[ssutton]

Hi Guys,
I have a question about using MDF as a sacrificial surface. My table is set up with a 2 zone vac system. The manifold sheet is planed flat and drilled with 1000 1/4" holes and a sheet of 1/2" MDF is on top of that. The vac draws thru 1/2" MDF and works great, if the material is at least 12" X 24" it is nearly impossible to move it on the table. Here is the issue; in my little experience with MDF it seems to be doing something strange, during the heating of the day and the cooling of the afternoon the MDF curls, this curling effect causes a loss of vacuum pressure and a non-flat surface. Currently, my MDF is not attached to the vac manifold, but is held in place by the vacuum itself, that way I have the option to remove it.

1. If I bolt the MDF to the vac manifold I know that it will not visibly curl but in your experience does it truly remain a machine flat surface?

2. Almost all of my cuts are profiles, cutting out parts from the sheet, so how much do you guys normally cut into the sacrificial surface to release your parts?

Obviously, the less that you cut into the sacrificial surface the longer that it will last, but if the MDF is curling say .025 over a 4 FT length then it must be compensated for in cut depth.

Anyone with direct experience please comment, I want to optimize the sacrificial surface so that I do not have to continuously change it out.

-Scott

[ger21]

On our 5x12 machine at work, we cut .003" into the spoilboard, and resurface it when small parts start to move.

The curling is due to changes in humidity.

If the corners are curling up, just flip the MDF over, and it should flatten out easier.
If you have enough vacuum, it should remain very flat with vacuum applied.
As soft rubber seal in a channel around the perimeter may help to pull it down better.

[Phife]

Try surfacing both sides of the MDF.

[Pplug]

How about a dehumidifier?

[mlabruyere]

After you resurface the MDF you could put a coat of sealant on it to keep it from absorbing humidity if that's your issue.

I found that when I surface a sheet of MDF it'll curl right away due to the difference in tension between the surfaced side and the non-surfaced side.

I drill through the MDF and attach it to the bed surface. Make sure that the bolt heads are at least .25" below the surface to start. Then I surface the MDF....stops the curl. I don't use a vacuum on the bed itself so I don't know if it'll actually hold a vacuum or not.....sorry.

[Connor]

After you resurface the MDF you could put a coat of sealant on it to keep it from absorbing humidity if that's your issue.

I found that when I surface a sheet of MDF it'll curl right away due to the difference in tension between the surfaced side and the non-surfaced side.

I drill through the MDF and attach it to the bed surface. Make sure that the bolt heads are at least .25" below the surface to start. Then I surface the MDF....stops the curl. I don't use a vacuum on the bed itself so I don't know if it'll actually hold a vacuum or not.....sorry.

You can't seal the MDF if your going to pull a vacuum through it.

[EBC]

You can't seal the MDF if your going to pull a vacuum through it.

...but you can seal the edges.......(chair)

[Connor]

...but you can seal the edges.......(chair)

But, I don't think that's going to help very much on the MDF curling...