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Making a hinge mortising fixture.

Router Guide Bushing

Router Guide Bushing

In computer programming there is a term called “stuck in an endless loop”.   Usually programs do the same thing over and over using different numbers and if you somehow forget to tell the program how to stop executing the loop it just continuously keeps doing the loop forever.   Sometimes us humans feel like we are stuck on one of those endless treadmills.   I thought about calling this post, “Making a tool so I could make a tool”, but the current name is more descriptive and the procedure I used can be used for other projects as well.

If you remember in the last post, “The Hinged Plate for the Metal Brake”, I have decided to make the metal brake out of wood and I will be using salvaged door hinges for the hinges.  I spent a lot of time in that post describing how the hinge pins need to be centered at the joint between the unhinged part and the hinged part.  Now it is time to do that mortising of the hinge.   There are several possible ways of accomplishing that.  The first way is the old fashioned way of using a wood chisel and removing the wood.   That probably requires more skill and patience than what I have.  The second way is to hand guide a router to remove the material.   The third way is to build or buy a fixture to guide the router.   I chose the third way because once you have the fixture it is repeatable and the skill level is a lot less.

The router base without the bushing installed

The router base without the bushing installed

The first step in the process is to install a guide bushing in the router base.  These guide bushings are specific to each brand and possibly each model of router and usually come in sets of several size bushings.  I chose the smallest size bushing out of the set I had purchased for my router.  The first concern I had was how thick material I would use would have to be.   As I show in the first picture the material can be very thin for this bushing.  The three holes in the router base are counter sunk so the busing is centered in the router base.  However, it turned out the router base was not centered on the router frame.   The three holes where the base mounts on the router are in large flat spots so the base can move some until it is tightened.  The base has been removed several times and until now it was not that important that it be centered exactly.

Centering the Router Bushing Guide.

Centering the Router Bushing Guide.

To center the bushing exactly I used a digital caliper and checked the depth between the edge of the guide bushing to the shaft of a router bit mounted in the router. I set the router with the bit extended past the cutting surface so I would be measuring against the round shaft. Three measurements were checked 120 degrees apart by taking the measurements over each of the three router base screws. I then loosened the screws and adjusted the base until all three measurements were within a couple of thousands of an inch to each other.

The Fixture Design.

The Fixture Design.

Since I have never done anything similar to this before, I went to CAD and laid out the design. As I will describe later, I did not completely think out everything. Sometimes you get lucky and even with my “screw-up” I was able to cope. With the hinge pinned together, it measured 4″ X 4″. I used a 1/4″ router bit and since I measured the distance from the bit shaft, also 1/4″ to the bushing O.D. I was able to create the drawing of the bit path and the dimensions of the hole in the fixture to guide the bit.  My mistake at this point was to not worry about the dimension shown on the drawing as 2″.  I knew I just had to be greater than two inches and I would worry about creating a stop in the future.   As we say here, I got the cart ahead of the horse.

Layout out the fixture

Layout out the fixture

The material I used to make the fixture was polycarbonate window glazing. It is 0.090″ thick and this is greater than the 1/16″ protrusion of the router guide bushing. I put masking tape over the area where I would be cutting and made the guide marks.   Because I was afraid I would break the plastic, I cut the hole in the middle of the plastic.  The initial cuts were made to the inside of lines.  It is always easier to trim off more than it is to try to figure out a way add more material if the hole is cut too big.

Cutting the fixture.

Cutting the fixture.

To cut the fixture I used a Dremel with a metal cutoff wheel. This worked well except where I needed to make the rounded corners. I cut a diagonal line there and rounded it off by again using the Dremel but I used a sanding drum to remove the material. I would not use a grinding stone to do this work because the soft plastic would probably fill the stone and make it useless for any other purpose. The sanding drum and cutoff wheel lasted throughout the whole job. The final bit of material at the square corners was removed with a hacksaw blade and I thought I was ready for testing. That is when I realized I had overlooked a major problem. How was I going to attach the fixture to the board I was going to mortise? If I used screws the screws had to be flush or inset into the fixture. That was not likely because of the thin material. I needed some way to clamp the fixture to the side of the board. I thought of attaching a small board to the fixture, but again I could not use screws and any glue would probably break loose.

Heating the plastic to bend it.

Heating the plastic to bend it.

I solved this problem by actually bending a right angle in the plastic sheet. To do this I clamped the sheet between two boards. The boards need sharp corners and not the factory milled round corners like is found on 2X4 boards. I used a heat gun to heat the plastic. However I could not effectively do it with the nozzle as shown on the heat gun because the gun needed to be closer to the plastic. Once the plastic is hot use a small piece of wood to bend the plastic to a 90 degree angle before the plastic cools. The plastic is hot, do not touch it with your bare hands.

Final Fixture

The Final Fixture

Bending the right angle should have been the very first step when working with the plastic for the guide.  I consider myself lucky that everything lined up because the corner of the bend has some rounding to it.  It would have been much more accurate to bend first then cut out the hole.

 

The routed mortise for the hinge.

The routed mortise for the hinge.

I then used the fixture to test route a mortises. I did this to adjust the final depth of the router and also make some final adjustments to the fixture. Remember I had cut to the inside of the lines and needed to make the fixture a little bit wider by using the Dremel and sanding drum. The front part of the board doesn’t look very nice. I had to cut away some because for my uses I want the hinge pin to be centered over the joint between the two pieces and the hinge material around the pin required some clearance. I did a very rough job of removing that material because it was not necessary to be accurate for that part of the job. I have checked two of these test boards butted up next to each other and the depth of the mortised hole and it fits very well. I am ready to go on to the real thing.

One final note. One my fixture I wanted the boards to butt up next to each other with the hinge opened flat. If I were to use this fixture to mortise a door and facing for hinges, I would place a 1/2″ square dowel rod between the fixture and the door because doors have the hinge set with the pin about 1/2″ on the side of the door that the door swings into. (Look at a door to see what I am trying to say, because I did not say it very clearly.)

Gary

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