How to cut a long arc

[Cal]

I am building a kitchen island with attached bar height seating counter.

This counter will be 6 feet long x 18" at the ends.  I would like to put a curved top on this that would add 6" at the center point.

Laying the cut out is no problem.  Rough cutting the mdf top will not be a problem either, but I would like to get a "perfect" edge if I can.

Using my rudimentary math skills I have determined that a router tramell would be right at 13 feet long!

 

So what technique might you suggest to cut this arc?

Does a router tramell have to be a solid board?  What about using a cable/wire for this purpose?

Thanks,

Cal

[Dadio]

If I were doing it I would make a plywood template and cut it with a router and a pattern bit w/top bearing.

Herb

[Cal]

Herb, That is a-ok; but how do I cut that "perfect" arc on the plywood to begin with?

Cal

[Gene Howe]

How about the string and pencil method to draw the arc on the pattern? Cut it with a jig saw and file/rasp/sand it to the line.

[PeteM]

Take a thin (1/8") strip of wood, like pine, a bit longer than the curve you want to draw.  Slot the ends of the strip, and insert a knotted string in one end.  Run the other end of the string through the wood strip's other end/slot.  Tighten the string to bow the wood strip until you get the curve you want.  Tie off the string and use the bowed wood strip as your template.  Make sure the wood strip is uniform in thickness because the arc of the wood depends on the thickness and the tension working together (OTOH, if you want a unique curve, use a slightly tapered wood strip!)  Use the strip/string arc on a piece of hardboard, cut out the hardboard with jigsaw or bandsaw, then sand smooth.  That gives you a template for your router guide bearing.

[HARO50]

Attach a router to the 13' board and cut the arc with straight bit.

John

[Stick486]

Drawing_Circles_by_Keith_Mealy.pdf

Edited December 13, 2016 by Stick486

[lew]

Here's an image of how I do it-

 

I have a long piece of straight grained maple- but any wood will work- instead of the nails, I use a couple of clamps as stops. 

[kmealy]

Yeah, that Keith Mealy is a smart guy.  A friend of his recommended one of those methods to his neighbor who wanted to cut an arc in the top of fence that would have taken a 35' trammel.

 

While those (last two) methods are used for marking, there's no rule saying you can't use your sabre saw and/or router in the place where your pencil will be.

 

I don't really like the string and stick method because it relies on a very uniform stick.  If there's any difference in grain or stiffness, you won't get a perfect arc.  And there is some question as to whether that curve is part of a circle, a parabola, or a catenary.  A jet engine mechanical engineer and I had that discussion once and could not come up with a provable conclusion.    All of which begs the question, "Is it close enough for the eye?"  Or, "Even if not a circle, is it pleasing to the eye?"

Edited December 13, 2016 by kmealy

[HARO50]

With all due respect, Cal's question isn't about DRAWING the arc, but CUTTING it. If I have it right, he's worried about the slight irregularity that can creep in when following a pencil line with a jigsaw or other tool. Attaching a router to a straight, solid board, fixed to a pivot at the other end, would allow him to swing the router in the required arc, leaving a clean-cut curve. Or am I mis-interpreting his question?

 

[kmealy]

10 minutes ago, HARO50 said:

With all due respect, Cal's question isn't about DRAWING the arc, but CUTTING it. If I have it right, he's worried about the slight irregularity that can creep in when following a pencil line with a jigsaw or other tool. Attaching a router to a straight, solid board, fixed to a pivot at the other end, would allow him to swing the router in the required arc, leaving a clean-cut curve. Or am I mis-interpreting his question?

 

True, but is there a way that's not awkward given the length of the radius (13') the size of the counter-top (6' x 18") and keep it all solid and stable while making the cut?  How many shops have the swing room and a 15" level work surface? (for using a trammel)?

 

 

Edited December 13, 2016 by kmealy

[lew]

5 minutes ago, HARO50 said:

With all due respect, Cal's question isn't about DRAWING the arc, but CUTTING it. If I have it right, he's worried about the slight irregularity that can creep in when following a pencil line with a jigsaw or other tool. Attaching a router to a straight, solid board, fixed to a pivot at the other end, would allow him to swing the router in the required arc, leaving a clean-cut curve. Or am I mis-interpreting his question?

 

I'd cut outside the line and then use a belt sander to bring it to the line. Or you could use the method in the above image- only a thicker board with supports on the inside. Use the outside edge of the board as an edge guide for the router base. 

[Stick486]

draw the arc...

close cut to the line w/ a jigsaw..

fasten a flex ruler to the line to act as a template guide......

add filler behind the flex ruler to carry the router perpendicular to the surface....

use a top bearing bit to follow the guide...

[HARO50]

42 minutes ago, kmealy said:

True, but is there a way that's not awkward given the length of the radius (13') the size of the counter-top (6' x 18") and keep it all solid and stable while making the cut?  How many shops have the swing room and a 15" level work surface? (for using a trammel)?

 

 

You mean to tell me that not EVERYONE here has a 48'x64' shop???

John

[Fred W. Hargis Jr]

I think the cable would work, working slowly. Unless you know someone with a CNC to do it, I think your stuck with some kind of trammel (board or string). You could do this outside (maybe) if you're space constrained.

[HARO50]

To my way of thinking, a cable would work, but would require a constant pull to get a smooth arc. This is fine on a VERY large workbench, but iffy if you have the pivot on a separate structure. With the solid trammel, you could place (for example) two picnic tables 10' apart, and not have to worry about pulling the "pivot" table closer as you worked.

John

[Dadio]

I have laid out a 45' radius inside a small space with the bent stick method before. Once the radius is laid out then there are numerous ways of cutting it. Even a skilsaw will cut a 13' radius ,no problem. Then sanding or planing to fine tune the template.

Herb

 

[Nickp]

Maybe the question is "why not use a 13' trammel"...make a template outside (hopefully it's not cold) with the long trammel and attach the router instead of/in conjunction with a pencil...usual template making process...

 

...I have always found wire to be a bit tricky...suggest a 13' 1x2...and have at it...

 

I'm sure you've already considered the arc for the middle 6" overhang section will be different than the inner and outer arcs...

 

Best...

 

[Cal]

Thank you all for the suggestions so far.

Drawing it out is not the issue, I will do that on my vinyl cutter and either draw or cut the arc out for a pattern.

I have a few days to firm up my plan of action, as I hope to do it Saturday.

Keep those suggestions coming!

Cal

[kmealy]

3 hours ago, clhyer said:

Thank you all for the suggestions so far.

Drawing it out is not the issue, I will do that on my vinyl cutter and either draw or cut the arc out for a pattern.

I have a few days to firm up my plan of action, as I hope to do it Saturday.

Keep those suggestions coming!

Cal

I'd say we all show up at Cal's shop Saturday morning.   Have the coffee and donuts ready!