I just resided a dormer today. It was WAY beyond rotten. I have the second one to do tomorrow. Theey each have a round top window with a diameter of 34 1/2 ” . It was trimmed with a 3.5″ trim board but it was so rotten I couldnt use it as a template. I wanted to use solid wood and segment them togther with biscuits and poly glue. What is the formula for figuring out the angle for each segment. I figured it out by trial and error. I literally drew the radius on a peice of hardie and just tried diffent cuts. I used 1×6. I think my angle was 25 degrees. But there must be a way to use the same angle and all the peices will be the same length.
The very ineffecient carpenter
Replies
I'm presuming from your description that the window is a half circle -- in which case 180o (half a circle) divided by the number of segments gives you the angle at each joint and you divide this by 2 to get the angle of the cut on the end of the segment.
However, the fact that the correct angle for the first dormer was 25o leads me to suspect that you haven't got a half-circle.
Edited 4/2/2002 11:56:47 PM ET by IanG
I think you have to divide by (number of segments +1) to figure out how to divide the arc. (?) .
Phill Giles
The Unionville Woodwright
Unionville, Ontario
Phill,
Providing the arc is a segment of a circle, it is sufficient to know how many degrees of the circle it is.
I suggest the length of each segment is decided by the width of the timber from which it is to be cut, since it is that which decides how much of the curve you can cut from one piece. Dividing the circumference by the length of the segment gives you the number of segments needed and dividing the degrees by that number gives you the angle of each joint -- divide that by 2 to get the angle of each cut.
I've had to deal with this sort of problem often when cutting curved borders from parquet blocks and this (maybe ad hoc!) method has always worked for me.
Of course the method doesn't apply to an elliptical arc.
What you've forgotten is that each segment has two sides Ian. If the two "ends" are square-cut, then your number of angle-cuts is one less than the number of segments, if the two ends are part of the arc, then the number of angles to cut is one more than the number of segments.
Think about the simplest case, 2 segments turning 90º ; it has three cut angles: one at the start, one between the two segments, and one at the end. If the two termination angles are each 90º (i.e. square-cut), then you have one (1) angle to cut, 90º/2, or 45º degrees for each side. If you choose to originate and terminate with angle cuts, then the number of joints is 3 (# of segments +1) and each angle-cut is (90º/(# of segments +1))/2, which comes to 15º cuts to produce 3 30º joints. .
Phill Giles
The Unionville Woodwright
Unionville, Ontario
Phill,
It all gets complicated when I haven't access to a program that I can draw with but I don't think the first piece has got a square cut.
Draw 2 concentric half-circles to represent the outer and inner circumferii of the trim and divide the 'trim' equally and radially to represent the joints and you'll see that the cuts on both ends of the first piece are at the same angle.
I think!!
Phill,
I'm getting nothing else done trying to sort this out!!
Taking your example of the 2 sections = 90o, then a complete circle = 8 sections = 8 joints at 45o = 22.5o as the angle of the cuts, not 15o.
My head hurts.............
No, what you've left out is that with a part-circle there's one more IMPLIED piece, the rest of the trim, which you semi-circle must meet in two (2) places, not one (1)..
Phill Giles
The Unionville Woodwright
Unionville, Ontario
Is there a profile to this trim? Is it flat. Perhaps on a piece of plywood you could swing a router from a trammel point, there will be some waste, if you could use whats leftover somewhere. Much faster, IMO.