Touching Off Tools: Spoilboard or Material?

CNC owners discuss whether to zero their tools to the spoilboard, or to the material they are machining. July 8, 2014

Question (WOODWEB Member) :
In the past six months we have replaced our old Multi Cam CNC with a new one. We are also in the process of upgrading our software. With our old CNC and old software we would always put the material on the CNC and touch off from the top of the material. Now with the new software we are using the techs recommend that we touch off from the spoil board. The material thickness is changing all the time so I think when you put a piece of material on the CNC and touch off from the material that is telling the CNC the exact thickness of the material for every job. When we touch off from the spoil board it is relying on the exact thickness that we have entered in the software. Is this correct? We have had more material thickness issues in the last month than we've had in the last eight years. Any input would be appreciated.

Forum Responses
(CNC Forum)
From Contributor J:
When I cut through I will zero off the spoil board but if I need to pocket or engrave I zero off the material.

From the original questioner:
So when you are cutting out cabinet sides and doing operations you prefer to zero off the material because that gives you the exact material thickness, correct? We had a tech at our shop and he insisted we zero off the spoil board for everything.

From Contributor J:
Yes that is what I do.

From Contributor D:
I can't see any reason to reference off the material. Sheet goods can have different thicknesses in the same sheet, especially if it's been around a while. For cabinets, if you have your decks and tops butted up to the sides, it's the design software that will determine the length of the decks and tops. You'll only be as accurate as the data. For shelf and slide holes +/- 0.020 in thickness makes no difference. If you are dadoing in the decks and tops, referencing off the table is better. The width of the cabinet is based on the length of the stringer plus the dimension of the end after the dado. If the material is supposed to be 0.750 and you want a 0.250 dado, you will end up with 0.500. If the material is actually .765 you will dado 0.265 and still end up with 0.500 instead of 0.515 if you reference off the table. You might say that 0.015 is not that much, but 0.015 on each end is 1/32 per cabinet. On a long wall of six cabinets you will have an increase in width of 3/16. That also doesn't seem like a lot until you top it off with MDF cabinets that are the correct size. Then that 3/16 is huge. Also, think about training. Doing it the same way every time is easier to teach.

From contributor W:
We touch off on the spoil board. We usually have several different materials in a single job. I would want to have to retouch all the tools every time we change materials.

From contributor S:
I set my bits to zero on the spoil board like the others say. My post processor asks for a "Z shift" which is the nominal material thickness, the controller reads that number and shifts the z up by that amount. The only time I worry about actual thickness versus a nominal and variable thickness is when I want to pocket something in the material that has to be flush with the surface (an inlay or piece of hardware maybe).

When you say you have had "material thickness issues", what exactly do you mean? Are your cabinets coming out the wrong size or are you trying to sink something in flush with the sheets surface? I think you need to think and draw your parts up by remembering that your rabbet or dado size is what you have left over after you have cut the part not the nominal depth of the dado from the top surface of the sheet. For example, I use a lot of 1/8 deep rabbets on cabinet parts to spot locations of other parts. I set my depth of cut at z -.125 and the nominal z shift or material thickness at say .75. I almost never care how deep the cut is, only that what I have left over on the table is .625 thick. All the other cabinet parts are drawn and cut assuming that .625 is butting up to that piece.

From contributor S:
I guess what I am/was saying is what Contributor D is saying too. If material thickness is screwing you up put a rabbet or dado in the piece and you will end up with a known and not variable dimension. We are able to keep finished sizes of cabinets dead on this way.

From Contributor A:
We also have a newer Multicam and new EnRoute software. We touch off on the spoil board once in the morning and whenever we spoil our boards off. Touching off on the spoil board basically tells the machine to "cut only to this depth every time". After that, we set our max depth about .010" into the spoil board. Now it doesn't matter what thickness material we cut as long as the CEL of the tool is sufficient. The tool always goes to the same depth. Now, like Contributor J said, if you're cutting pockets or other cuts that don't go through the material, you would touch off on the surface of the material and set your specific depth.