Thread: Drawing large arcs

Quite clever! Shall have to remember that, I usually find myself on the opposite side of the room with long bits of string etc.

Incidentally, I don't think you need any maths. You could use that trick to fair any three points together in an arc, even if it's not symmetrical.

I have been doing such calculations and cutting of arcs like you are doing there champs although a bit different .
I have been cutting the arc into a disc spinning on a lathe , my radius's have been from 15 to 50 feet and the width of the disc or the chord is 600mm . I built the lathe to do it on with the two pins to slide the cutter through to cut accurate radius's

That is called a ship rights compass, It's one name for them there could be more . Its accurate and the way to go. mounting a router to the right sized jig sounds interesting . You will have to show that off

The way I was getting segment heights [ your C to D ] and chords [ your A to B ]sorted was with this link

http://www.1728.org/circsect.htm

How long is your chord ? that you want for a 2.4 M radius

Rob

I was doing it old school (calculator not trig tables) - nice online calculator though - thought I'd leave the maths out as the only thing which might be needed is the centre of the chord
eg curve the end of a dining table, you would set this compass from the endpoints of the arc and the centre of the end of the table

This is more or less what I used:
The formula for the radius is:


W is the length of the chord defining the base of the arc
H is the height measured at the midpoint of the arc's base.

For router use the pins could be almost anything, it is the two straight edges attached to the router with an adjustable angle I'm still thinking over. It would be nice two have something which could stay assembled and just drop the router on. The lock on the arms would also need to be pretty rigid. I know it would be easy to just make single use angles but it is way more interesting trying to make a nifty tool.

For the ends of tables .

For me to be able to cut a 600mm chord I needed the pins to be set at 800mm for my machines arms to slide across those both have to remain in contact at the same time with the pins , so the thing ended up being about 1800 wide pretty heavy [ Too heavy , good for when I'm using it but a nightmare to move around ] and adjustable. I was shown this and worked it out from there.

For cutting and moulding round tables and the half round ends of extension tables I do I stick to the basic compass style arm with a router and a steel pin 12 mm for it to pivot off . I like that way for tables.

Nice toy!
I had trouble finding it - Stanley calls it an angle divider hence the right angle end for measuring half the alternate angle. (I think)
http://www.jonzimmersantiquetools.co...ures/no_30.jpg
Does give some interesting ideas, I can see how you could use it to our purpose.
Would solve the lock but adds a lot of bulk.

Yeah nice toy . When trying to figure out how to solve the radius cutting problem, a friend showed me the Stanley 30 and it grew from that . I did my first prototype out of Masonite and a pencil.
I wouldn't be doing it for table ends though . you would be spending many hours making something that can be done with a more simple solution which is lighter and much more easy to construct . The single arm of a pivot.

Rob

For really big arcs or ovals, where you can't reach the centre to mark out, try these calculators

http://www.blocklayer.com/arc.aspx

http://www.blocklayer.com/oval.aspx

.

http://www.straordinaria.ch/taller/compass/#a.jpg

I had to make a series of bending forms for chair back rails. I used a CAD program to calculate the radius. (3/4 inch mid height from chord) A trammel for the router made the task simple to cut the forms.

(Tip: Make the first cut using a 3/4" router bit and then all subsequent cuts with a 1/2" router bit. Use a finishing / flush trim bit with the bearing riding on the 3/4" cut piece to trim the subsequent pieces until the needed height is achieved.)