Here you can read how Phil Moore of PMinMO.com takes care of the hold-down 
problem:  
http://millpcbs.com/index.php?option=com_content&view=article&id=17&Itemid=46.
No need to make it more complicated than necessary. 

>________________________________
> From: gene heskett <[email protected]>
>To: Enhanced Machine Controller (EMC) <[email protected]> 
>Sent: Tuesday, January 10, 2012 11:53 PM
>Subject: [Emc-users] Correcting for workpiece warpage.
> 
>Hello all;
>
>In carving these pcb's for the encoder, I only have room to clamp one edge, 
>the back edge of the pcb.
>
>I carved a dead flat pocket about 30 thou deep and slightly larger than the 
>raw pcb material, gouging out the left front corner so the board, when 
>pulled left & forward, isn't riding the corner, but is located by the left 
>& front edges of this plywood containing the milled pocket.  But when I add 
>a small board across the rear edge of the board and clamp down on it, the 
>front edge of the board rises slightly, about 5 thou I'd SWAG.
>
>Is there a way to couple the y axis position to a small z axis shift so 
>that the engraving tool will stay within a thou or so of the correct depth 
>relative to the warped pcb's surface by driving the z deeper as y moves the 
>table forward to work on the rear portion of the pcb.  Something on the 
>order of 5 thou down at the rear of the board, over a y travel of 1 inch, 
>would help, a lot.
>
>I've read the pages about mapping screws, but ANAICT, that can only correct 
>the same axis for errors and wear in that axis's drive, but I don't see a 
>way to make z move as y moves.
>
>Is it there & I'm too dumb to find it?
>
>Thanks everybody.
>
>Cheers, Gene
>-- 
>"There are four boxes to be used in defense of liberty:
>soap, ballot, jury, and ammo. Please use in that order."
>-Ed Howdershelt (Author)
>My web page: <http://coyoteden.dyndns-free.com:85/gene>
><Kensey> RMS for President???
><RelDrgn> ...or ESR, he wants a new job ;)
>
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