HDU holdown

[conceptmachine]

Just curious to what method's everyone uses to hold down hdu.Will a vac table hold hdu down? So far i've used carpet tape it does good on smaller signs but is a pain with large signs.
--shawn

[donchapman]

In the past, I mostly used screws from under my MDF table into the back of 1.5" HDU, but I now use my inexpensive Fein vac hold down system, which works great because HDU is closed pore so air doesn't pass through it, plus I can do double-faced signs with no screw holes.

[jhicks]

We have had good results holding HDU on our vac table but I suppose the zone size and material size are key to success. I think you can get very good results especially since it cuts so easily with little lateral torque. I would suggest either closing off or sizing an appropriate zone to get maximum holding power. Maybe even priming 1st to get a smooth surface on the vac but its really not necessary. After that cut or profile the outside cut out as the very last step and leave an onion skin to either cut in a shallow final pass or even remove and break away from base stock. Its very easy to break away parts by either running through a wide belt sander or manually removing the part and sand edges. You should not have any problem doing this on a shop vac.
But as always, run a few tests on scrap before diving into the real thing.

[conceptmachine]

Don,Jerry,
Thanks for the advice,what size zones do you use? Since signs vary in size would a 4 zone setup be sufficient?
shawn

[donchapman]

I routed the standard 4 zone grid into 3/4"MDF using the file provided by ShopBot and can make any of the 2'x4' zones smaller by weaving gasket material in the waffle grooves wherever needed to seal the particular size piece of HDU, plywood, MDF, etc that I happen to be routing. This system has worked fine for all the different material sizes I've routed from about 4"x10" to 4'x8'.

[jhicks]

We set up multiple zones shown here in 11" x 46" and 11" x 58" on the ends. The entire bed is controlled with 2 shop vacs and has 8 zones. Each section has Allstar Adhesive .062" thick gasketing around the perimiter to isolate each from one another. The grid is our plenum grid cut with a 3/4" ball nose 1/2" deep in 3/4" mdf in squares approximately 3/4" ID. Then the plenum is sealed with shellac top, bottom, and sides. Trupan is layed on top and vacuum sucks through it quite well. On top of that is another sheet of trupan we level and surface repeatedly until too thin, then replace it. The Trupan on top of the plenum is never cut or screwed into so we never have to refabricate the base table, only the spoil board on top of it.
Colors show single and combined valved sections which are controlled independantly or all at once for full 5' x 10' sheet. The 6 zones on the left and right each have their own shop vac over the 60" x 60" surface and can also be joined with a shut off valve connecting them together for full 60" x 120" table.
(3 yellow & 3 Pink are one valve each but ported through 3 sections so any one can be covered with PVC sheets independant of the others)This means two lower yellows, two lower pinks, and all 4 blues = one 4x8' sheet when we seal off top yellow and pink sections
When using material smaller than a single zone, we simply lay down an 1/8" sheet of PVC to cover the area not under the stock. For solids, we lay down a sheet of melimine 12" x 96" and suck it down, then screw the solid on top of it so we never put screws into the spoil board.
Net effect is a versitile vac table to run many sizes from 12x48 all the way up to 60" x 120 and all the combinations inbetween. Hope this helps but for us, it works great.

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