ShopBot Decking Material

[dttworld]

Hi All,

3 questions about building the 4' x 8' deck for my ShopBot.

1) The assembly manual suggests first layer should be "3/4 inch shop grade or cabinet grade plywood or better (birch, maple, poplar.... hardwoods)" attached by carriage bolts to the frame. Is this plywood something I can buy from Home Depot or Lowes? I remember seeing expensive plywood that was really smooth on both sides.

2) For the next layer it suggests gluing 1/2" or 3/4" MDF and then leveling using the TableTop Surfacer Virtual Tool. Would anyone recommend screwing down this layer with counter sunk screws instead? This would allow me to remove the MDF or carriage bolts if ever required in the future. Example: maybe I want to add mods later for a vaccuum table.

3) How do you'll attach the final spoiler board to the bed? Seems like using screws from the top down would eventually wear out the middle layer MDF. Wonder if long screws from the bottom up would be a crazy idea.

Thx

[CNYDWW]

The reason it's suggested to glue the spoil board down is it helps combat expansion and contraction of the bed materials. I bought my machine used and it has a 9' 6" x 5' 6" bed. Two layers of 3/8" HDO plywood then my spoil board glued down and i still get some variation. The machine came with the HDO already in place with a vacuum plenum on top. That's partly due to my material being particle board. I find it takes screws for hold down much better then the junk mdf that's much more expensive.

In your case, you can get 3/4" hardwood ply of any material, oak, maple, etc etc. I would prime it to seal it against moisture and recess holes for the heads of the carriage bolts. make sure that the head sits just shy of the surface. Trick with carriage bolts. The holes that you drill all the way through the plywood should be the proper size of the bolt shank. The square base on the heads will sink and lock into the hole in the material. Keeping the bolt heads under that surface will let you mill off all but a small amount of the spoil board when it comes time to removing the bed when you upgrade. Leave an 1/8" or so of the spoil board then give the bolts a smack with a soft head hammer from the underside and they should pop right through.

When gluing everything down, use a lot of weight and sink screws to help with clamping. Pile whatever you can on the board, the more contact and pressure the better. Once done, remove the screws and surface the spoil board flat. I usually then check my router to tune it perpendicular to the board to get a good flat finish cut. I also run a file that drills 1/16" deep in various places on the spoil board. Before i surface initially i run it at 0 being 1/16" above the surface to make sure that milling it 1/16 will take out all variations. I'll run it again after to set the reference marks. When i fire up the machine for the day i'll run it with a typical 1/4" bit. If i hear it cut anywhere, that means i need to resurface.

Regards
Randy

[jTr]

Dahn,
Welcome - I just completed setting up my new bot a few weeks ago. This is what I did after condensing many searches on this topic in the forum:

1> 3/4" MDO plywood. Not hard to find - smooth surfaces, made to be stable outdoors, recommended by many for first layer.

2> 3/4 MDF is fine for layer 2. Per Brady Watson, NO SCREWS! Per Gary Campbell: No water base glues.
2 part epoxy optimal, but expensive. I used plastic resin glue as it is readily available, lots of open time to assemble, and it reacts with the water you mix with it, unlike pva's which have a harder time setting up sandwiched in that large surface area. Use lots of weights or bent cauls to distribute pressure. Spread glue on BOTH surfaces, using a foam roller. End result: structural grade rigid glue line nicely complimenting the properties of the MDO.

3> NO SCREWS! Glue on MDF same steps as above. Many other options beyond scope of your question, especially if you are contemplating vacuum hold down. I advise you to research this option - just got mine runing this week, and WOW! I would never be without it for sheet goods.

In the end, you have a very thick layer that can be infinitely machined flat. OR: layer 3 can be machined into a vacuum hold down grid, and even include pockets for aluminum T-track.

When thickness is diminished to a point of concern, glue on another layer. This way, you always have option of changing your methods of hold down with less liklihood of a complete tear down of the table.
By the way, router bits break when you hit screws

Best of luck!

Jeff Roltgen
Roltgen's Woodworks
www.roltgenswoodworks.com

[myxpykalix]

I think all i have is one layer of 3/4 ply and one layer of mdf. Keep in mind that for every layer you put on you reduce your Z height by that much.

What i would do is attach my first layer of plywood with roundhead bolts countersunk into the plywood to your frame.

Then i would run one of the many plenum vac designs into that plywood face. Make your hole for your vac hookups in the table (included the plenum file) and attach your connection to the bottom of the table for a vac setup.

Lay your mdf on the table and use a temporary hold down so that you can take a thin layer off one side that will be the mating side to the face of the plywood where the plenum is cut.

Use titbond glue to the face of the newly cut surface and the plenum and apply together. Take several cauls and clamps to hold down the plywood and mdf. Let dry.

The idea here is to do all the setup work for a vac table so that 6 months from now when you decide you want one, you don't have to skin your mdf off and do all that from scratch. All you would need to do is attach your pvc line for your vac table and you are done.

Also when you do get around to making the vac table take a tube of silicon caulking and apply to the thin edges of the plywood and mdf to seal them up because you lose a lot of vac from the edges.

After the plywood and mdf are bonded take and run your surfacing routine so that your carriage and table surface are parallel to each other. If for some reason you feel compelled to use bolts to hold the mdf down (which we all discourage) because it will allow the table surface to "pooch" in the center, you could always use those nylon bolts that connect your toilet seat to the toilet.
From there you should be "In like Flynn"

[dttworld]

.... In your case, you can get 3/4" hardwood ply of any material, oak, maple, etc etc. I would prime it to seal it against moisture and recess holes for the heads of the carriage bolts. ....

Good point about the primer. The A.D.D. in me would have just thrown a piece of plywood onto the bed. The ShopBot is in my garage so there are times of high moisture during the summer. I'll probably hook up a portable AC unit to reduce humidity.

[dttworld]

..... In the end, you have a very thick layer that can be infinitely machined flat. ....By the way, router bits break when you hit screws

I see the big picture now. Basically no screws because the spoil board is periodically machined down to create a flat surface. It's also aparent that I need to invest in a dust collection system.

[dttworld]

.....I think all i have is one layer of 3/4 ply and one layer of mdf. Keep in mind that for every layer you put on you reduce your Z height by that much.

What i would do is attach my first layer of plywood with roundhead bolts countersunk into the plywood to your frame......

OK. Think my approach will be to attach a 3/4" layer of plywood. I'm gonna skip the other layers and come back to them later after I do more research about vaccuum tables.

After I attach the plywood I'm going to assemble the rest of the machine so I can start messing with cuts on a temporary spoil board clamped to the plywood.

[myxpykalix]

ok well we will be right here when you have your next question....

[michael_schwartz]

I recently redid mine, and I used.

2x 5x5 sheets of 3/4" baltic birch plywood cut to make an oversize support board spanning between the table legs, measuring about 104" long. I joined the two sections using biscuits. I bolted this "support board" to the table using every available bolt hole, which was somewhere in the neighborhood of 50.

Prior to bolting this the layer down, I sealed both sides of this "support board" with a coat of alcohol cut shellac, followed by two generous coats of waterborne urethane applied to the bottom, and edges. I did not apply a coat to the top, since I would have needed to strip it off prior to gluing down the next layer.

For the next layer, in my case the plenum, I glued down a 49x97 sheet of regular density MDF to the support board using weldwood plastic resin glue. I scuff sanded the top prior since I applied the coat of shellac. I routed a grid on the bottom of the plenum board, and used vacuum to clamp it down, rather than using a million clamps, and weights. I borrowed this idea from David Buchsbaum.

Since I decided to add a vacuum table, I then routed my plenum into this layer. More on that later.

If I were not going this route, I would have glued a second sheet of regular MDF the first.

With my first spoil board I made this mistake of screwing my two sheets of MDF to the support board. The sheets will eventually curl, and lift in-between the screws. I had to resurface often due to this effect, and even still I don't think it was possible to maintain flatness. I would not be surprised if this is a common mistake.

[steve_g]

Danh

I second the Baltic birch recommendation... It's the most stable material I have used. I don't have a vacuum hold down and use Baltic birch even for my spoil Board. Unless you intend to Air condition 24/7, you'll need to resurface an MDF spoil board every time you use it due to uneven swelling (and even some times in the middle of a long day!)

Steve