Plenum Issue

[sevans]

Hello. I just removed my spoil board this evening for replacement. There were some torn areas in the MDF plenum where the silicone pulled away, which led me to surface the plenum. I removed about .02" in the 1st round, and realized there was a huge dip around 28,75....a large area of it. So I zeroed there and took off a little more. Two questions here: will this surfacing of the plenum significantly affect my vacuum hold down? Also, why the dip? When did this happen. How can I avoid this??? Thanks much! -Sarah

[Gary Campbell]

Sarah...
You dont say how thick your plenum is/was. Thinner spoilboards do not have less hold down, but they are prone to allow more leakage with the same open area.

What this means is that when completely covered prior to cutting, both will have the same vacuum, but as you cut thru your part(s) your vacuum level will reduce quicker with a thinner spoilboard.

Somewhere between 3/16" and 1/4" is considered the minimum effective thickness for Trupan.

Your "low spot" can be caused by a slight bow up in the spoilboard during the initial surfacing. When the spoilboard is not covered you may not develop enough vacuum to suck down a bow in it. Humidity changes and regular use with material covering the spoilboard allow it to sink down to the plenum. It also could have simply warped due to humidity change.

To try and avoid this:
Eyeball the sheet and put the bow up to surface off on the 1st pass.
Gary

[beacon14]

Gary, Sarah was talking about surfacing the plenum layer, not the spoilboard.

Surfacing the plenum should not affect your vacuum hold-down unless you significantly lessen the depth of the grooves, in which case you can re-rout the grooves. A few hundredths shouldn't make a difference.

Dips happen - was anyone standing on the table to work on wiring, etc.? What is the table construction? Unless is moves repeatedly and a lot I wouldn't worry about it. When you surface your spoilboard any non-flatness (within reason)of the table becomes a non-issue.

[sevans]

Perfect! I did not take too much off. Yes I have stood on the table many times. I guess I didn't know that was a bad thing. I'm a pretty small girl, or am I? Anyway. Thanks so much guys for the good advice. I have had so many problems with pocketing (uneven cuts, etc) and I think I've found the culprit....or at least a fraction of it. I'm praying that surfacing the plenum can aleviate some of this. Over and out, Sarah

[Gary Campbell]

Dave...
Thanks for saving my tail again... when its after 11 PM I better learn to read the post twice!

Sarah..
Sorry if I confused you.

Gary