Zeroing From The Table w/ Tall 3D Parts?

[DanThomson]

I am having trouble understanding how it is possible to zero from the table(which I find more accurate) when machining tall 3D pieces such as 3",4", etc.

The part I am having trouble with is understanding whether I have to set the safe Z pull up height to an amount that after zeroing the Z axis to the table will bring the bit above the material height in order for me to zero the X and Y over the material?
Otherwise how after zeroing the Z to the table are you able to move the bit over the material and zero the X and Y?

I would love to hear from guys on here who machine a lot of 3D parts.
I have been getting more and more of this type of work and would like to streamline my process.

Thanks in advance!

[garyc]

Dan...
One way that this can be accomplished and stay with common convention is to:
Zero to bed
Note the design thickness of the model (for this example 3.875)
Make sure model has designed with zero to top
MZ, 3.875 (moves the z to the design height)
ZZ (to zero to design height)

There are no accuracy differences between zero to bed or top of material in design. In most cases the innacuracies are caused by zeroing the bit to a section of the material or model where the actual thickness is not known. Different hold down methods can warp (up or down) material, as can humidity or vacuum.