How to set out this curve

[Mcluma]

How to make this curve

I’m preparing replacement architrave to match in with the rest of the house and above the windows we have the following type of architrave

This particular moulding is long 3900mm by 40mm

How would you set out the curve?

Thanks
Chris

 

[CNC Paul]

Make a template or a half from the existing window

 

[Harbo]

At 3.9m x 40mm it's very long and narrow?

I would try to draw it out to a suitable scale to get some reference pin coordinates then use French/Railway Curves for the ends and a Piano Wire for the middle bits.

Rod

 

[Jacob]

Drawing doesn't match the given measurements. If that's 3.9 long then it'd have to be 400 to 500 wide.
Whatever it is - copy the existing, either by scribing on to a board, or taking it down and drawing round it, or making a rough copy from measurements and offering it up and trimming until it matches.

I wouldn't attempt the geometrical methods describes as it was probably freehand in the first place and nowhere near a perfect ellipse. Either way it's easier to copy rather than re-construct.

 

[ondablade]

I might fake it the way Harbo says, but it looks like it's half of a long and narrow ellipse. If so you could probably plot the profile using the pin and string method described half way down here: http://en.wikipedia.org/wiki/Ellipse

It'd take a bit of fiddling to get the string loop and pin spacing to the right length, but it'd give a nice smooth curve for a template or something...

 

[Mcluma]

Trammel of Archimedes

That will work

http://en.wikipedia.org/wiki/Trammel_of_Archimedes

Will be interesting to build something like that

 

[Mcluma]

Ellipsograph

 

[ondablade]

Hmm. Not heard of Mr. Archimedes trammel before....