If you mean a half round (3/4" diameter), use a 3/8" radius bit. Duh.
The trick is the technique, not the math. If you can attach an
auxiliary board to the routed board for the second side, that's the
easy solution. In my case, I used a steel straightedge to align my
incra fence with the bearing on the bit to within a thou (yay for the
micro adjust!) and just routed both sides - the board rubs along the
fence at JUST the tangent point, and you end up with an essentially
perfect 3/4" half round.
Uh huh. Anyting less than 3/8" and you will have to leave a
flat spot somewhere on the edge, or that 'jag'. You can use
larger radiused bits, then the curved edge may be tangent
to either face, with a sharp edge at the other, or tangent to
both with a sharp edge in between.
A roundover with a 'jag' is called a bead.
Draqing the profile on graph paper can help a lot.
Please explain. The only factors I know which will cause a jag are bit
height or fence setting relative to the bit. The radius has nothing to do
with it. The radius choice is not related to the thickness of the wood,
other than cosmetic preference.
The dial is marked in 1/1024 inch units, and yes, "off by one" makes
the difference between "straightedge rocks" and "straightedge doesn't
rock". You do realize that your fingers can feel a 0.001" offset,
right? A thou really isn't as small as you think.
Besides, that was kinda the whole point of spending over a thousand
dollars building an ultra-precise router table.
Actually, I justified it as "practice for the dining room cabinets"
and "I need this to *build* them too" :-)
I was using it this weekend to make tenons for the face frames, and it
turned out that a few thou made the difference between "too tight" and
"just right". Eventually I figured out that the tenon had to dial in
at 0.238" to 0.243" to get that "sweet" fit in the mortises. The
mortises were all made in a batch first, so they're all essentially
the same, but the wood for the face frame is not consistent thickness,
as the store had S3S at the time, and it doesn't matter as long as the
*front* lines up, so I used the router table to cut the front cheek of
each tenon FIRST, in a batch, without adjusting the router height
at all, then used the dial calipers and PRL's thou indicator to custom
cut the back cheeks individually.
Cut. Measure: 0.256". Raise bit 0.016". Recut. Measure. Perfect.
Of course, this was after the usual shoulder cuts on the table saw and
rough slabbing of the cheeks on the bandsaw. A few extra steps, but
each step was fast and easy and it went pretty smoothly.
 The mortises were started at the router table, with the front face
against the fence every time, and finished with a hollow mortiser,
again with the fronts against the fence. The tenons were cut
fronts first, so the front faces all line up. This way, I don't
care how thick the boards are.
 The router table couldn't go deep enough, but my drill press is a
cheap store brand with lots of quill wiggle. So, I route first,
deep enough to guide the hollow mortising chisels. That way I use
the strengths of each tool to avoid the weaknesses of the other.
FYI I learned the "reference face" technique at last year's GNHW
Joinery Symposium. http://www.gnhw.org/joinery-2005.html - the
"machine mortise and tenon" session. DVD number 05-JS5 if anyone
wants to buy a copy (http://www.gnhw.org/library.html ).
I've been a machinist and toolmaker for 19 years. I have a pretty good idea
of how big .001" is. Your setup is not ridgid enough to hold it. Nothing on
that fence is strait enough to reference. Your adjustment screw probably is
accurate (precision screws are pretty easy) but that alone won't get you
that kind of precision.
Have you seen one in person?
Besides, I can only speak for my own experience. A thou on the dial
is a useful increment for that kind of setup. A thou on the lift is a
useful increment there. On my setup, a thou sometimes makes a
I like the look of a 3/8" myself. Any size properly adjusted won't leave
any marks at the top. You can use a 3/4" also, but that tends to make it
look more like a full rounded edge rather than just a nice break of the
Depends on what you want as a final result.
Thanks. But what I want is half that. So the bottom is flat and the
edge curves up to the the top. It will be the top of a tongue drum. I
believe the first post was correct with 3/8" (I won't comment on the
"duh"...;+}) I guess it is obvious when you think about it.
So I went and looked at bits... wow. $30!
If you want a 1/4" shank, much cheaper right now
Still a good price here
Both brands are top quality bits. The 3/8" roundover is one of the most
used bits in my shop so I don't mind paying for one that will last a long
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