Arches

[Daglad]

Hi everyone,

I am trying to make an archway for the garden. The problem l have is making a template for the actual arch. The problem is marking out a constant depth to the template. It's a bit difficult to explain but the depth starts at 100mm and spreads to 110mm in places and then back 100mm or less. All l really need is two parallel lines in a constant arc. I have tried a piece of string from a static point and also a piece of wood with holes in for a marker.

Any other suggestions?

thanks
:confused:

[silentC]

Draw your first arc, move your centre point up 100mm and then draw the same arc (same radius). That will give you two arcs of the same radius separated by 100mm. I think that's what you're after.

[Barry_White]

silentC

I don't think that is quite right. To get 2 parallel lines in an arc you need to draw them by using the same centre point 100mm apart.

What I don't understand what he means by going from 100mm to 110mm.

Perhaps a freehand sketch of what you mean Daglad

[silentC]

Quite correct Bazza, I shouldn't reply to these things when I'm still half asleep. Never do mental geometry before lunch.

[Barry_White]

Daglad
Is this the sort of thing you are trying to do.

[Daglad]

Hi guys,

Thanks for your response's, I have done the 100mm apart center points that silent C recommends. I am sure this is the way to go. I guess l have the hand and eye co-ordination of a handcuffed crab!

I am back in my shed tomorrow so l will post my results (Or lack of) tomorrow night.

Thanks

[journeyman Mick]

To get two parrallel arcs 100mm apart they must be of different radii but use the same centre point. If you draw to arcs of the same radius with centrepoints 100mm apart they won't be parrallel. Use the same centrepoint but with one arc's radius 100mm less than the other. I've got a formula somewhere for how to calculate the radius of a curve if all you have is its length and height.
All of the above only works if the curves you are using are arcs of a circle, and not parabolas or catenarys. If you want a parabolic or catenary curve the easiest way is to hang a piece of chain up betweeen two points and adjust the distance between the points and the length of the chain until you get the desired result. The you put a piece of ply or mdf against it and trace the curve.

Mr Curvey (Mick :) )

[silentC]

Hi guys,

Thanks for your response's, I have done the 100mm apart center points that silent C recommends. I am sure this is the way to go. I guess l have the hand and eye co-ordination of a handcuffed crab!

I am back in my shed tomorrow so l will post my results (Or lack of) tomorrow night.

Thanks
Don't do it that way, I was wrong. Bazza and Mick are on the money: same centre point, different radius.