router jig for large arcs?

[bugbear]

Has any one taken this (marvellous) idea:

made a router jig, for accurately cutting (as opposed to marking) large arcs?

BugBear

[Pop]

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 drawn. I can see uses for it though not for a router.

Pop

[bugbear]

When making an arch in a building (e.g. doorway), you often know the width and height but NOT the radius, so the technique is directly applicable.

That'd be maths (or as it's called in the USA, "math")

BugBear

[Josh]

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 degrees apart.

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 a router.

Josh

[bugbear]

(maths agreed with and snipped)

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.

BugBear

[bremen68]

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 face plate.

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.

[Josh]

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 in.

Don't get me wrong. It's a good idea. Just different than what Bugbear was talking about. Unless I misunderstood.

[bremen68]

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.

[Robatoy]

Tip of the hat for this. What a nice way to mark the end of peninsula countertop. I really appreciate this.

Rob

[bugbear]

Heh. The technique I posted at the start of the thread will happily do a 1" high, 6' wide curve.

would require a 54' trammel bar, which might be a little unwieldy for people with smaller workshops.

BugBear

[bugbear]

You're welcome. The idea isn't mine; it's VERY old. I'm told it's been in FWW too.

If you make a router jig - tell me!

BugBear

[Josh]

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 outside one).

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 trigonometry needed.

Josh

I think you could just screw on a couple of temporary wood blocks to the countertop

[bugbear]

I hadn't thought of that - I was thinking of the bit as a point (which is wrong, of course).

BugBear