what a day - table build suggestions?

[ddrumguy]

well i am finally hitting the brad nails that we used to hold things together on my table. I know (bonehead move) we put this together a few years back and were scrambling to beat the glue drying between layers. Needless to say looks like i'm gonna have to start over as i cannot remember if we just used nails/screws on the plenum or also embedded in the trupan and i really don't want to deal with any more fires from embedded nails/screws.

i've already got the docs from shopbot on how to do the plenum/table build but since i can't get replacement wood til next week i thought i would reach out to anyone that cares to chime in on tips, tricks, suggestions or latest thoughts on doing plenums/vac tables etc.

I am planning on redoing the table just like it is now with (2) 3/4 birch panels for the base, one 3/4 mdf panel for plenum and one 3/4 panel trupan for spoilboard.

i plan to make a new plenum before tearing it all apart so it is ready to go and while the machine is still currently capable of cutting it. shopbot folks tell me they usually have base and plenum wood installed before cutting the plenum - would it be better to do this way instead?

also looks like my vac holes dont line up with their drawing so i may have to redo some plumbing or take some very careful measurements to adjust the file to match mine.

[coryatjohn]

I installed the base sheet, then cut the plenum after that. Just be really careful about your Z. The last thing you want to do is cut through the base sheet and hit a steel cross member.

On the plenum, the channels only need to be 1/8" deep. Cutting deep channels into the base sheet only weakens the table. It does zero in regard to hold down or airflow.

Be sure to seal the plenum as air will leak through the sides of the base sheet if you don't do it well enough. I used four coats of heavy urethane floor finish to seal the plenum. It was nice and shiny when finished. Your spoil board should be well sealed on the edges as well. I suggest sealing the edges of the trupan sheet before gluing it to the plenum and be sure it is well sealed or you'll have a significant leak from that source.

A final suggestion would be to use 2" flex pvc instead of rigid pipe. It's about 100 times easier and requires almost no exact measurements or cutting. It attaches just like rigid pipe and is impervious to collapsing under vacuum. It is a bit more expensive but is well worth the effort. As a side benefit, the flex PCV will have a lot fewer (like zero) elbows so all turns will be gradual and cost very little in resistance in the flow.

[ddrumguy]

Thanks for the response, some good tips there. I hadn't thought about the depth of the plenum cuts if i recall mine were pretty deep (someone else cut the plenum for me when i bought the machine). i just used titebond glue on the sides of the plenum and the trupan, seems to work fine.

Few other questions / thoughts if anyone would care to chime in

- Currently wondering if the 2 sheets of 3/4 birch are necessary? it is a loss in overall Z to some degree. i see the shopbot doc only calls for one sheet for the base.
- Also wondering about using hdpe or machine grade pvc (instead of mdf) for the plenum. Any value in that?
- Is there anyway to hold down the trupan instead of gluing it to the plenum? oring material, gaskets? just trying to think outside the box if im going to have to redo this thing

I am now cutting on the other end of the table until i can get a final plan together

[coryatjohn]

I used 2 sheets of high quality birch plywood. The top sheet is where the plenum is cut, then on top of that, a layer of trupan. I have a 12" Z so an extra 3/4" wasn't missed. For a 6", that might be a lot. Three layers of base and a trupan layer would seem excessive. That's 3" of wood. A lot.

I don't think it's worth the expense to use plastic at 10x the cost. Good quality (not home depot) birch plywood seems to be fine. Remember, the machine has a lot of wiggle to start with so overdoing one aspect will generally only make it more expensive, and not more accurate. If you had a welded all steel frame and beefed up bearings and guides, maybe but an aluminum bolted together machine is never going to be that rigid.

Gluing the trupan is the standard way of doing it. You want to be sure that the trupan sheet is solidly attached to the plenum so it won't bulge or move when the humidity changes.

There are certainly many ways to skin this cat.