Uneven table

[dauteuil]

Hi fellow shopbotters
I have been following this forum for over a year now, but haven't posted much since my PRT 48x96 with 4g upgrade has been mostly trouble free. Bought it used a year ago. Couldn't be happier with this bot. We have been turning out stairs, furniture components and many signs with it. All turned out excellent or beyond our expectations.
We installed our first 3D sign last friday and am waiting for the next sunny day to take decent pictures. I will post them so I get some of your constructive critics.
But... a problem with the flatness of the table still persists after all my attempts to fix it. When cutting through all the way to the spoil board, a section of the table (near center) seems to be lower because a "skin" always remains in this area.
Doesn't matter if it is solid pine, furniture grade veneered plywood or as the pictures I post here, Gatorfoam.
The material is held by the vacuum pump and I resurfaced the table before proceeding with this cut of nested letters.
What am I missing or how should I proceed to remedy the problem?
Thanks in advance for your help.
Bernard d'Auteuil

[dauteuil]

Forgot to mention. The third photo shows when the cut is OK. The fourth one shows the problem.

[gerryv]

Hi Bernard, A couple of thoughts.

You may have noticed that the latest machines have a set of centre legs. That may point to the centre portion of the earlier tables being more sensitive to being "pushed down" when subject to cutting loads, especially with certain types of bits? A simple test might be to use some scrap or make a temporary hardwood leg that fits snugly against a square piece of heavy plywood under the centre of your table, do a light resurfacing of the spoil board and then see if your results change?

Another thing to consider is what's different when you're levelling and when you're cutting - vacuum? Perhaps a small gap between the spoil board and the base that is pulled closed by the vacuum?

[Brady Watson]

"Screws are bad news"

Your spoilboard 'layup' should consist of the following:

1) A good quality layer of plywood bolted to the steel crossmembers. This is your support board.

2) A sheet of Medex or MDF GLUED to the support board with wood or Gorilla glue. This is your vacuum plenum layer that you'll machine the zoned grid into.

3) A sheet of Trupan or Ultralight MDF GLUED to the plenum layer. This is your bleeder/spoilboard.

If you have screws on any layer, the material WILL buckle & you will be plagued with unreliable flatness in your table until the end of time. The ONLY way to remedy this is to rip it all off & start over. I have seen this problem over & over again...

-B

[nailzscott]

Bernard,

Do you resurface the spoil board with the vacuum on? Even though I have the standard setup (ply, mdf, spoil board), I noticed that I will have similar problems if I don't surface the board while the vacuum is on. My low spot is not in the middle but I definitely have an issue if not using the vacuum during the surfacing.

[gene]

If you have to machine the spoilboard with the vac on then there is something loose between the layers. there should be no flex in the spoilboard . when i replace my spoilboard i surface one side of the spoilboard , then flip it over and run a series of .375 holes 1.5 in deep roughly every 12 " in a square pattern. then i use the holes to insert wood dowels and glue to help hold it down. i also glue a little bit around the edges and between the zones and then after i use the vac to pull it down i put some weight on it to hold until the glue dries. When dry i surface the spoilboard and go on from there . Just the way i do it . someone may have a better way but this works for me. Hope this helps

[matt_raymond60]

I have a question, I thought the spoilboard was supposed to be loose so that you could change it out easily when it gets too scarred up to use. I have the second layer (MDF glued to the plywood deck) with my grids machined into the top with a piece of Trupan laying on top of it then the workpiece on top of that. Overall it works pretty good but maybe it could be better.

[dauteuil]

I just love that forum.
So many answers and solutions in just a few hours.
You guys are really helpful.
Indeed when I surfaced the table, the vacuum was not on.
So as suggested, there must be a gap between the MDF spoil board and the under layer (the plenum). But I did resurface the underside of the spoil board before turning it over and siliconed it to the plenum edges and zones dividers.
Then I surfaced the top of the spoil board.
But the first reply from Gerald makes a lot of sense. I did not notice how the new machines were assembled. Since this is the simplest solution and the quickest to verify the results, I will try that first.
If the problem persist, I ought to rebuild the whole table the way Brady suggested.
When I first changed the spoil board I noticed it was made of a sheet of
plywood (very dense and shiny smooth. Don't know the name of that.) with glued slats for the perimeter and zones and a bunch of evenly spaced little square blocks also glued on to create the plenum.
In the rebuild I will certainly also use Gene dowel solution. I like the idea.
So thanks to you all. Your answers are really appreciated.
Bernard.

[Brady Watson]

Carefully inspect your silicone around the perimeter of your bleeder board. There is a good chance that it's adhesive property has failed. Turn on the vacuum and place a sheet on top - build vacuum, then squeeze the perimeter of your bleeder to see if there is ANY movement. I'm not a big fan of silicone because it doesn't adhere well & is akin to placing your bleeder on a slab of jello. It's worth a look.

-B

[dauteuil]

Thanks Brady for the advise.
Will switch to glue (Polyurethane that is.)