Spoil board not staying flat.

[erniek]

I have noticed that the spoil board will lose it's flatness over the course of one day of use.
I am using 3/4" MDF as a spoil board,it is glued down. I surface it flat in the morning and in the evening it is out maybe 0.02" in spots. I mostly cut melamine cabinet parts so this becomes a problem when some of the parts are left with a bit of a melamine onion skin from not being cut deeply enough. I thought that it could be the table not being secure enough so I built a wooden support down the middle. Then I thought it might be the hot air from the vacuum pump so I put a pipe out past the edge of the table. Now I'm thinking it might be the MDF taking on moisture from the air that's being sucked thru it and swelling up in places. Any ideas or suggestions would be welcomed.

[daski]

zero to the table, then cut into the table 15 thou.

[ron brown]

You have been observant Ernest,

I haven't had a good rant on MDF and its' qualities in a while. When I try to find something good to say about MDF, my memory fails.

Here is as close as I can find to a stable spoil board: Epoxy coat your 'sleepers' on three sides and bond to the cross-pieces leaving the uncoated side up. After these cure, surface the top edge and epoxy coat that, let at least partially cure before you place the 'pre-coated' stable plywood down and bond to that surface. Add a second layer of edge-coated ply using epoxy as the bonding agent and let cure. Surface this flat with your spindle/router and a large bit. (I like the large Her-Saf indexable bits for this, expensive for first purchase, cheap to run.)

Now you have as stable of a wooden surface as I know without using individual layers of vertical grain stable wood and epoxy between each layer. When you drill the penetrations, be sure and coat the grain where they go through.

A table so made will stay flat for extended periods. Even so, after a long period of no use, mine needed re-flattening.

There are some aluminum filled urethanes that claim to be better than epoxy for moisture exclusion. I have a quart of such a product and plan on using it in a future product. Eventually, I may use it as a table under-sealer. That will be AFTER I find out if it really works.

As a side note, I am told the filler in epoxy responsible for its' water exclusion properties is Mica. The microscopic 'plates' require any moisture to go through a 'maze' before it can effect the substrate.

Ron

[dray]

Did you surface both sides of the spoil board?

[beacon14]

What is the table construction under the MDF layer?

[erniek]

Thanks for the tips Ron. I may have to rebuild the table from the bottom up. The first layer is 3/4" lacquared plywood. I surfaced the layer of lacquar off the plywood and glued a layer of 3/4" MDF to the plywood. This layer of 3/4" MDF has the channels cut into it for the vaccuum distribution. Then a layer of 3/4" MDF with both sides surfaced is glued on top and this layer is used as the spoil board.

[erniek]

I should add that I sealed the edges of the MDF layers with white wood glue.

[joe]

My experience with substrates is a little different than my good friend Ron. He should post more often.

I think there are two different issue's.
Consistant material thickness.
Swelling due to moisture.

I've found .75" HDO to be evenly flat.
HDO is water proof. Which means it stays flat.

This material is made for the concrete industry to be used for building forms. It has 18 layers. It cost $60. for a 4'X8' sheet. The front and back surfaces from are very slick and if it's to be used with a vac, holes would need to be drilled.

Happy Routing

Joe Crumley
www.normansignco.com (http://www.normansignco.com)