Continued:

Cut piece D to a length about 8" longer than the width of the "U" shaped assembly.

Next make a block to help align the clamping board square with the back of the mortise and insure that the mortise is cut to allow the proper offset of the hinges.

To determine the thickness of the block:

Subtract 3/16" (the offset) from the thickness of the door. E.G. (1-3/8" - 3/16" = 1-3/16") Then add one half of the difference between the diameter of your router bit and the outside diameter of your guide bushing to this distance. E.g. 1-3/16" + (1/8" / 2) = 1-1/4"  After you have finished planing and dimensioning the stock for this block cut off a piece about 5" long for use as a spacer.

Fig. 3

To finish assembling the jig, place the clamping board (D) in the vise flush with the surface of the bench, as shown in Fig. 3.  Place the spacer block in the center of the clamping block and tighten the vise.  Next place the "U" around the spacer block with the best face up. Clamp the spacer block to Piece " C ".   Pre-drill for and drive four (4) screws to fasten the "U" to the clamping block.

Remove the completed jig from the vise and sand off all of the sharp edges.  Finish with a coat of clear Watco Danish oil and enjoy cutting perfect mortises for hinges whenever necessary.

Any questions or comments contact me.