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1. 
   4DThinker | Oct 24, 2003 06:33am | #1

   To make an ellipse jig tight, you'll have to find materials for the sliding parts that can be snug in the slots but still slide easily.  Usually this requires dis-similar materials such as plastic in a metal slot, hardwood in a plywood slot, plastic in a hardwood slot, etc..  And the pivoting pins and trammel need to have the same snug fit but ease-of-motion.  I came across some 1/4" teflon dowel once that I use for jig pins.  It fits snug in a 1/4" hole but will allow easy pivot of any material around it.  I have also used 5mm shiny chome shelf pins in a 5mm hole.  I find that if I'm using metal rods or wood dowel for pins I need to make the hole 1/64th oversize, and that allows slop in the mechanism. 

   And dovetails are hard to get sliding smoothly in a dovetail slot with no play. I've had better luck using oak slides, 3/8" square, sliding down a 3/8" slot cut into Baltic Birch plywood.  The difference in surface density between the hardwood and the plywood usually lets them slide well against each other. You'll get less play if the slides are longer, too. And I sand the leading and trailing edges of the sides to help them not get hooked/snagged/stopped in the slot.  They don't need to be trapped in the slot so long as you don't lift up on the trammel during a cut.  I make my trammel as wide as my router base (6" usually), and can easly keep one hand holding down the center over the slides as the router travels around the elliptical path.

   If you want to make more extreme ellipses, you might want to read my thread on "ellipses, upside down".  Basically by inverting the jig and using the table as the trammel, it lets you move your board on an elliptical path through the stationary router bit on a router table.

   4DThinker