Thread: Is a spiral upcut bit a better choice?

Downcut will leave a very clean virtually splinter free finish on the top surface.

The basics are:

A spiral upcut actively lifts the shavings out of the cut, but in doing so also tends to raise splinters on the upper surface of the timber. Primarily used where shaving clearance is essential, or for through cuts where the router is operating from the back side of the material and the presentation side needs to be absolutely clear of defects.

A standard straight flute bit does nothing to actively dispose of shavings, and might leave some splintering on the upper surface, caused by pushing fibres away from the cutting edges of the bit instead of slicing through them. That's a function of how sharp the flutes of the bit are, bit RPM and feedrate, the actual material in use, and the direction of cut relative to the grain.

A spiral downcut actively pushes the surface fibres down into the body of the material, which backs up the fibres during the cut, resulting in minimal surface damage. However it also tends to drive the shavings down into the groove and can compact them fairly solidly in instances where the bit is making a groove one bit diameter wide. However compaction should be less of an issue in your instance as the area being rebated is significantly wider than the bit, requiring one pass to create the outline of the rebate, then multiple passes to clear the field, which will break up most of the initial compaction and avoid creating further compacted shavings.

Thanks Malb
that’s very clearly explained.
Cheers Jim

To give you an idea of the difference, Jim, have a look at the attached pics.

never mind. mis read post.