Virtual Zero and Tool Changes

[AZRoger]

I'm doing 3D cutting from Aspire 4.0. There's a roughing tool path, a finish tool path, and two "rest" tool paths.
Each of these uses a different cutter and therefore Z0 has to be reset for each change.

This project is also tiled along the Y axis.
The Virtual Zero function seems essential for getting the edges to line up after moving the material.
Setting the Z0s for the corners after the part is clamped to the bed should get them to align properly.

With Virtual Zero, there are 5 sample reference points. After the roughing pass, the middle reference is gone.
Is there a way to use only 4 corner points and skip the middle?
Can I just set Z0 for one corner and have it adjust the rest from that one measurement?
Do I have to make a spacer that will bridge the part to put under the touch plate for the middle point - and the corners?

Thanks for your suggestions. Roger

PS: I ordered the touch plate and SCP2 combo pack. The touch plate was Dead On Arrival .
I've contacted Next Wave for a replacement cable. I haven't been able experiment with V0 yet.

[Joseph Poirier]

The Virtual zero process uses triangles, so just taking 4 points would not allow the process to work correctly.

If you are already doing a roughing pass to smooth the entire material work space, why do you need virtual zero at all?
VZ is designed to allow you to cut material that isn't 100% flat without doing a rough cut of the entire material surface.
Once you set VZ, you do not need to reset it for the material you are working with, you only need to run [Detect Touchplate] to alter the zero location, and the system allows you to do that from any point on the board.

[AZRoger]

OK. My tool change need is satisfied already by the system. That's very good.

Now, to list my three most important reasons for using Virtual Zero are tiling, tiling and TILING.

The VZ is needed to account for warping of the machine bed under clamping pressure.
The blue aluminum bed isn't flat or completely perpendicular to the z-axis to start with.
If I drive the cutter around the whole bed, there are differences up to 1.5 mm in Z height.
And when the work piece is clamped to the bed using the t-bolt hold down fixtures it's different.
The Oak and Hard Maple I mostly use is stiffer than the aluminum. I need to register Z0 to the wood.

While cutting a long piece - say 7 feet long - I need to tile this into 4 separate runs.
I have locating blocks bolted to the bed and index marks so I can move the work piece along Y and keep it "in line" with the previous step.
So for X there's no change and Y is manually adjusted to get the work where it need to be. However, Z is a different situation.

What I'm trying to do is set Z0 exactly for the 4 corners of each "tile". Here's the scenario.
Do Virtual Z with the board in the first position. Run the toolpath.
The Minimum Y edge will be machined exactly to the model - we have good Z0 value for the Minimum and Maximum X for that bottom edge.
The Maximum Y edge will also be machined exactly to the model - again we have good Z0 values for the extremes of X for the top edge.
When the part is moved, the prior top edge will be the new bottom edge. Next do the VZ process with the board in the new position.
This should assure that the Z height of the new bottom edge will exactly match the Z height of the old top edge resulting in a perfect match.
I can finesse the middle point - but I have no way without VZ to get a perfect match at the edges of the tiles.