I know some people carve by setting their Z zero to the spoilboard surface, and managing their operation start height.
One advantage to this is, you don’t carve up your spoilboard. If Z zero is accurate, and you never carve at negative Z, the spoilboard should remain without carve lines.
The downside is, if you are carving a pocket to a precise depth, you risk error in the depth of cut - because the depth is now dependent on the precision in measuring your material thickness, and the tightness of your clamps (since wood compresses).
But if all of your cuts are through cuts, and the accuracy of your partial depth pockets is +/- 2mm, why NOT use a workflow that relies on Z zero being the spoilboard surface?
This is a great question. I’ve worked in a production environment doing lots of programming, and training new machine operators, along with programmers for many many years. I’ve thought about this long and hard. All your points are valid.
The policy on how to program was always being refined and improved. The latest programming strategy always indicated the Z0 position on a setup sheet in one of two ways:
Set Z0 at top of material, or
Set Z0 at a specific distance off the spoilboard (normally whatever the material thickness is).
If the material was 0.5" thick, it would be common to set Z0 exactly 0.5" off the spoilboard.
It would have been nice to always use one strategy, but as you mentioned, there are benefits to one method over the other, so using both was required.
Most of the time referencing the spoilboard was used, and is generally more useful. The exception is when the cut needs to be a specific distance from the top, in the case of an engraving, or something like that.
We would not set Z0 at the spoilboard. Doing so invites the possibility of an operator accidentally setting up a job using one method, when it should have been the other. The result is often a disaster resulting in damaged tools, material, or machine. The method I posted above means that if an operator mixes the two up, there’s no drama at the machine. Sometimes you get lucky and the parts are still within spec, and can be used anyways.
Many programs have a facing routine built in, and referencing off the spoilboard is better, because the final thickness is not subject to the variation in material thickness. The parts are always the correct final thickness regardless of how they started.
This.
I know this is more in line with machining than woodwork, but the math is the same. And it has made me rethink when to use each method.
If I want a board/part/piece X thick with a Y thick rabbet/rebate around the edge, do I really care where that material started out? or where it ended after machining?
Great, I went -Y from the top surface, but what if X was off by whatever margin?
If I measure from the bottom up, both X AND Y will be on.
As long as there is a top surfacing operation to follow just to be sure, I suppose…
This statement is not intended to diagnose, treat, cure, or prevent any machine or operator condition or deficiency.
Pretty much 80% of my work is profile cutting shapes.
I zero at the spoil board raise up the thickness of the material being machined plus 4 thou
Rezero and away I
I do the same if i am machining pockets. I produce my own material to thickness from rough sawn so can control its tolerance
My machine is used in a production shop with only me working it, i have tried many work flows and now stick with where i am