Has any one taken this (marvellous) idea:
and made a router jig, for accurately cutting
(as opposed to marking) large arcs?
: Has any one taken this (marvellous) idea:
: and made a router jig, for accurately cutting
: (as opposed to marking) large arcs?
Interesting, if you know the three points needed. But, I don't
see how it could be used to draw, say, a portion of a 15' radius.
eg, I don't see how it's related to a specific radius to be
I can see uses for it though not for a router.
This is indeed an old trick, though there's nothing "magic" about it as
the link says.
Also, the link says that the "long compass" thus constructed is
universally applicable to arcs of a particular radius. That's not
true. It could be used to draw arcs of any radius, but the arc length
subtended will stay the same. In other words, when you construct your
original three points, let's say you put brads at 10:00, 12:00, and
2:00 on the perimeter of a 15' diameter clock. The angle between the
outer two points would be 4/12 of 360 degrees = 120 degrees. At that
point, in order to trace out the 120 degree arc, you'd need to remove
the middle brad and trace the path of the crotch as you rotate the
"compass" from one brad to the other. For that matter, you could take
ANY two brads any distance apart and use the compass to trace out a 120
degree arc between them. The radius would not be fixed; it would be
whatever radius is required in order for those two brads to be 120
If you want to know the "maths", the radius can be found as follows:
R = D/(2*sin(arc/2)), where D is the distance between the two outer
brads, and arc is the arc angle (120 degrees in this case).
Nuances notwithstanding, this would be a pretty good trick to use with
Indeed; that was my thought (for a carpenter friend,
actually - I'm a pure-bred neander).
I just wondered if anybody had already designed
and built such a thing, and I could benefit
from their thoughts and experience.
I made my own offset router face to do arks. I took a regular face
plate off my router and traced that onto some graph paper, then decided
how long I wanted the new off set plate to be. Then I put another
circle, using the graph paper as a rough guide (I used the inside
circle on a roll of tape that I had handy to make the other circle)
about 8 inches away from the main face tracing. Then I used a ruler to
line up the outside of the circles. (kinda picture a bicycle chain on
a small and big sprocket for this) Cut out the form, traced it onto a
spare piece of clik flooring that I had laying around, traced the holes
for the screws onto this piece and drilled those....attached it to the
router and used a small plunge bit thru the flooring to drill a hole so
that I'd have a center point. Drew a center line down the plate, bored
the main hole out to allow the bit clearance and there was my offset
After it was made I drilled holes along the center line every 1/16" so
that I could pin it in place and make my ark just like a big compass.
Worked like a charm. I used pin nails to hold it down onto the
material I was drilling so the resulting hole was easy to cover.
So does that mean you can do circles with radii of up to about 8"?
Bugbear is talking about doing radii of tens of feet or more. That
would take one hell of a face plate, and a whole lot of space to use it
Don't get me wrong. It's a good idea. Just different than what
Bugbear was talking about. Unless I misunderstood.
My misread on that one. :-O I was reading 15" not 15'.... Sorry
about that. Thanks Josh.
Depending on what materials Bugbear is working on he could do a similar
set up with a 15' or more long aluminum piece...Mount the router on a
piece the width of the router face, 1/4 thickness, length to whatever
you think the maximum length would be. Find your center line and do
the same thing.
Depending on your tool availability/experience you could make it out of
aluminum sections that you could lock together, then you could
add/subtract pieces depending on the size of the ark you want to make.
I'm thinking aluminum for strength/weight.
Heh. The technique I posted at the start of the thread
will happily do a 1" high, 6' wide curve.
Which would require a 54' trammel bar, which
might be a little unwieldy for people
with smaller workshops.
So the biggest problem I see with making a router jig is the location
of the router bit. If you want the jig to be variable, at least one of
the two straight pieces has to be able to pivot, but the router bit is
sitting right at the pivot point. If it pivots about any other point,
it will only line up with the edge of the router bit for one particular
angle. Otherwise, the edge of the bit will be offset from the "crotch"
of the V.
One solution is to let one arm pivot an inch or so away from the bit
and just compensate with the placement of your guide nails, but it
might take some practice to figure out how to adjust them.
I think the better way to do it is to fix the arms with bolts which
pass through slots in the base plate . The slots would be arc-shaped
and concentric about the router bit. Personally, I'd cuts arcs that
ran a full 45 degrees so that the arms could be adjusted from straight
apart (180 degrees) down to 90 degrees apart. At 90 degrees seperation
of the arms, you could cut a perfect half-circle. By slightly
oversizing the slots relative to the bolts you could have a little
extra "slop" so that you could line up the arms with the either edge of
the bit (depending on whether you were cutting an inside arc or an
If you were cutting a peninsula countertop, you could simply mark the
"corners" of the arc, screw in some temporary wood blocks that line up
with your marks instead of guide nails, mark the furthest point you
want the arc to hit out at the end of the counter, and adjust the angle
of the arms until the router bit reaches just that far. No
I think you could just screw on a couple of temporary wood blocks to
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