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# Cutting arc with large radius

## Cutting arc with large radius

#1
11-19-13, 11:35 AM
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Cutting arc with large radius

I'm looking to cut an arc in a poplar board that has approx a 12 ft radius : 48" wide and 2" high at top of the arc. I'm looking for a way to do this that would produce a smooth finish. I've seen people say they would use a jigsaw or bandsaw then smooth it by sanding it. I don't have any fancy sanders so I'm not sure if that would produce result I'm looking for. I do have a plunge router but I'm not sure if/how that could work either. Any ideas or tips??

#2
11-19-13, 01:40 PM
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Might be time to buy a sander I doubt your router would help much as it has to have something smooth/even as a guide. Personally I'd use my jig saw and then sand it smooth. I'd only use a router afterwards if I needed a profile on the cut edge.

#3
11-19-13, 02:38 PM
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What in the word are you trying to make?
Often times explaining the big picture helps come up with better ideas.
Not sure what you consider a fancy sander but for something like that I'd use a belt sander to get it close and a random orbital sander to clean it up.

#4
11-19-13, 04:13 PM
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Well, I did this exact thing once, while making a garden arbor for my sister in law.

You probably already know it, but for others, the formula is: R= (C²+4x²) / 8x²

where:
r= radius
c= length of chord
x= the 2" height of your secant

What I did was I laid out the board I wanted to cut, and then butted a long 1x4 into it. In your case, butt a 1x4x12' into the bottom of your poplar board so that it is perfectly centered at the midpoint of your 48" long board. You need to ensure that the 1x4 and your poplar board are perpendicular with one another... so measure from each corner of your poplar board to the far ends of the 1x4, and adjust it left or right as needed until those diagonals are equal. Then clamp it or temporarily screw it down so it can't move.

Your radius is actually 145". So cut another 1x4 roughly 148" long... but drill an 1/8" hole in the center of it at 145". Measure from the top of your poplar board down the centerline of your 1x4x12, and make a mark (x) at 145". Place a screw through your 1/8" pilot hole and screw the 148" board to the 1x4x12 exactly on your mark. Leave the screw a little loose so the board will turn. You can now use the board as a spacer to trace the radius on your board. I would probably carefully jigsaw the resulting line, and then belt sand it.

If you prefer, you could do this on another board (as a test run) and use the resulting shape as a fence for a router. Tack the "fence" to your work so that the router will follow the shape exactly. Use your router and a 1/4" straight spiral bit... the router bit will probably produce a smoother cut than your jigsaw... but how smooth it is will depend on how well you did cutting out the fence in the first place.

#5
11-19-13, 04:16 PM
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I would use a jigsaw and a router. I like to make a template out of 1/4" MDF for this type of work first and then use that to cut the board.

Cobble some 3/4" plywood together so it is about 13'-14' long. Mount your router to one end and stick a screw 12' from the edge of your router bit at the other end. Position your piece and cut away. The 1/4" MDF can be cut in one pass. Use that to mark your poplar board, rough cut with jigsaw and then flush trim to final size.

#6
11-20-13, 05:31 AM
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I followed most of drooplug's explanation, but might add one additional option. To steady the jigsaw, at least on mine, I can remove the plastic base plate and attach a piece of 1/4" plywood or equivalent. That would allow me to firmly attach the jigsaw to the 12' swing arm and give me a way to guide the saw for a smoother cut.

As for sanding, use a section of the material you cut away to form a sanding block with the same curve. Maybe 2 or three thicknesses.

Bud

#7
11-20-13, 06:22 AM
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Personally I would skip the jigsaw & template idea unless you plan to make several of these arcs.
Lay 2 sheets of poly insulation end-to-end on the floor as a 16X4 foot sacrificial work surface. Make the router trammel that drooplug describes. Hot glue or carpet-tape your poplar board right to the foam board & use the router to directly cut out your arc. Glue or tape a block the same thickness of your workpiece to the foam at the pivot location for a level trammel and a strong pivot.
The final pass can be cutting into the foam--save the hacked sheets for the next project or throw them away. I like to keep one around to use when rough cutting 4x8 sheets to size with a circular saw.

#8
11-20-13, 12:25 PM
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I'm not sure the point of all the foam is.

I would use to jigsaw to rough cut the board close and then clean up with the router and template. I like to use the template because you will get a nicer cut on your board and be able to place the arc more accurately. If you cut the board directly, you will need to make several passes and the cuts may not all line up. You are also putting the router bit under a much heavier load. That will wear the cutter out rather quickly. Using the jigsaw, you can leave about 1/16" to 1/8" left and easily removed that in one pass with a flush trim.

I've had to make cuts like this many times over the years and find this way works best for me.

#9
11-20-13, 02:03 PM
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If you have the room to work (probably outside) you can fasten the jigsaw to a 12' radius arm Swiveling on a central pivot. You can use sawhorses and boards to make a temporary work tee shaped bench for this. The cross of the tee holds the board to be cut and the leg of the tee the pivot arm.

#10
11-20-13, 07:29 PM
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I'm not sure the point of all the foam is.
So you can work on the largest, flattest available surface--the floor or ground.

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