What size spiral bit to get?

[jussi]

Looking to get my first quality router bit. Hartville is having a sale on their Whiteside and Freud bits. 25% off using coupon code TS0925W. I'm looking to get a spiral upcut bit to make dados. What size should I get. I don't have a project in mind yet so I'm looking to get for general use. What is the actual thickness of 3/4" and 1/2" ply?

[LarryG]

Most plywood is about 1/32" undersized, but this does vary somewhat; and Baltic birch plywood is metric and does not exactly correspond to non-metric sizes.

So I would suggest you buy a 3/8" bit and plan on making two passes. That will allow you to cover any flavor of 1/2", 5/8", and 3/4" plywood, whether nominal or true to size, as well as the two most commonly-used sizes in Baltic birch, 12mm and 18mm.

[JimD]

My 1/4 inch spiral is the only spiral cutting bit I have. For dados, I use much cheaper straight bits. I use the spiral bit for mortises and for making holes for shelves with a homemade jig. When I do mortises, I make them wider when appropriate by making multiple passes.

Jim

[LCHIEN]

i seem to use the 1/4" a lot for cutting circles, cutting items from templates, large openings etc.

I seem to use the 3/4" to cut slots that need to be exactly 3/4" and to cut recesses that are wide and need rounded corners, like handholds etc. that might be an inch wide and 4 inches long and 1/4 or 1/2 deep.

[AlanWS]

Why upcut? It will clean out the chips better as you cut, which is more important for deeper cuts like mortises. The edges would be cut cleaner with a downcut bit, which would seem more important for dados. An upcut bit also puts a force on the bit that can tend to pull the bit out of the collet, which can ruin your workpiece or worse if it actually moves. For this reason Pat Warner actually prefers straight bits. See his point 12 of his FAQ:
http://patwarner.com/faq.html

[crokett]

I would get the 3/8" bit. I prefer spiral to a straight bit. A spiral bit leaves a cleaner edge and is more versatile.