Don

If you look at the construction of the bits, the horizontal cutters are
typically angled to begin with to provide as much sharpening edge as possible
and as much support as possible. This does produce the slight dome and that’s
normal. You can also find some that will cut a flat surface but they were made
for that purpose.

The skew cuts across the grain more efficiently and that’s the reason for the
shallow added angle. You do have the option of sharpening to reduce the skew but
that will increase the force needed during drilling if you’re using a wood
framed bit brace.

I’ve read different numbers on angles, which I believe has more to do with the
type of wood being bored than with any rule as well as, again, whether you are
using a wood framed brace or a metal framed brace. Shallow angle for wood framed
and a slightly more aggressive for a metal frame. But still, these are fairly
delicate bits so in general, look at the existing angle of the cutter and just
sharpen the edge only as much as is needed, forgetting about angles.

Gary
...............................
Gary Roberts
http://toolemera.com
http://toolemerabooks.com

"I'ld rather read a good book, than write a poor one." Christopher Morley




On Apr 23, 2014, at 3:31 PM, Don Schwartz  wrote:
> 
> I've made some progress, getting some of the larger bits cutting nice clean
holes, and adjusting their sizes a little, but the holes they're producing
aren't flat-bottomed. They have shallow domes in the center. This doesn't seem
critical at the moment, as I can always pare or rout the bottoms if need be, but
I was wondering if I should aim for flat-bottom holes?  I'm thinking that can be
achieved by filing the 'horizontal' cutter or router edge furthest from the
center, to bring the edge nearer to square with the center of rotation. Most of
them seem to droop or angle downwards.
> 
> Also I'm wondering about the bevel angle for the upper surface of the router.
Does30 degrees sound good?
> 
> Finally, some of the horizontal cutters have a back bevel, sometimes quite
large, and it seems to me this is just wrong, because it will reduce clearance
there. Should I aim to eliminate it completely?