What Router Bit to Surface a Slab?

A reader recently wrote to me asking for advice on what router bit to use for surfacing wood.

He has been using standard straight bits that have been effective at levelling the Douglas fir end grain but left some tearout which required a significant amount of extra work to remove.

What determines quality of cut?

When it comes to selecting a router bit to control tearout while surfacing there are two main things to consider – cutting angle and sharpness. Sound familiar? These same factors apply to nearly any cutting task whether with hand or power tools, wood or metal.

The geometry factor

Lower cutting angles on a spinning bit generally tend to cut more cleanly because roughly half of the cutting is done across grain, one quarter against the grain, and one quarter with the grain.

Most straight router bits have carbide cutters oriented parallel to the shank and therefore cut at a 90 degree angle to the surface.

Shear bits cut at a slightly lower angle (I measured 75 degrees) and spiral bits less again (I measured 45 degrees).

Some surfacing bits with face-mounted cutters have effective cutting angles under 45 degrees. I would estimate the cutting angle of this Dimar surfacing bit to be around 35 degrees (link).