Bit Selection for Contouring a Wood Chair Seat

Choose a bit radius that approximates the curve you need to create. July 25, 2010

I have several hundred seats to machine each month and I'm looking for a chair seat contour bit for my CNC (or any other ideas).

Forum Responses
(CNC Forum)
From contributor K:
What do you use at the moment? I use a PCD roundnose bit at 40mm diameter for sign manufacture. I've done loads of work with it, and it's still sharp.

From the original questioner:
I am thinking I will use an endmill to do the lowest, flat portion and then a 2 1/4" fairly flat cove for the outside. The ball nose would work too, but with perhaps some extra finishing or more passes. I saw an Otoro seat contour bit online, from Japan - pretty cool, but they haven't got back to me yet. Thanks.

From contributor R:
A really big ball end mill would work perfectly well. I have used a 1 1/2" radius bit with replaceable insert cutters. Plenty big enough to hog out material. You could eliminate having to use two tools for the finish cut.

Maybe a custom bit, a large diameter core box type with a 1/4" or 1/2" flat spot in the center? Would allow for bigger steps between passes on the flats, but still do the curves well.

From the original questioner:
Thanks. Where did you purchase your 1 1/2" radius replaceable cutter bit? Sounds like that would do the edges well.

From contributor M:
For any dish shape you wish to cut, you use the least amount of passes when the diameter of the cutter matches closely the diameter of curvature you wish to cut. On a chair seat, the curvature is sometimes constant, but usually variable. Find the tightest curvature and use that - maybe that is 10 inches radius, maybe some other number. Call any custom tool shop and have them make a 2" diameter tool with a radius end in replaceable tip carbide tooling. Vortex Tool will be happy to help you out. I can't of course quote for them, but I bet a custom body with inserts will be around 400 to 500 bucks with a few sets of inserts included - not a bad deal! I have had this type of thing done before (different configurations for roughly this price zone). Have them make the tips only 1/2" or 3/4" wide at the outside end of the radius. You don't need the center tooled unless you are plunging really close to vertical, and the cutter will just dull up and get hot there anyway. Just approach the cut at say 10 or 15 degrees to parallel and travel. The bit will look like a 2 inch fly cutter with a slight radius to the end.

From contributor S:
At Active Machine we have made many CNC bits for this application.