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what bit is best to use for routing the binding channel. i am doing it with a lam trim router that has an edge guide for curves and such. i am gonna just make a donut for the radius and was wondering what bit will get least tear out. spiral cut down?

In my somewhat limited experience, a spiral down cut works fine.

Same here.

What type of edge guide do you have, is it just a single point or bearing that registers off the sides? It is far easier to keep the trimmer parallel to the sides if you have two points (over / under), or some other dedicated binding jig.

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Rian Gitar og Mandolin

You are right. Spiral down. It cuts down on the wood and leaves a cleaner, less fuzzy edge. Upcut is good for saddle slots as it clears the waste up out of the slot (a bit any way).
But with a lam trimmer I use a rebate bit with a bearing to set the horizontal depth. One bearing to cut the shallower purfling slot then with a larger bearing and
more vertical depth, the binding slot/"shelf"

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Greg
http://garibaldiinstruments.com/