Someone posted a question about scribing a large radius of 30′ and not having enough room to do it. I just did two 41′ radius for a store front a couple weeks ago and I scribed them in the parking lot which worked out great for me. Even when I was doping that I was trying to figure out a different way other than that.
I always thought that there was a connection with the use of a framing square on how you can make a small circle with the framing square.
I went back to the job today and laid out a small 36″ wide by 6″ height and tacked a nail at each end and at the 6″ height mark. I just did it with a small jig real quick because I was busy.
The pieces to use should be longer than the width.
I cut a couple pieces of 5/4 around 5′ long and used 3′ as the width and 6″ as the height which would give an 30″ radius. I then tacked my three nails in place at the two width ends and the height and slid the two pieces of 5/4 so that they touch all three nails and overlapped at the top and then I scribed the 5/4 and dadoed them out and then nailed them together flush and pulled the top nail out and stuck my pencil right inside the corner and proceeded to pivot the 5/4 jig with it still touching each width end nails from left to right and then back from right to left and it made a perfect arch.
So if you have a large radius to make you can do the same steps as above but I would nail a piece at the bottom so it doesn’t move.
I hope this makes sense because this worked out really well.
Replies
Thanks joe, I'm glad you followed along with the pictures. Now I understand perfectly.
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Once I scribed it and took the jig away I new I was going to check it anyway with my tape but when I took the jig off I knew it was right by the way it looked. I checked with my tape anyway and it was perfect.Joe Carola
Using the angle relationships in this sketch, it should be possible to work out the angle needed on the jig for a given chord and radius.Joe Bartok
Joe,Why don't you try doing it the way I did and see how nice it works.Next time I get a large radius where I don't have the room I will do what I did yesterday.Joe Carola
Edited 1/13/2006 5:51 pm ET by Framer
Joe, here's what I meant in yesterday's post. It's the same thing as what you are doing but I'm solving the triangle without laying out the radius. (A lot of people want the houses we make wa...aay out in the boonies ... no driveways or other flat surfaces except for a sub-floor).
The triangle can be re-calced beginning with the same radius but a different chord. The result is a different triangle but it will still trace the same arc. Everything can be worked out using the Pythagorean Theorem.Radius Jig Calculation
Joe Bartok
I saw your pics over at JLC of that building you're doing right now. So when you built those two large arches, you laid out a 41 foot radius circle in the parking lot and then scribed the top part of the circle onto your ply?
Thanks for posting more pics of your jig. After we get off the roof I've got about four arches to build...and it looks like I've got a new method.
I snapped a 41' line on the parking lot and perpendicular to that line at the end I snapped an 18' 4" line square to the 41' radius line. I went 18' 4" because the radius started 2" in from each end.I tacke two full sheats of plywood down with a 2'4" sheat in the middle and then put a nail at the other end of the 41' line and then hoooked my tape on it all the way to the 2" mark on the plywood and scribed my radius. That was it.The first picture is a drawing of what I did. The second picture is the finished store front.Joe Carola