Arc Cutting Jig?

[mrmortise]

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? Sincerely, Michael

[George]

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.

[Swingman]

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".

[Pat Barber]

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...

Sw> I do this a lot and my solution has always been a bit of simple cave man

[bridger]

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.

[George]

Spokeshave. And only half the curve.

[Swingman]

MDF template, like this http://65.201.81.222/images/EndTemp2.jpg ,or this http://65.201.81.222/images/Trestle%20Table3.jpgFirst 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.

[Pat Barber]

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".

Sw 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.

[Swingman]

Ackshully, I prefer to think of it as that old, slightly imprecise, "made-by-hand" look that is both desirable and damned expensive.

[Pat Barber]

Okey Dokey...

"Hand Crafted" always brings the big bucks...

Sw> Ackshully, I prefer to think of it as that old, slightly imprecise,

[Rodney Myrvaagnes]

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

[Chris Melanson]

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 one 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)

Chris