After wrestling with many mission style projects involving arched
rails, I would like to know if any of you might have some good ideas
concerning a universal jig to accomplish the task of cutting gentle
and shallow arcs. And don't flame me with the "do it free hand, then
spindle sand to the line".......You guy's know what I mean! I read a
recent article in Woodsmith Vol26 / No. 153, June/July 2004, where a
clever band saw jig is explained. The jig is easy to make and does
work well. Seems though, that the jig is made for a specific size
piece of wood. If you want to use a different lengths or widths , the
problems arise. I understand concept of the circle cutting jig, the
large trammel, making it easy to produce a nice round piece of varying
diameters. To do the same thing on an inside arched cut would be very
nice and simple too. I can conceptualize the same result with a
router jig set up too. Sounds easy but I'm having more trouble
realizing the actual circumferences. Is it a matter of long, long
trammels? Or is there another formula to cut nice smooth reproduceable
arcs? Or does one need to make different jig for every new project?
I'm sure others would like to know the secret as well. Upon looking in
a router book, the oval cutting trammel has possiblities, but it
looks mighty complicated as well. Any thoughts?
Make ad-hoc jigs. They're anything but complicated. A miter bar reference
and plywood, two holes the rail distance plus 1/4" apart, and you make a
nice rough to rout to final dimension on a table or shave/sand as pairs by
deleting the 1/4".
I prefer to use the bushing and spiral cutter on a demi-jig which keeps the
curve symmetrical and allows me to plan hold-down places and cut down hill
on end grain. There I find it's as easy to make the template with my
scrollsaw, shaving or filing/sanding for a fair curve.
The quality of the template reflects directly in your rails, so get a fair
curve and a sanded edge on it. If you have a bit with good shear, you
really won't need to sand.
I do this a lot and my solution has always been a bit of simple cave man
technology: I use what appear to be the bow part of an archery set to draw
these curves on Mission rails and stretchers.
The "jig" is simply a long (about 40") thin (1/8") strip of wood about 3/4"
wide, strung up with a string, just as you would a bow. Affixed to the
string is a small piece of wood that allows the string to slide through for
adjustment, but friction will cause it to hold tension on the string, much
like a friction belt buckle.
Simply adjust the string til' the bow reaches your desired radius, then
clamp the bow to the piece and outline your curve, cut it out with the
bandsaw, and "sand to the lines". :)
Alternately, and if I have a bunch of these things to do, I will sometimes
make a template of mdf using the above method, then use the template and
router pattern bit as a guide for routing, instead of "sanding to the line"
as the last step.
It the above is unclear, just holler and I can post a url to a picture of
this highly sophisticated "jig".
Having done a end table(s) with curved legs, I can say
that "sanding to the line" is easier said than done
on any kind of arch.
I would like to hear your version of how you get a
template perfectly smooth with nary a "flat spot"
that glares up at you like a bad penny.
Getting that "fair curve" is quite a trick...
The "bow and arrow" thingy does work...
First cut out with a bandsaw; sanded "close to the line" with an oscillating
spindle sander; smoothed with long strokes of a pattern makers rasp; then
finally sanded with a thin strip of wood with sandpaper attached that will
fit into the curve to get rid of the dreaded "flat spot".
Then it is a matter of a good router bit to reproduce the curve on the piece
you want - as in Amana #57186, 4 1/2", four flute, top bearing flush trim
bit with 1/2" shank.
That said, the OP was talking mostly curves in table rails ... since they
are usually different sizes and one jig won't work for them all, I forego
making jigs and just use the "bow and arrow" tool for the curve, then do the
above method for each piece without making a template. After all the curves
are generally on different sides of a table, won't be seen side by side, and
don't have to match precisely ... but I didn't say that.
Ahhhh.... the old "indian trick" where they can't see
both curves at the same time ????
The mdf is always the way I try to make any curve that
is going to be repeated more than once.
Getting that VERY first template can be a very interesting
test of one's patience. The sandpaper on the thin stick
sounds interesting... I have tried them all and curves can
be a stone cold bitch to get right.
A bad curve just sort of peeks out at you and says,
"loooooook, I'm not a smooooth curve".
After all the curves are generally on different sides
of a table, won't be seen side by side, and don't have
to match precisely ... but I didn't say that.
This is true. The eye can spot a curve that is not "fair" in boatspeak
to a very fine tolerance.
I think a fair curve could be described mathematically as one whose
derivatives are continuous.
Anyway, you can produce one with a batten that is uniform enough in
stiffness. Since you can see if it isn't fair, just plane a strip of
wood and bend it. If it looks good, trace its curve. If it doesn't
look good, try another piece.
Rodney Myrvaagnes J36 Gjo/a
"We have achieved the inversion of the single note."
__ Peter Ustinov as Karlheinz Stckhausen
On 2 Aug 2004 22:02:30 -0700, email@example.com (mrmortise) wrote:
yep, long trammel.
you can make a series of center pin holes for the radii you need as
you need them, but better is to make a method of fine tuning the
length.I've seen a number of approaches that work. I usually rough
slightly oversize on the band saw and jig up a router on a large flat
surface. whatever the bar of the trammel is, make it massy enough that
flex and chatter aren't a problem.
When using a router set up a way to figure out radius is as follows
For an outside radius measure from the inside of you router bit to the
center of your pin and for an inside radius measure from the outside for the
router bit to the center of you pin.
All you need to do is make a circle jig with a movable pin point and
make some reference marks for each project. If you would like I could send
you a drawing of one so you get the idea. Just let me know and I will send
You will have to let me know through the group as the email addy is not
valid (to much spam on it so I changed it)
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