Infeed/outfeed table surface construction questions.

[Jim O'Dell]

OK, I'm getting close to starting the multifunction station wall units for the shop. For background, this will be 2 cabinets, each 121" long and 21" to 23" deep. The in/outfeed tables at the front of the cabinets will be 13" deep, then a 4" or so bump up for a built in fence with T-track embedded into it for stop blocks, tie downs and feather boards, and 8" to 10" surface back to the wall. (I have a 6" DC pipe that comes down the wall and goes into the cabinet to port the many different tools that will reside in the 3 1/2' gap between the benches.)

Will a double layer of 3/4" MDF with a skirt on the outside edge, and a 3/4" MDF stiffner daddoed and glued to the middle stay flat enough? Or should I build a torsion box with 3/4" MDF on the skins, and 1/2" MDF as the internal structure? Can I get away with painting the underside and back edges to seal it, or will I need to use laminate surface on all edges to keep it from moving? I'm leaning toward the torsion box design as I think it will solve some of my design problems for how to brace and adjust the top to keep it flat. (Like virtually eliminate the need for it:D ) My cabinet structure will be 3/4" plywood panels cut to the profile of the bench shape (front to back) with 2X4 material daddoed to form a frame around the plywood. Kind of like a wood I-beam is constructed, only enclosing the ends as well. (I've used this to build platforms in the van to raise dog crates up off the floor for storage with great success. Strong and lighter weight than anything else I could come up with.) I will use doubled 3/4" plywood to join these individual frames and give a relatively flat surface for the top to sit on, and plan on 24" on center with them (6 panels for a 121" length of bench.). If I were to use the doubled thickness MDF, I'd want some sort of screw adjustable leveling method that starts to get real complicated, but can be done. The top will be the last thing that goes on but I need to know the thickness of the top to plan the braces.

So, any ideas? What have you used that has stayed flat over the years?
Thanks! Jim.

[Tyler Purcell]

A torsion box would be your best bet for a truly flat table that will stay flat forever. However, MDF is fairly sturdy provided that you seal it up properly. MDF is like a big sponge, so it will take many coats of whatever you use to seal it up with. I like to use shellac, but I have also used a 50/50 water and wood glue mixture to seal it as well. Either one will take many many coats so be ready for a day of sealing.

[Jim Becker]

You need at least a frame under the MDF to keep it level...with or without the torsion box technique.

[Jim O'Dell]

Thanks for the quick responses guys! Jim, think of the panels I'll construct as the sides of a normal base cabinet, then the doubled 3/4" plywood, probably 3 to 4" wide, could be the face frames, top and bottom, front and rear. It just wouldn't have a back on it. Instead of individual cabs, it will make one large cab. I've even considered putting braces across the top, front to rear, between the panels to give contact points like five 24" wide cabinets would have. Would this be adequate for the framewwork? I wish I could get myself to sit down and learn Sketch-Up, so I could "show" what I'm talking about.:) Visuals on this would be soooo much better!

Tyler, I'd probably use 2 or 3 coats of Kilz primer to seal it off it doesn't have to have laminate. It has sealed wood very well for me in the past. Jim.

[Jim Becker]

24" OC framework under the surface may be fine for infeed/outfeed use, but I'd still go 16" OC for those strips.

[Howard Rosenberg]

I wouldn't use two sheets of 3/4" MDF though.

But a single skin with a skirt and ribs will be just as effective.

One 4 x 8 sheet of 3/4 MDF weighs 96 pounds!

If you ever need to move you OF table, you're gonna need to call every friend neighbour and anyone else you've said hello to in the past 25 years to move it!

You could do tis with 1/4 melamine coated hard board, squared off every six to eight inches and you'd be fine.

But I WOULD use 3/4" plywood for the skirt and ribs though - it'll take screws better - best to think of any longterm mods you'll make.

My 2C
Howard

[Jim O'Dell]

Jim, if I put a cross piece between each panel, then I'll have support every 12". Trying to put them at 16" would be more trouble.:D
Howard, the torsion box tops will come off just like they go on. In fact the whole thing will unbolt and and break down for moving if need be. Although it's doubtful I could fit them in anyplace I were to move to, but it's possible to cut them down the way I'm building them.
I've got some pictures of the parts that I'm about to put on my shop rehap thread if you're interested. Jim.