Vacuum table flatness issues

[3axisinc]

We have a 7 year old PRS Alpha that we have just about run the wheels off of. We use it a ton and have done lots of 2D and 3D work on it.

We just moved buildings and are having major issues with the bed of the CNC now. Previously the bed was constructed as:

Layer of 3/4" MDF for spoil board
Layer of 3/4" Polyethylene for the plenum
(as seen here, we removed the spoil board for the project for the extra Z clearance)
Layer of 3/4" Baltic birch plywood for the plenum support.

We had the Polyethylene bolted though the plywood into the supporting angle iron rails. The MDF spoil board was then caulked and screwed to the Poly/Ply plenum (screwed from the bottom, so our spoil board was limited to using about 3/8" because the bottom 3/8" was screwed into.)

Our new setup is:
Layer of 1 1/8" MDF for spoil board
Layer of 1 1/8" MDF that has been sealed and secured as the plenum

the top spoil board was then caulked only to the bottom layer. We don't have to use screws this way as the top layer of un-sealed MDF 'glues' to the bottom sealed layer. We now have all 1 1/8" of spoil board.


Now to the problem. We can surface the spoil board, and then next run a part file - setting the bit to cut .005 (like we used to) and it won't cut all the way though in the center of the bed. We have even tried setting the bit to .015 over and still not cutting though. It's like the center of the bed is at a different elevation after surfacing and then placing material on it. Yes we surface the bed with the vacuum hold down pump on.

the only other change is our vacuum hold down pump. We switched from a 'Roots' type rotary screw to a rotary vane pump. Our old system would pull about 8" to 9" of mercury and the new system will pull about 13" to 18". Lots more pressure for small parts.

we have tried everything we can think of. We have been scratching out heads as to what might be causing the problem because after we surface the bed - the bed has to be deflecting in the center - and we can't figure out why.

any thoughts??

thanks-
Brad.

[jerry_stanek]

I ran into a problem where the center was not glued down and there was a small gap between the plenum and spoil board. When I would put a full sheet on the table the center would pull down and the tool would not cut all the way through. I ended up removing that spoil board and regluing a new one down using the vac and a sheet of sintra to help suck it down until the glue is dry

[2006prt]

make sure you are running the vac when you surface the spoil board

[jerry_stanek]

make sure you are running the vac when you surface the spoil board

Even running the vac won't always suck the spoil board all the way down. It helps but still there may be a small gap between.

[3axisinc]

As i mentioned in the original post - we surface with the vacuum pump on.

When i set the spoil board here is the process I use:

surface .01 off both sides of the new MDF spoil board
remove the old spoil board
apply thin bead of silicone - (line of about 1/8")
set spoil board down with help to avoid dragging the silicone
set a sheet good (usually 6mm Sintra) on the new spoil board
turn on the vacuum pump
let it sit with the pump on for 2ish hours.

(BTW - our new pump will pull 20" of mercury which is a LOT of pressure when the spoil board is completely closed off with a new sheet. As we cut though the pressure will drop to 13ish".)

b.

[jkaras]

If you surface with the vacuum table then you have to run it with the vacuum table.

[CNYDWW]

Cheap paint can be your friend. Latex paint (yes the mdf will swell a little). Paint the spoil board lightly. Create a pocket run an offset mill from the center out. The paint should help the spoil board stay down in the center. Milling it first should take care of your issues.

Regards
Randy

[Brady Watson]

remove the old spoil board
apply thin bead of silicone - (line of about 1/8")
set spoil board down with help to avoid dragging the silicone

This may be your problem. I have seen & corrected this on a few machines in the field. Unless you have also applied a bead to the center of the plenum (like in between zones), you will be left with a floating gap between the bleeder board & plenum that will never go away. Surfacing with the vacuum on is a moot point because I doubt it has enough pull down to overcome the difference between the sealant bead (probably greater than .005") and the center, which is now floating - UNTIL you get something on top of the bleeder to start building some real pressure. This of course prevents you from doing a proper surfacing.

One of the key things you must always remember with vacuum is, whatever you do to the perimeter, you must also do in the center - or you will have a concave surface.

If possible, slip a drywall/putty knife in there & work the sheet off. A utility knife may also help. Then clean off all the sealant. Either use the Bot or do it by hand with a scraper. Then re-apply sealant, including the center areas where you deem prudent to reduce the chances of dip in the center - if you insist on even using a sealant.

I gave up on using sealants, caulks and gasketing (except for tiny or special production parts) - years ago. It seems that nothing is as consistent as using nothing at all - flat material to flat material. Then focus on sealing those edges with foil tape, 50/50 mix of wood glue & water or any jungle juice or paint you want to use as a sealant. To keep your bleeder from walking, drill it around the perimeter with an 1/8" bit (deep enough to go into plenum - say 1.06") and then countersink for screws .75 to .875" deep. I like the 1" Kreg pocket hole screws for this from Lowes. Space them 18" or less apart & you should be good.

-B