a) mark it out accurately and saw / grind/ file to the middle of the marked line. To mark out, pointed dividers set to radius with a vernier or digital caliper will be quite accurate. locate one point of dividers in a centre punch divot and scribe your line. To help see the line better, colour your plate with permanent marker or marking out fluid or engineers' blue prior to scribing. Also, when approaching the line in final filing, use 10X optical magnifier of some kind.

b) send a DXF file for it to your local wire edm house, arrange money transfer, and wait for the postman

Good luck JD

You could try a form tool but sounds hard work.

I have tackled this sort of job by creating a spreadsheet of feeds to form a semi-circular arc from the circle equation and used the handwheels and a lashed up DRO using a digital caliper for the longitudinal axis. That works fine – gives you a stepped approximation to the curve that you file to a smooth shape afterwards.  I can give you some further information on doing this if you want.

Nowadays I'd just give the problem to my CNC…

Edited By John Haine on 26/01/2017 16:00:06

As John H says work out a few co ordinates at regular intervals which will give you the basic shape then blend further by eye and finish with a file.

This shape

use these co-ordinates

It really isn't that difficult. I cut a profile that method on one of my first projects..(and I still don't know what I'm doing most of the time). No cad software so I drew it out on a scabby bit of graph paper and worked out the depth and length of cuts from that. Yeah, a lot easier with a DRO t keep track of the numbers but more than doable without. You can see i crossed each out as i got to it:

Instead of calculating co-ordinates just use a fixed template (use your tailstock to mount it, it can be a drawing on card!) and a wire bent pointer from the tool-holder,

– Nick

I've never seen a brake wheel before . What is it for ?

Easiest way for a beginner to get the job done with limited machinery might be to set the aluminium disc up in the lathe, probably mounted on a rough mandrel made from a bolt and nut, then by setting the top slide to progressively different angles, go around and machine a series of small flats on the OD approximating the shape you want. Start by setting the topslide to 45 degrees and make a large chamfer across half the OD surface, then move the top slide around to 30 degrees and make another smaller chamfer, then 15 degrees and repeat. Then move the topslide around to the the other side of the job and repeat teh process. Once you have a nice series of flats approximating the radius you want, you can finish the job off with a file while the job is spinning. It will help to make a rough radius gauge by drawing up and cutting out a piece of cardboard or plastic ice cream container etc etc just to make a reference to work to.

This is what the guy from eccentric means by a form tool, it is a piece of metal thet is ground or cut to the negative of teh shape you want to form. If small they can be ground from an HSS toolbit, if large as in this case then better to use gauge plate also known as ground glat stock.

To do a half round drill or bore a hole in your gauge plate, saw inhalf across the centre line of teh hole, use a dremel to put relief on teh underside of teh cutting edge then harden and temper.

As your rounded edge is quite wide and the job of a large dia you would probably be better off making the tool a quater circle and then when one side is done reverse the work in the lathe and do the other side. Still going to be quite a wide cut so you will need a reasonably sized lathe.

If you are not happy grinding a tool then I think RDG do radius forming tool bits ready ground or as suggested above a rounding over router cutter can be used.

You probably don't really need to have as rounded an edge as the one in the photo, just turn it like a disk flywheel and round off the corners to say 3mm radius so you don't rip up your hands anymore than needed.

J