JamulBlog: FJ Cruiser: Hidden Hinges...

Sunday, June 2, 2013

FJ Cruiser: Hidden Hinges...

The platform I'm building will have two hinged doors, one on each side of the truck. The door will open up, like a chest, to provide access to the storage area below. The top of these doors is the platform for our dogs to lie on. This posed a bit of a design challenge: we wanted the doors to be hinged, but we really didn't want the hinge to be sticking up out of the platform, as an ordinary hinge wood.

I found a hidden cabinet hinge that would work for our purpose, made by SOSS. They're available in multiple sizes, including one that was perfect for us. Installing these would be a bit tricky, though – you can't just mount them to the plywood that makes up the door. I got a piece of red oak 2 x 2, finished on all four sides (it's really 1 3/4 inches on a side). That's plenty strong enough to mount the hinges into, and easy enough to attach to the plywood (some screws and glue will do the trick). But the hinge needs to be mounted with its centerline just 1/2" below the surface of the door. Since the door is made of 1/2" thick plywood, that would mean that the hinge's centerline would be right at the joint between the oak and the plywood, and half the hinge would be in the plywood – that's not going to be very strong.

So I decided to make an oak “biscuit” for each hinge. This biscuit would be a half inch thick (to match the plywood's thickness) and about 5 inches long. A couple of matching notches in the plywood would allow the plywood and the oak to mate up properly. Then the hinge would be mounted in solid oak, nice and strong.

Time to fabricate this:

First step for the oak biscuits: rough cut a piece of 2 x 2 to give me boards about 3/4" thick...

To mill the boards to 1/2", ideally I'd use a planer. I don't have a planer, so I opted for a redneck mill instead. I swung out the drill press' nice, flat table, then used a couple pieces of oak 2 x 2 to hold up the router. This securely holds the router precisely level, while allowing me to adjust the bit to any depth I'd like.

Close up view from the side of my redneck mill, where I'll be putting the rough-cut boards...

After a little practice, success! The end of this rough-cut board has been milled to exactly 1/2"...

The same board, now completely milled...

After milling the two boards, I looked like this from head to toe...

The two milled "biscuit" boards, next to a pair of 2 x 2s. The red oak is a pleasure to work with...

The biscuits are cut; now it's time to mount them. I'm drilling the pilot holes for the #12 wood screw; absolutely essential in hardwood...

Drilling the second screw hole, pinning the first hole with a screw to ensure proper hole alignment. Notice that I'm using a tapered drill bit, shaped specially for wood screws. The resulting tapered hole allows the wood screws to be easily screwed in, but still provide all of their purchase. Using a non-tapered bit in wood like oak would me compromising the strength or making it very challenging to screw in...

A mounted biscuit, glued and screwed...

Laying out the hinge locations for a pair of oak frame pieces - one for the door, one for the stationary part. These must match up precisely, so the layout must be made with great care...

Preparing for milling the hinge mounting hole. The short piece of oak is a “fence” set up precisely 1/2" from the drill bit's centerline. The drill bit is a 5/8" Forstner bit, which will make a clean, straight hole.

Let the drilling commence!

First I drilled three deeper holes, to match the deep part of the hinge. The drill press makes it easy to drill to a particular depth with its adjustable drill stop...

Next comes four shallower holes to match the rest of the hinge...

Then comes some tedious work with a chisel and hammer, to make the shape of the hole match the shape of the hinge...

After a few minutes of chisel work, the hole starts to look like the hinge might actually fit in it...

The end result: a nice, tight fit for the hinge. It's actually quite strong even before putting in the screws!

Now its time to cut the notches in the plywood to fit the biscuits. Here I've made the first cut with the router (taking out a bit of my sawhorse along the way!), and I have the fences set up to make the final two cuts...

A biscuit notch!

Then two notches...

But the notches have round corners, and they need to be square. Time to sharpen up the chisel! The perforated metal is the surface of a diamond hone, which works beautifully on the chisel...