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Mobile Torsion Box Workbench #3: Torsion Box Top

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Blog entry by Ron Stewart posted 551 days ago 4524 reads 5 times favorited 6 comments Add to Favorites Watch
« Part 2: Design and Features Part 3 of Mobile Torsion Box Workbench series Part 4: The Stand/Base »

The top of my workbench is a 4” thick torsion box: 3/4” birch plywood panels on either side of a frame of 2.5”x1.5” yellow pine members.

When designing the top, these were my primary considerations:
  • I wanted it to be flat and strong, and not too light.
  • I wanted an array of dog holes. I decided that spacing them 6” center-to-center would be good enough for my surface vise.
  • I wanted every dog hole to be in, or backed by, solid wood. That way, even if the holes in the plywood got chewed up, the holes would still be usable.
  • I didn’t want any exposed screw heads on the top of the workbench (and I didn’t want to have to drill and plug counterbores).
  • I didn’t want sawdust and chips falling into the top dog holes and filling the box. (Dust and chips—or Carolina Wrens—can still enter from the holes in the front and back, though.)
  • I knew I couldn’t drill 4” dog holes all the way through the finished top, so I decided to drill them in the frame, and use the holes in the frame as a template for the plywood panels.

This rendering shows the basic design of the torsion box. The spacing between the braces is 4.5”.

I needed one full sheet of plywood for the top and shelf (full-resolution image). That’s why my top is 1/4” shy (both length and width) of six feet by two feet. (In the beginning, I thought the shelf panel was going to be as wide as the top, but I changed my mind later.)

I needed three 8-foot 2”x6” boards for the torsion box (full-resolution image). Why 8-foot boards? I can’t fit anything longer in my Honda Element.

Building the Torsion Box Frame
I won’t detail every step, but here are some key ones:
  • Use the table saw to “mill” six 2.5”x1.5” boards for the frame.
  • Use a miter saw to cut the frame pieces. It’s easy because all 14 cross pieces are exactly the same length.
  • Use pocket holes and 2.5” screws to build the frame. Each cross member has four pocket holes (one on the top and one on the bottom at each end).
  • Use 1.25” pocket holes to attach the top plywood panel to the frame. It’s important to drill these pocket holes before assembling frame.
  • Create a template from one of the interior cross braces and used it to drill the dog holes in the other 11 braces. The trick in creating the template was marking the holes from the center out. Mark the center, measure 3” out on either side. Then mark 6” out from those marks. Then use a drill press and 3/4” Forstner bit to drill the holes.
  • Drill the holes in the front edge the saw way. Start in the center (where a hole will be) and keep measuring out 6” from there.
  • Assemble the outer frame first. Make sure it’s square.
  • Add the cross braces from the center out.
  • Glue the support blocks for the stand/top bolts, 4” back from the outside edges of the front and back.

This photo shows how I started adding the cross braces from the center. I had marked the centers of the front and back of the frame, and of two the 4.5” long spacer blocks. I clamped those blocks in place and screwed in a brace on either side. After that, it’s child’s play. Just clamp the spacer blocks beside the just-installed brace, butt the new brace next to them, clamp, drive the screws, and keep going—no more measuring or marking needed.

Here’s a close-up of the pocket holes used to assemble the frame. Note how the pocket holes intersect the dog holes. That’s not optimal, but I don’t think it really hurts anything.

Here’s the finished frame assembly, upside down. Note all of the pocket holes for the screws that will attach the top plywood panel to the frame.

And here it is, right-side up.

Drilling the Dog Holes in the Plywood Top
This photo shows how the assembled frame acts as a drilling template/guide for the plywood panels. I placed the assembled frame on the top panel (both upside down), with the bottom plywood panel on the bottom. I clamped all three pieces together, then used a corded hand drill to drill the holes all the way through the top panel, and just into the bottom panel beneath. I stacked the plywood pieces to reduce tear-out, and to position the holes in both panels. Also note how the plywood panels slightly overhang the sides of the frame.

After that, I removed the frame, put the top plywood panel (upside down) under the partially drilled-through bottom panel, and finished drilling the holes. Again, the idea of putting the top panel underneath was to reduce tear-out.

All of the above sounds a bit involved, and it was. One tricky part was keeping it straight in my head which piece needed to be right side up and which needed to be upside down. The hardest part, though, was keeping the Forstner bit perpendicular when drilling the holes in the plywood. I learned that I’m really bad at that. The holes in the top are okay. On the bottom, some are okay, and others are slightly off. It’s not a big deal, because they’re on the bottom, where they aren’t visible and won’t really affect the function. I just had to ream out the holes (from the top down) to make sure a dowel (or future holdfast or other accessory) would go all the way through without interference.

Here’s what I mean. Most of the dog holes on the bottom panel look like this:

But a few of them look like this:

Attaching to Top Panel to the Frame
This part was easy. Lay the top panel on my “assembly table” upside down. Add glue. Lay the frame on it, also upside down. Use two short pieces of 3/4” dowel to line everything up and keep things from sliding around. Then screw in all of the pocket screws.

Attaching the Bottom Panel to the Frame
This part was also easy. Flip the frame+top assembly upside down. Place the bottom panel on it. Use the dowels again to line everything up. Mark the locations of 1.5” wood screws that will attach the panel. Use a countersink bit to drill pilot holes. Then add glue, use the dowels again, and drive all the screws.

Finishing the Top
With the panels in place, all that remained was using a router and flush trimming bit to remove the slightly overhanging edges I mentioned before.

Finally, I use a router and chamfer bit, set to a very shallow cut, to bevel the edges of the dog holes (Schwarz also recommended this) and all outside edges.

Here’s the finished top, sitting on top of my old “bench.” (I did not drill the holes and counterbores for the bolts that attach the top to the stand until the stand was finished.)

-- Ron Stewart



6 comments so far

View DIYaholic's profile

DIYaholic

12982 posts in 1280 days


#1 posted 551 days ago

A very well written and photographic description of your bench build.
BTW: The bench is done very well also!!!

I’m thinking about a very similarly constructed bench. This gives me ideas as to how to go about accomplishing the design & build! Thanks for such a detailed blog of you build!!!

I’ll be following along!!! I certainly hope you don’t mind me looking over your shoulder. Just shove me to the side if I get in the way!!!

-- Randy-- I may not be good...but I am slow! If good things come to those who wait.... Why is procratination a bad thing?

View shipwright's profile

shipwright

4848 posts in 1403 days


#2 posted 551 days ago

Very clear and detailed description Ron and a fine bench build.

My only concern relates to your comment about the Carolina wrens. It made me think of who else might find this a wonderful apartment complex. It may be an idea to have removable covers on those edge holes to keep Mickey and Minnie out. They could have a field day in there especially with lots of readily available sawdust and shavings.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fiberglass trees. http://prmdesigns.com/

View jap's profile

jap

1225 posts in 659 days


#3 posted 551 days ago

very nice

-- Joel

View roofner's profile

roofner

95 posts in 888 days


#4 posted 551 days ago

I have been studying this project for several months and look at a lot of the same material. I will be following your blog closely. I don’t have heated garage so this time a year I think about projects like this. Very good design. I need a mobile solution your’s looks like a good option for me .

View Boatman53's profile

Boatman53

823 posts in 801 days


#5 posted 550 days ago

I’ve been concidering making a top like that to put on a workmate base. I am concerned about the screws you used if you ever have to drill another hole in the top.
Jim

-- Jim, Long Island, NY Ancorayachtservice.com home of the chain leg vise

View Ron Stewart's profile

Ron Stewart

27 posts in 1109 days


#6 posted 550 days ago

Thanks guys.

Paul: I did wonder about mice, but I don’t anticipate any problems. The cat that adopted us spends a lot of time in the garage, and she’s a good hunter. Also, the cavities are narrow tunnels, and I think I could use my hand to plug the hole on the back and put the shop vac hose on the corresponding hole on the front and clean out the cavity pretty thoroughly.

Jim: Yes, that’s a valid concern. I hope I never need to add any more holes (and I’ll be happy not to see a Forstner bit for a while). You could skip the screws and just use glue, or you could position the pocket holes more strategically to leave some open area for future holes.

-- Ron Stewart

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