I have always set my zero at the top of my material. Should I try setting zero at the bottom, I heard you can save more of the spoil board this way, The starting point for a complete cut through should be minus the thickness of the material. Is this correct?
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Correct. You just have to make sure your material thickness is accurate in setup just as you would when zeroing off the top. The way I see it is if the material has slight variations in thickness you could end up with cutting into the spoilboard anyways or not cutting all the way through.
I don’t always run my material through the planer so if I cut into the spoilboard… oh well… MDF is cheap 
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Nailed it. I vastly prefer zeroing from my work table these days.
Whether to measure from top-of-stock or bottom-of-stock depends on where you want your accuracy.
If you are cutting through material, then you probably want to know where the bottom of the material is most accurately.
On the other hand, if you are doing a V Carve, you want to know where the top surface is most accurately.
In either case, the surface you probe to will be as accurate as the machine can probe. The other surface will be as accurate as you, the designer, can measure the thickness of the stock and ensure the design has that value.
In most cases, I would think that the surface that you probe will be the more accurate one.
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Michael is correct. It’s mostly arbitrary.
When zeroing at the bottom, you can set the Z zero 0.001" above the table so you never cut into it.
But you can do the same with a top zero by accurately measuring the stock. After you set your Z zero, jog the cutter down to the table and that is your stock thickness. It requires an extra step in going back & setting your stock thickness in CC to match the actual stock thickness on the machine.
I’ll generally use some 2-way tape to lift the stock about 0.004" off the table. Then I set my stock thickness to cut about 0.002" above the table, and 0.002" into the gap.
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Thank you all!
I use two-sided carpet tape, so I like the depth cut advice to account for the thickness there.
Every comment has a tidbit of sound advice.
Thanks again,
Eric
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I agree with other posters here. I use bottom of material for any cut that will cut through. I save my spoilboard by using bottom of material. Like others I use top on projects that will not cut through. So there is not 1 answer. It depends. As @mhotchin indicated accurate measuring of your material is very important. For top of material it important so your first cut is not too deep and/or not touching at all and you miss fine detail. For bottom of material it is important so your first cut does not plow into the material too deep or not cut deep enough on things like vcarve.
What I like about using bottom of material is there is minimum destruction of my spoilboard and there is no onion skin left I have to remove. I measure my material and enter that in CC. Howevere I almost always use painters tape and super glue and the two layers make my material slightly higher and the onion skin left is non existant. I find it tedious to remove parts that are not cut all the way through. I seldom use tabs because the painters tape and super glue work so well and I hate cutting the tabs out. Some projects are just too large for the tape and glue so I will use tabs.
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