Burned plenum, lesson learned...

[geneb]

Using a 1/8" downshear bit to drill holes in 3/4" plywood leads to bad, bad things.

http://www.flickr.com/photos/33471283@N05/5340963973/

Never, EVER, leave your 'bot unattended! Had I not been paying attention I could have easily lost my whole shop.

So now that I've ruined my vacuum plenum, it's time to make the new one better. :)

My plan is to remove the t-track and then "pocket" away the old plenum using a 1/2" cutter. I need to keep the zone count the same because I don't want to re-plumb the whole thing, but I want to change the material & pattern of the plenum.

I'm thinking instead of using 3/4" MDF again, I'm going to try a 3/4" 7 layer AC "cabinet" grade plywood that I recently got a deal on at the local BORG.

Is there a reason I shouldn't use that material for the plenum? I figure properly sealed with a few coats of Killz (oil based) and chased with an oil based gloss white should prevent any air leaks through the material itself. This is something I fought constantly with the old plenum because I never got it properly sealed.

I'm also planning on isolating the zones with 1/32" 3/8" gasket, similarly to how I have it now. I'm going to add 1/2" "piers" to support the gasket going into the zone so it'll support the spoil board in the center of the zone better.

Instead of having the T-Track span the table width, I'd like to break it up into two sections, inset from the table edges to eliminate the air leaks caused by having them at the edge of the table.

Any suggestions on what I can do to make the plenum better beyond what I've outlined here? Is there a favorite grid pattern that seems to work better than others? I'm planning on cutting the grid to .2" instead of the .25 the last one had - just in case I want to shrink the zone down by using an inlaid gasket material.

Thanks all!

g.

[chuckster]

I have been in the same situation a couple times. Nothing like "the smell" when one is at the opposite end of the shop! I am thinking it may not be a good idea to use Plywood, since plywood will actually start on fire if this happens again. I drill a ton of holes, but just dont use the vacuum during the process, I clamp the material to the table, and when it comes time to cut then I turn on the vacuum hold down.

[geneb]

I wasn't sure what exactly was going on at the time - I could smell a smoky odor, and I was checking really close when the 9-15 vacs started pumping smoke into the shop. I actually broke the fire extinguisher mount when I pulled the FE off the wall. :) Fortunately I didn't need to use it.

I suspect the MDF would have caught fire had it been able to get any air - turning off the vacuum cut the available oxygen supply to what it could get through the 1/8" hole that started the mess to begin with.

g.

[billp]

Gene,
After looking at your pix you WERE lucky...
I think you'll be surprised at how much better a good plywood plenum is compared to MDF. You've already nailed the major issue; trying to seal MDF (and then not breaking that seal through the course of slamming stuff onto the table, deeper than anticipated Z plunges, and rogue tool paths, etc...).
If you use your primer, and then heavily paint the plywood you should be fine.
Many of us just use multiple coats of shellac, or water based poly ,because of their short(er) drying time, but just about anything that you can hit the piece with on ALL sides (and edges!!!) will be fine...

[geneb]

I take it the plywood won't swell like MDF does with water based finishes? I've never used a water based polyurethane (Polycrylic?) on plywood, so I don't know.

The plan was to sand it with a 400 grit paper and the coat the "A" side and edges with paint, then glue down to the base table & cut the plenum. After cutting I'd then go back and paint the cut areas. I take it the same effect I'm after could be obtained using just a water based poly then?

Would you recommend coating the side that would be glued to the table as well?

Thanks Bill!

g.

[waterwheelman]

About a year ago I was cutting some hardwood parts, and I had the same problem. It didn't get to the plenum but it did cause the spoiler board to start glowing with hot embers. When I turned off the vacuum motors, the embers quickly cooled down. I later cut out the scorched section down to the plenum and replaced it with a new piece of MDF. The lesson I learned, was that when cutting hardwood, it's wise to use the sharpest bit in the shop.

[myxpykalix]

This is one reason i don't use my dust collection as much as i used to when cutting wood and do use it when working with mdf but monitor it while i am.

[billp]

Gene,
Yes ,I would coat ALL sides of the plywood. This should even out any tendencies of the sheet to warp .
The ply will not suck up the finishes the same way the MDF does, and this means minimal swelling ( if any).
I would always seal the ply, fasten it to the table supports, and then cut the plenum channels as you describe above. Then I would immediately hit the channels with the shellac, or water based coating before the piece of ply even tried to "adjust itself" to the new stresses. I used to follow that up periodically by inspecting my channels, and if there was even the slightest ding, hole, etc. I'd seal it. Once the moisture gets UNDER your surface coating it CAN swell the outer veneers.

[geneb]

I am planning on gluing the plenum down like I had the last one, so I probably shouldn't paint the bottom of it. :)

I've got two coats of Killz water based primer on it now and I'm going to hit it with a couple coats of an oil based gloss next. The good news is that I was able to get the t-track out of the old plenum without bending any of it. I still have about 1/2" of plenum left to remove - it takes a while with a 1/2" end mill. :)

g.

[tgm]

Gene,

More than us have our fair share of 'close call' and for the life of me I still don't understand why some folks insist on using a 5mm cutter so that they can cut out parts and use it ti drill holes. Even after all of the postings they still buy the tools and use them to drill holes. It can be bad enough without a vacuum table but the table only makes it worse as the 'mini forge' action starts to take place where the material meets the table. Usually when you see/smell the smoke it's already cooking.
Glad you caught in time.

Tomk in PA

[geneb]

At some point I need to get off my butt and get my air drill lined up so I can use that - or get a full set of ER25 collets that match the common drills I normally use...

g.

[geneb]

Here's a video I did on the incident. Hopefully someone will learn from my mistake. :)

http://www.youtube.com/watch?v=Ub6X8pjo5ls

g.

[gerryv]

Thanks Gene!

[geneb]

You're quite welcome Gerald. I hope this saves someone else from doing the same stupid thing I did. :)

g.

[bleeth]

OK: True confessions time!
I did this a couple of months ago to a brand new spoil board. We are one of the shops that drills and cuts with the same 5mm mortise bit. It happened while cutting center cabinet drawer dividers that needed to be drilled through for hardware on both sides in assembly. Luckily I caught it fast and only did in a bit of the spoilboard that was easily repaired with bondo. I decided to let them put one of the drawer dividers in with regular screws in the future. Changing bits every sheet takes more time (I've tested it) and standard motors just won't handle the added force moments to hang a drill out in front of the big Columbo Spindle.
Any drilling with a cutter should always be done with a peck strategy but deep drilling with a small bit is tricky.

[geneb]

Dave, I'll see that confession and raise you one. I've got an air drill installed that I really don't use. :)

Something knocked it out of alignment at some point and I just haven't spent the time to adjust it. That and it can't reach the edge of the material towards the X 0 end of the table.

g.