Dining/Game Table - #10 Building a Flitch Beam & Installing MDF Playing Surface Support

The height of the table is fixed so it is comfortable to sit at, the bottom of the rails are fixed to allow for leg room below and the depth of the playing well is also fixed.  What remains is the depth of the structural support for the playing surface.  The perimeter pieces are screwed to the inner rail which will provide stiffness but the intermediate pieces are on their own.  With a span of 42” and only 1¼” wide they are not stiff enough to provide adequate support for the playing surface to feel solid.  The fix is to borrow a technique from building construction used in the houses, decks, and other wood-frame structures called a “flitch beam”.  It is typically made up of a steel plate sandwiched between two wood beams held together with bolts.  Now I will not be holding up a house but the principal is the same.  I am going to add a 1/8” thick by 1” wide piece of wide aluminum in each of the double cross pieces.  Here is what a completed assembly looks like along with the pieces need to make the second one. 

Flitch Beam & Component Parts

I use 7 screws to hold the assembly together.  Four on one side and three on the other.  Once I have the parts clamped together with the ends squared I laid out the screw locations then drilled a pilot hole down through the aluminum.

Small Pilot Hole

Next is to drill the remainder of the pilot hole and a countersink so the screw will be slightly below the wood surface.

Countersink for Screw

Last is to drive the #6 X 1 ½” flat head screw in. 

Installed Screw

With that done I could start assembling all the individual parts into the structural assembly.  I used a single #6 X 1 ½” screw at each end.  For the pilot holes I drill a small pilot hole then a counter sink and larger pilot hole in the outside piece.  Last is to drive the screw home.  Here is the setup for putting it together.  I have all the pieces held in place with a couple long aluminum clamps and a nylon band clamp.  The pipe clamp on the diagonal is to assure that everything is square and stays that way.  On the left there is a marking gauge, a drill with the pilot hole bit, another drill with the countersink/pilot hole bit and an impact driver for driving the screw in.  The flat plate vice-grip style clamp at the far left front is to assure the top and bottom edges are flush.

Structural Support Assembly

Once I had all the pieces screwed together I set the assembly in the table frame, centered it and cut the first piece of ½” MDF which will provide the base for the playing surface.  This is what that looked like.

Bottom Layer Structural Support in Place

With the MDF in place I did some testing by leaning on the surface to see how much it flexed.  I had hoped that the flitch beam assembly would be rigid enough to help the adjacent cross-pieces resist deflection which they did.  However, it was not enough and that means I will be adding the aluminum bar to the other three intermediate cross-pieces.   The process will be very similar to the three-piece flitch beam I had already built.  I started by laying out the screw locations and using the drill press to drill the pilot holes the aluminum and into the wood.

Setup for Drilling Pilot Holes in Aluminum

Next was to use a drill the same size as the outer diameter of the screw threads for a clearance hole which only goes through the aluminum bar.  Last the screws are installed from the aluminum bar side.  The screws need the underside of the head to be flat so it fully bears against the aluminum bar like the screw on the left.  The one on the right is a typical flat head screw and would not work the way I wanted in this application.

While I had the structural assembly apart I laid out and partially drilled the 22 holes for the screws that will secure the structural assembly to the inner rail.  First was using the drill press drill a pilot hole through the perimeter structural rails then drill the countersink.  When the MDF playing surface support is installed I will extend the pilot hole up through it.  After the inner rail is set in place I will extend the pilot hole up into it.  With that done I will remove the inner rail and drill a clearance hole through the MDF and the structural support.  I know it sounds like a lot to go through but here is what I will end up with.

Section Through Table

Here is how the structural assembly looks with the additional aluminum bars in place.  This time when I stressed the surface it was quite a bit more rigid.

Aluminum Reinforcing in Place

This is what one end of the single cross-piece looks like with the aluminum bar screwed to the cross-piece.

Reference Notes & Aluminum Bar Installed

When all the parts are screwed together I added some more reference notes so I could assemble all the pieces back together in the right place.

More Reference Notes

I could now layout the locations for the screws that will hold the MDF in place.  With all the holes I had already drilled in the structural pieces I needed to make sure these did not conflict with the existing ones.  With the hole locations laid out I could carefully square up the table, clamp the first piece of MDF in place, drill pilot, clearance and countersinks holes for the four screws that will lock that piece in place.  From there I measured the center MDF piece cut it to size then went through the drilling and installing screw routine to lock the second piece in place.  Same procedure for the third and last piece of MDF.  The photo below shows the three pieces of MDF locked in place.

MDF Locked in Place

To  make the installation of the remaining 88 screws easier I moved the assembly out of the table and onto my work bench where I could work at a comfortable height.

Installation of Screws thru MDF in Progress

Next Up – Inner Rail & Structural Support Assembly