This is more of a curiosity question than trying to solve a problem. Part of my quest to learn more about using my machine…
Let’s say I have several pieces of stock ranging from 0.75" to 0.79" thick. For each piece, I want to run the same job of making a cutout all the way through the stock.
Now let’s say I set
Thickness to 0.79"
Set the Zero Height to “Bottom”
On the Contour toolpath I set the Max Depth to t-0.79
Would this cut all pieces of stock (assuming max thickness is 0.79") without cutting into the spoil board? If so, this would save me from having to reset the Thickness for every piece of stock. Or am I missing something?
If you are doing a cutout, sure. If you are pocketing there would be a difference of .04. On the cut out you would be cutting air for .04.
Could you run the stock through a planner to make everything uniform?
CrookedWoodTex
(Tex Lawrence (Don't Mess With My Texas!))
3
Dave, you’re not going to find a process that works for all situations. It ain’t gonna happen. @Zman
shows a good case on point.
If I were doing your example (and assuming that you don’t have access to a planer or drum sander to normalize the thickness), I would sort my stock (and label it) out by thickness and make separate gcode for each thickness (also assuming the rest of the toolpaths are identical.) Name the gcode files so you can clearly match the file to the stock thickness on the machine.
Definitely Z-zero to surface that the stock you’re cutting mounted to (spoilboard.) The only time my cutter digs into my spoilboard is when I screw up, so I’m rarely resurfacing my spoilboard. Last year I did a PVC project that was painted red before I cut it out with a downcut bit. There is still a red paint outline on my spoilboard. That’s how close my bit was to the Z-zero surface.
I do have a planer. I’m not trying to “find a process that works for all situations.” I’m doing what I stated in my first sentence of my post - trying to learn the limits and capabilities of the machine so that I can make better use of it. It’s a curiosity question, an exercise in learning. It is not a problem solving question.
I have always set height from the top. But Carbide Create gives the option to set height from bottom. Why? Is a way to leverage it to make my work more productive and more efficient?
The scenario I gave was completely made up for the sake of thinking through the question, not an actual situation I’m facing.
Maybe a simpler approach is - can someone tell me why anyone would want to set zero height from the bottom if not for the scenario I presented?
Setting height from the bottom makes it less likely that one will cut through the material and into the baseplate.
It is useful for part geometries which benefit from using the bottom surface of the part as the reference surface — for example in joinery, when a V endmill is being used to cut a miter — getting that height wrong results in the part being the wrong size and making it possible that the joint won’t fit right.
As pointed out, the decision to set Z zero at the top of the work or the top of the wasteboard is driven by a number of factors.
For me, I set mine to the top of the wasteboard because my projects are all dependent on the thickness being constant and at the specific thickness. I first use a flattening pass to get the specific thickness, using the top of the wasteboard as the reference. Then, everything else can be done using either top of the work or top of the wasteboard; you just have to be consistent. Since I start with top of the wasteboard, I continue using it so I don’t either cut air or drive too deeply and break bits.
Wood is notorious for not being consistent in thickness. Even wood that has been previously planed can be off enough to make a difference if some time has passed since the planing. Moisture can make the planed wood differ by a couple 1/100" fairly easily, depending on the type of wood. That’s why I surface the piece just before doing any carving. Almost always, I keep it in place after surfacing, as I already have the zeroes, load the new program, and let it rip. It has worked great for me.
As always, YMMV (Your Mileage May Vary). This is what works for me; you might find a different method that works best for you.
1 Like
CrookedWoodTex
(Tex Lawrence (Don't Mess With My Texas!))
9
So sorry to have misled you with my post. I don’t think I’ll do again.
Like Tex, I now almost always zero off the spoilboard, because it’s easier to cut all the way through a part internal or perimeter, ensure the thickness of a pocket in the part and not chop up my spoilboard too often.
When the job makes more sense to zero off the top, I do that too, for example, I was making some mortices in some 2x4 for a shed door, and figured that was a job for the Shapeoko (any excuse).
But seriously, with the very limited Z clearance I had to set the retract height to 3mm above the stock, which was inconsistent thickness by a couple of mm and I wanted consistent depth mortices. I zeroed X and Y off the angle plate and a dowel you can’t see behind the 2x4 and then Z off the top of each piece of stock I fed in.
resurfacing my spoilboard. Last year I did a PVC project that was painted red before I cut it out with a downcut bit. There is still a red paint outline on my spoilboard. That’s how close my bit was to the Z-zero surface. 
