Problems grasping Z-* when zeroing to spoilboard

[scottp55]

Hey All, I've started dimensioning some of my boards to exact thickness by zeroing to spoilboard and then entering my cut depth in VCP as Z-(the thickness I want) and love it. BUT just made a brandy new spoilboard that is bigger than my cutting area and while trying to trim my 0,0 fences managed to put 2 nice gouges in the slight pocket of cutting area caused by surfacing.
I'm referencing to bottom of material, entering .685"(BBPly fences) as my material height, start cut 0---depth of cut Z-.01 and just doing an 18X24 profile cut inside the line. Is there a tutorial somewhere I can't find? or am I making ANOTHER newbie mistake.:) Thanks in advance

[Joe Porter]

Scott, I don't think you are supposed to put a minus sign in front of your depth of cut. Also, I have found that when you get to your "Home" position, you are supposed to add the thickness of your board to the total Z height. Hope this helps...joe

[scottp55]

Thanks Joe, Going out now, will try suggestions. Probably cheat and bridge Z-plate between fences at 0,0 for now and work from top of material as this has slowed me down a couple days and I'm way behind.:) I should have experimented on old spoilboard first, but totally forgot my fences would be outside cutting area:(

[srwtlc]

Scott, post your original file that made the problem if you can. Not quite sure what you did. From what you say, you should have been 0.010" above the spoil board.

[scottp55]

That's what I thought Scott, which would have trimmed the fence and been .01" above surface of pocket. Think I just added an extra toolpath to my XXXDIMENSIONING file and after second shot just exited without saving in frustration. Should have saved as "Garbage to inspect" :)
So I did it right? but did something wrong?
Worked perfect yesterday trimming dowel tops and surfacing all those wedges and fences to same height so a quick visual reference tells if board is flat or lifting.
Thanks

[adrianm]

If you're zeroing to the spoil board surface/material bottom it shouldn't really be possible to go into the spoil board like that as no Z move will be below the 0 point.

One thing to make sure when using the calculated depth of cut feature such as Z-0.01 is that it's still set when you've changed things like ramps, tabs etc.

It will revert to the actual value rather than the calculated value which can cause issues if you subsequently change the material thickness as it's not picked up when recalculating the toolpath.

That won't be the issue when zeroing to the bottom though.

[scottp55]

Whatever I did before wasn't repeated. It was getting late that night, If I had typed a non capital "z-.01" could that have done it? I found myself doing it today.
Thanks for the warnings Adrian. Only ever used it for surfacing before, but with new set up may have occasion to use it more so good to know. Time to do the rest of them without having to creep up on it with offsets:)
Thanks

[feinddj]

I don't know if this will help you but its what I do:

Skim the spoil board so it's flat.
Screw a bit of ply to the outside edge, I generally use half inch as it will be out of the way for most cuts.
draw an L. put a filet in the corner so that your stock will rest against the L. the corner will be 0,0. cut all the way through the ply and maybe a small amount into the spoil board. Now you have a easy spot to rest against and place your stock.
say the board is 875 thick. Make the math easy and tell your program the wood is 1 inch. As we are planning to a specific thickness, subtract that from one and that's your depth of cut. ie: to get a 5/8 board, .375 depth. when you are planning the second side, start by using the depth of cut at .375, and then depth of cut at your pass depth. ie: .375 start, .125 depth gives a finish of .5. This avoids extra air passes without changing the cutter profile each time.
The other trick is to set the pocket to raster in the grain direction. Oversize your cutting area so that the direction changes take place in the air so you don't get the marks that take quite a bit of sanding to get rid of.

David

[scottp55]

Hi David, That's the way we were doing it, but measuring each board and then subtracting the depth we needed, as well as having to Z in an exact spot on one board out of 4, and then measuring again when flipping, was a pain and not always perfect. This way we can trim rough down to say Z-.79" on the backside, and then flip and cut Z-.75 without measuring anything, and only Z-zero the bit once for as many boards as we want.
I hadn't even seen it until ScottW mentioned it.
http://www.talkshopbot.com/forum/showthread.php?t=19671