ShopBot Decking Material

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

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

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:eek:

Best of luck!

Jeff Roltgen
Roltgen's Woodworks
www.roltgenswoodworks.com

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.:eek:

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":D

Quote:

---

Originally Posted by CNYDWW

.... 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.

Quote:

---

Originally Posted by jTr

..... 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:eek:

---

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.

Quote:

---

Originally Posted by 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......

---

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.

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

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.

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

Quote:

---

Originally Posted by 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.

---

I was wondering about the bed size. In the assembly manual there is a CAD drawing that shows the mounting of a 4' x 8' sheet of decking for the PRS Standard 96-48-6. For your 10' x 5' deck did you need to move the table cross supports on each end farther out to support the bigger deck? Also, do you think a 4' x 8' bed is good enough?

Quote:

---

Originally Posted by 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

---

I just bought the most expensive piece of plywood I could find from Home Depot. It is "3/4 inch Birch Hardwood Plywood". Cost $47. Don't know if it's Baltic but that's all they had. Sounds like the MDF could be problematic in my garage. Maybe I'll go with another layer of plywood. Wonder if the resurfacing might leave behind surface splinters.

Quote:

---

Originally Posted by dttworld

Maybe I'll go with another layer of plywood.

---

Danh - please do not reinvent the wheel when it comes to the table layup. You will be disappointed...

ShopBot officially recommends 'cabinet grade plywood' for the support board that bolts to the chassis of the machine. Then, MDF for the next layer, which will be called your 'spoilboard'. Both can be sourced from the big box stores, although I must say that it is not my preferred choice.

Glue both of these together, clamp all around the edges and put weight (lots of weight) in the center and let it sit overnight.

On my personal machine, I use 3/4" MDO from HarborSales.net, and Medex moisture resistant MDF as my spoilboard, also available from Harbor. I *always* sand the MDO paper to give it tooth before I glue anything to it. Epoxy would be best, Titebond and even white glue will work, as will Gorilla polyurethane. Use what you KNOW will work, or what pros have recommended to avoid getting the sawzall out and starting over again...

Drywall screws are the devil - when used on ANYTHING other than drywall, and should NEVER be used on your CNC for ANY reason.

-B

Quote:

---

Drywall screws are the devil - when used on ANYTHING other than drywall, and should NEVER be used on your CNC for ANY reason.

---

I agree. It was pretty sad to watch the sparks as a drywall screw destroyed the very expensive bit we used for surfacing the spoilboard. :eek:

We ended up tearing everything out and re-doing the setup as ShopBot recommends.
:(

Quote:

---

Originally Posted by bradywatson

....I use 3/4" MDO from HarborSales.net, and Medex moisture resistant MDF as my spoilboard, also available from Harbor .....

---

I should be ok with the Birch plywood because it's also recommended by ShopBot. How much was freight from HarborSales.net to NJ for something like a 4'x8' sheet of Medex MDF? May not make sense for me if I don't buy in bulk to offset freight charges.

Quote:

---

Originally Posted by dttworld

I should be ok with the Birch plywood because it's also recommended by ShopBot. How much was freight from HarborSales.net to NJ for something like a 4'x8' sheet of Medex MDF? May not make sense for me if I don't buy in bulk to offset freight charges.

---

You'll be fine with the Birch.

Overnight shipping of any qty sheet good(s) is free...You need to set up an account to get pricing etc.

-B

Quote:

---

Originally Posted by bradywatson

You'll be fine with the Birch.

Overnight shipping of any qty sheet good(s) is free...You need to set up an account to get pricing etc.

-B

---

$55 for the Medex MDF. Not bad with free shipping. I'll get 2 for the ShopBot and 1 for my mill. Formaldehyde free is probably safer than the stuff I was buying from Home Depot.

I should clarify. I used 2 5x5 sheets worth of material, but I cut them down so that I could make the support board as large as it could be. The actual size was not 5x10. I can't remember the width off the top of my head, but it spans in-between the table legs leaving about 1/4" of clearance. I did this for a number of reasons. Mainly to keep dust from falling to the floor under the table,and to have something sturdy on the sides of my actual spoil board to accommodate various means of hold down, because I don't want to screw into my bleeder board when not using vacuum.

Personally I would use either MDO, or Baltic Birch. I didn't have the time this time around, but the next time I rebuild mine I am going to coat the baltic birch support board top and bottom, with a generous amount of marine epoxy.

I no longer buy any sheet goods from home depot at all with exception to construction material such as CDX, OSB, and of course Drywall, and perhaps underlayment grade particle board, or melamine in a pinch. From my experience they just do not sell any cabinet grade material that I would be comfortable using for anything. The last time I was there there were at least 3 skids of their higher priced material material improperly supported leaving all of the sheets with 4-6" of bow. My guess is that they do not even try.

Its worth taking the time to do a nice job with a support board, since you can keep using it for a while. I did not do a good job with the spoil board I built when I setup my machine. At least it was easy to rip up when I had to rebuild it.

I am assembling my Prs Alpha 120 after some delay. I was going to snug the deck in tight between the rails but every photo shows a gap of a few inches either on one side or the other. I can find nothing in the instructions that cover this. Any ideas?

My deck goes the full width, minus maybe 1/8” because MDF swells…
I wanted to minimize the debris underneath the machine because I use that area for lumber storage.
I don’t have vacuum hold down.
You can’t surface the spoil board the entire area, so I made mine separate
pieces around the edges.

i got the good 3/4" plywood from a local plywood supplier. it's not "cabinet grade" but the material has a few sections filled in. Since it's not for appearance it's perfect and even came oversized by an inch which allowed me to keep seams to a minimum (6' x 10' bed). I only sized it per the directions so i do have a gap of a couple of inches around the perimeter. My machine came with the vac hold-down system. Silicone was used to attach the air layer to the plywood base and then also used silicone to attach the spoil board (LDF). On a side note, I always put a couple of screws in the corners of the sheet I'm cutting as an extra precaution keep it from moving at all. i sometimes cut 1/8" aluminum and those sheets are too expensive to waste.

As far as leaving a gap, IIRC the instructions specified exact sizes for everything. That's up to you. i don't store anything under the machine that will be hurt by some wood/metal dust.

I will second Harbor Sales (Baltimore area) for sheet goods, but I got my base plywood layer from Fessenden Hall in NJ. Harbor will deliver a single sheet of coroplast (that cheap corrugated plastic you see on lawn signs) for a regular customer without delivery charges. They have started to add a couple of bucks for a fuel surcharge.