I need a better way to hold my spoilboard in place.

[knight_toolworks]

I ended having to remove my plenum layer as it had popped loose and when I surfaced my table then used the vac of course it was not flat. this time I used the machine to mark/countersink the screws so I could find them again.
the problem is I have glued down the spoilboard but sometimes it comes loose to I end up taking some off the plenum when shaving it down. sometimes the spoilobard is too chewed up to glue another on top.
I don't always use a vacuum I use nails or a jig depending on what I am cutting. so I am worried if I don't hold down the spoilboard and don't use a vac it won't stay flat.

[nailzscott]

I have a vacuum plenum that has some gasket material around the perimeter and between the vacuum zones (between vacuum plenum and spoil board). My spoil board had trouble laying flat as I began skimming off material during resurfacing. This also forced me to run vacuum, even if the part I was cutting did not require it. To fix my issue, I use plastic screws around the perimeter and between the zones to pull down my spoil board. From the bottom of the table, I have drilled through the birch base and plenum and tapped into the spoil board to hold it in place. When I skim off the spoil board, the plastic screws are skimmed too. I did this about a 6 months or so ago and have not had any problems since.

[carlcnc]

Steve
My plenum is 2 layers of 3/4 birch ply.
screwed to the frame,sealed with several coats of shellac. for years all I did was lay the spoil board on top and tape the edges.Using that method I usually had to replalce the spoil board because of warping more than any thing else.
Last summer I glued the spoil board with a bead of polyurethane const adhesive around all the zones with several "dabs" near the middle of each zone.covered the whole thing with visqueen and left the vac pump on for about 2 hours.
When it is time to replace this spoil board [at about 1/8 thickness],I can plane it away while only doing a skim cut of the plenum to get rid of the old glue. I figure I will only loose 3-5 thousandths of the plenum each time I do this.It
will be a few years and many replacements of spoil boards before this will affect the plenum and I have to work on it.
I am using ultralight MDF,weird that it doesn't "mushroom" when it gets wet. the water just sits there!

[Gary Campbell]

Steve...
I agree with Carl 100%. Glue the spoilboard to the plenum. Surface it off when needed. We usually cut 500 sheets before replacement.
Gary

[shoeshine]

Scott, what is your source for plastic screws?

[coach]

I use painters caulk. I run heavy beads around the perimeter and 4 zones.
I have success with this.
I did have corners curling this time when the spoiler got to be around 1/4" to 3/8" thickness.

[nailzscott]

Chris, I picked them up at the local Lowe's store - in their screw/nut/fastener bins. They were 1/4" plastic screws and I think either 2 or 2 1/2" long.

I placed the spoil board on my plenum and clamped it in place. My original holes were drilled from the bottom using the tap size drill for 1/4 screws - I think 7/32 - all the way through the birch base, plenum, and spoil board. Then I used a 1/4" drill bit (with a stop on it so as not to drill too deep) to drill through just the spoil board and plenum. Then I used a 1/4" tap in my cordless drill (slowly in and out) to tap the holes in the spoil board. Next, I turned on vacuum, put in the screws, and tightened them with my drill (not too tight obviously since they were plastic in mdf), and then re-surfaced the spoil board.

When I re-do the spoil board, I figure I will just take it off, put it on top of the new board and use the old board's holes as pilot holes.

[knight_toolworks]

I think part o the problem gluing down the3 spoilboard is it eithe3r sucks up the3 yellow glue and does not glue well or the surface peels away.
I wanted to mark on the spoilboard after I surfaced it where the glue will go with a pen then seal that part with yellow glue. that way the surfaced is hardened and sealed so it should work. but I don't have a day to wait for the glue to dry.
So I drew out where the glue will go and I will pocket everything else. so that way I have a hard skin to glue to and the skin removed where the air will flow. a little more work but lets me get going today.
I had glued my mdf plenum with a water based glue/caulk and that came loose. even with brads holding it.

[mysterium]

Gary

First and foremost I have to say "Thank You!!!" I have read hours of your very helpful & well thought out posts.

In your CNC Musings http://shopbottools.com/garysmusings.htm you indicated that you have your Tee track proud of the plenum & thus it keeps your spoil board loosely in place & by being proud of the surface it stops the spoil board from moving horizontally. I had also read that because of the Florida humidity you were having issues of the spoil board curling like Steve's problem ( what I am also seeing with table that came with my machine) but once you turn the vacuum on it lays it flat.

So my questions based on your above post, does that mean are you now recommending to glue the spoil board down? And how are you surfacing the glue off the plenum between the Tee track that are proud of your PVC plenum when you replace the sections of spoil board? Or have you now gone back & recessed the Tee track flush with the plenum?

I am shortly going to rebuild the table/plenum that came with my used PRT standard. The new plenum will be a 8 or 9 zone design based on yours & David Buchsbaum’s examples thus my questions. I am planning using the 3/8" bull nosed bit that you used 1" o.c. with a gasket around the perimeter, but it sounds like you are saying to caulk also.

And again humbly "Thank You!!!"
jim

[Gary Campbell]

Jim...
I cant say what is neccessary for your conditions, but I will say that for us to have any chance of it laying flat, I glue the spoilboard down using Titebond glue every place it touches the plenum.

When I switched to the dual Z, it required a different table configuration than I had previously. We made a new plenum, and recessed the T track slightly to allow surfacing off the spoilboard.

A note on the gaskets.... vacuum during surfacing is always much less than when a sheet is loaded. Compression of the gaskets due to differing vac levels were causing depth inconsistancies which were solved by gluing the spoilboard.

I should have printed a retraction on the column!
Also, credit for the setup goes to Brady Watson as I tried a variation from his reccomendations, but later learned he had been right all along.
Gary

[knight_toolworks]

here is a picture. this was a bit of a hassle and next time I will just mark the areas and seal with yellow glue.

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