everything that isn’t in a vice, however, one does need to remember what dimensions are assumed vs. what are known. I still occasionally forget what I’m doing and end up putting an unexpectedly heavy chamfer on some 6mm Ply because it’s really 6.7mm or whatever.
In materials like veneered ply or MDF you can’t just face off the material to a specified depth. You know it won’t be the stated thickness, if it’s ply, it probably won’t even be particularly consistent across the board. This means some planning if you’re cutting down from the top face for features rather than all the way through. If you have one ‘finished’ face then skimming to thickness on the other side first can work.
In Aluminium, plastics, hardwood, un-faced MDF and other homogeneous materials I tend to set my finished workpiece to be slightly thinner than the stock.
I face one side flat with a 0.2mm to 0.5mm pass, you can do this with either the spoilboard or the stock top as zero. I then flip it and then work from spoilboard up to finished dimensions.
When workholding with blue tape and glue I put a double piece of blue tape on the spoilboard to set zero from as stock bottom is not at spoilboard height and I get enough blue goo on the cutters as it is.
Using this approach with fixtures introduces another little wrinkle, the fixture top face needs to be flat with the machine or you end up with a wedge shaped part. Even if you’ve previously skimmed the fixture flat, when you bolt it back down it’s worth taking an indicator to it or skimming it again to get it really flat with the machine XY plane.
Doing fine finish chamfers (say 0.5mm) means your workpiece really needs to be flat to the machine and the surface needs to be where the CAM thinks it is, doesn’t matter where you zero.
I carve mostly 3d relief, or 2d+. I typically use rough sawn stock and always set z from the machine bed. My process takes off the top few thousanths of stock 100% of time because I set it for that in Aspire. Since the machine bed is known the carve takes off any inconsistent stock thickness from the top.
@CrookedWoodTex I’m sure dimension matters.
I generally prep my stock with a jointer and planer before putting it on my CNC.
I don’t always cut through, even when cutting out, because it simplifies my hold down strategy and it’s usually pretty simple to cut through a thin bottom - and I’m always going to sand the piece anyway, so the final trim doesn’t matter.
As a rule, if I’m going to cut through a piece, I add an extra .0002 to my cut…just to kiss the wasteboard. My wasteboard is an extra, sacrificial MDF board with a Myers hole layout. I don’t care if it gets slightly nicked.
Myers did not invent 2" spacing. He did put together some files to cut a spoilboard but he did not optomize it so the whole surface could be surfaced. This procedure can be used by any size Shapeoko with a one piece spoil board. Shapeoko 3 and the newer hybrid tables.
spoilboard_considerations.pdf (1.3 MB)
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