I am trying to make a Gothic arch greenhouse (The classic Gothic arch is known as the quinto acuto or "pointed fifth").
The arcs are 4/5 of the span. The centres of the arcs are inside the span of the arch.
I have created a jig to make a sample arch. Everything was fine until I removed the arch from the jig. I did not wait at all for the glue to dry thinking the screws would hold the arch. But this was not the case, spring-back action is occurred. In this case spring-back was really high because I did not clamp the wood together properly and wait for the glue to dry. If I try to do again, I think the spring-back will be minimal.
My question is: How can I draw the jig taking into account of spring-back action? I am looking for a rough approximation. It does not have to be a perfect calculation.
I am located in Scotland and could not find the proper timber for this job. I have used this timber:
it is: 3 x 2 ( 38 x 63 mm) CLS treated 4.88m
I have used 2 of them to make one arch. I also used the same timber to put in the middle (15cm blocks every 30cm and at the each end 30cm blocks).