3.05.2021 Keep on trammelling!

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Monday 3rd, 44-50F, a fair day though with a light shower. Scrambled to put everything away.

I continued to work on the router trammel and arc support system. So I can start producing more accurate, plywood ring and rib segments without too much fiddling. 

I am not happy with the jigsaw cuts so far. Despite having decades of experience with electric  jigsaws. I could use a router bit with follower bearing but would still need an accurate pattern to trim the existing arcs. 

The image shows the basic set-up. A 2x4 forms the spine and locks the two [clamped] workbenches together. The trammel, radius arm is centred with a long wood screw down into the 2x4. Two flat boards form a V out to the ends of the plank supporting the arc to which they are screwed. Without which there is no rigidity in the system. G-cramps hold the free ends of the V boards to allow adjustment. Cheap, adjustable, roller stands provide support for the "wing tips." The arc is held down and fixed firmly in place, with countersunk screws down into the supporting cross board.

The trammel arm is an 80x30mm, rectangular, aluminium tube. Nicely stiff but not too heavy. The little Makita router is fitted with a CMT solid carbide, 8mm up-twist bit. The mains cable is supported overhead from a long bracket to keep it out of the way. Not quite as good a battery router but good enough. I have taken a couple of cuts of a pre-cut arc and it all went very smoothly. Leaving a perfect radius. Without the usual ragged edges common to birch ply and Bosch jigsaws.

The set-up could be further improved using 2x4s for the arc support and the V-boards. The thin planks I used split far too readily when I fixed them together with wood screws. They have to remain unmoving despite repeated use. Or the cut radii will wander. The idea is easy repeatability without time consuming set-up for each new arc. 

The router bit is made to just touch the curved face of the plywood arc at each end. Then a cut is swept out in stepped depths. If the cut is not completely clean, from end to end, then the ends of the V-boards can be tapped lightly with a hammer. To give a slightly deeper cut. Adjusting the router along the fence bars would alter the radius. Which is not the point of the exercise. 

While I could cut arcs out of whole sheets 1.5x1.5m [5'x5'] with the trammel system it would be very cumbersome. I much prefer to trim a [jigsaw] pre-cut arc. The pre-cut arcs are easily manageable and can be fitted to the trammel jig without effort. The jig saw produces an arc in only a few seconds. It just doesn't hold tightly to the line. The slightest bump on the sawn edge [of a ring] pushes the whole arc away from the inside of the dome.

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