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View Full Version : General Metalworking Auger Jig for the Pisgah Bridge



James Marshall
08-03-2013, 12:18 AM
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To erect the 86' bridge we had to drill over 250 holes to mount brackets on the 4" side of a 4x6 and 4x8 pressure treated lumber as shown above. Some were 3/8" and some 1/2" diameter. Ones like the ones pictured had to match up with a steel bracket on one side and a mating bracket on the other side. Imagine trying to drill that without a jig. Here Tom is drilling some diagonal braces.

I used the Bridgeport to get the holes accurately placed. Our brackets were made with holes consistently placed 3" apart, some 1/2" in diameter, some 3/8". They are centered on 4" (3 1/2" actual) timbers. This universal jig was used to drill over 250 holes various places. It was used on several different structural pieces, starting out as a simple angle and then being modified by notching out chunks to fit the next requirements.

Holes were drilled in the steel angle to take the 3/4" pipes. I milled out the radius so the angle would fit tightly on the wood. The pipes were then welded in at 90 degrees. Various PVC pieces (bushings and plugs from home depot) were turned on the lathe to fit tightly in the pipes top and bottom, drilled to fit the 3/8" and 1/2" auger bits. They were then driven in the pipes making a PVC guide for the bits top and bottom. I made two sets, one for the 3/8" holes and one for 1/2" holes.

Here Tom is drilling the holes through the diagonals. Something as simple as a c-clamp kept the distance accurate from the end. There are brackets on both sides. The holes actually come out the other side where they are supposed to. Try drilling these accurately without a jig.
Besides the actual woodworking, building jigs has been one of my contributions to the project.
Jim