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).
Lower cutting angles will cut end grain more cleanly, and Douglas fir (even face grain) can be very difficult to cut smoothly without tearout.
Sharpness is a factor
Sharpness is the other factor. Tearout, as the name implies, occurs when wood fibres are torn rather than severed cleanly. Sharp tools cut better than dull tools.
Not all router bits are created equally. Some carbide grades are made up of finer grains and therefore can be sharpened to a finer point. Sharpening with a finer abrasive will also result in a keener edge.
Don’t overlook wear-resistance
Just because a router bit starts out sharp doesn’t mean that it will stay sharp through the job. Try this: set a straight bit, cutting end down, on a flat surface and look at how it contacts the surface.
Many are ground so that the outer two points are lower to score the wood first to ensure a cleaner entry. When surfacing, these points are doing all the work, and therefore will wear (dull) quickly.
What router bits are best for surfacing?
A bottom-cleaning bit has one or more cutters aligned with the surface and therefore produces a more even surface and is able to distribute wear over a greater area and, therefore, stay sharper longer.
Start by looking for router bits for dish carving, mortising, or surfacing, or straight bits with a boring point as these are most likely to have cutters oriented flat across the face. Specialty joinery bits such as those for cutting finger pills and slots may also be suitable. Dish carving bits have rounded corners, so any overlap marks from the adjacent pass will be less noticeable than with a square-edged bit. This is a personal preference.
Here are two bits I use for surfacing.
And remember, shorter bits with larger shanks are more stable and more apt to produce a smooth cut.