Drywall Arch and Drywall Router

Rotozip is the tool, darn expensive, don't even know if it's still availble, spiral bits were expensive.

 

I've got a couple in the garage.
Didn't know people were still doing arched doors though

 

You could use a router and a sharp chisel

 

You can form the arch either as a unit to install or by marking onto the material and cutting out what you don't need. Either way you will need need to mark it.

Which you do by fixing a string on the floor at the centre of the space, running it out to the arch top centre. attach a pencil to the string and follow the arc making markings on the material. If the arc is too circular or too flat, adjust the position of the string so that it creates a set of marks you can join up to create the desired shape.

The advantage of doing it onto the walls, is you can see what you will get before cutting. The open ends will need to be covered in wire mesh and plastered neatly to form your arch.

if its plasterboard you can cut with a hand hole saw, if its mdf cut with a jigsaw. Rotozips make something look easy and useful on a tv advert. They are a toytool, and end up costing you money in silly attachments and accessories. Eventually they seize up.