As said a picture would help as I can't see how any size plate could have it's edge rounded by turning without hitting other parts. Your mention of coming from both sides suggests 30mm thickness plate but if holding plate at 45deg you will only be able to cover approx 20mm plate and it won't be a flowing roundover.

Where have you seen that size rounding cutter? it's going to be big with approx a 50mm dia head and maybe 25mm shank and also cost a lot, a 6mm radius carbide one for upto 45HRc costs £96

Edited By JasonB on 27/11/2020 08:17:54

When I had to round the ends of some crank webs I took advantage of the fact that these will be a pivot pin at the centre of the semicircle. So I made a pin of the right diameter to be a good fit in the hole and clamped the bar on to an angle plate so the pin was resting on the edge of the plate. Then milled a series of flats on the bar end with an end mill set at the correct height to give the desired radius with the bar clamped at different angles. Finally smoothed the polygonal surface by filing. From what I remember it was a pretty quick process. Could be done more precisely clamping down to a rotary table with the semicircle axis on the table axis. Nowadays I'd just use the CNC.

How do you propose to cut a 15mm radius with 6mm or 8mm radius cutters? I am confused as to exactly what you want to do.

The usual way of rounding the end of a piece of flat bar as shown in Jason's drawings is to clamp the flat bar to a rotary table in the mill, with any size end mill cutter you like, say 12mm diameter, and use the rotary table to rotate the job so the side of milling cutter cuts the radius on the job. The radius is determined by the distance from the centre pivot point of the rotary table to the edge of milling cutter.

You could do it in the lathe by mounting a small face plate to your dividing head and clamping the flat bar to the face plate. And hold any old decent sized milling cutter in the three jaw chuck. (If the chuck is not bellmouthed it will hold a milling cutter without slipping just fine, in my experience over some years now.

Define precision.

I wouldn't call radius cutters a precision tool. Let's ignore the confusion and assume we're talking about a cutter with a radius of 15mm. That's potentially a pretty large width of cut. I doubt a small milling machine will be happy. I don't have any radius cutters that are used vertically. But I picked up a set of radius cutters for the horizontal mill some years back. They work well:

But you need a rigid setup to avoid chatter and a poor finish. I only use them for making parts look pretty. If I needed a precision radius I'd use a ball mill on the CNC mill with a 3D adaptive stepover. In the absence of a CNC mill I'd use a rotary table as mentioned above.

A problem with using the above methods (except CNC) is that it is easy to go a thou or so too far with the cut. This leaves a small ledge or indent on the work which is suprisingly difficult to remove by filing. By the way I can't see sandpaper having much effect on gauge plate. Better to use emery cloth.

The method of rotating the work in a vice and using an endmill to produce a series of flats has never worked for me.

Andrew

The challenge with the 45 degree method (RHS of sketch) is positioning the cutter correctly so the mid-depth of the plate passes through the radius centre. Easy to do on a picture, some thinking needed for real metal objects.

The LHS will only work if the cutter radius is half the plate thickness. Otherwise the curve produced will always be imbalanced on the plate.

Edited By DC31k on 27/11/2020 14:26:43

Genuine question, does anyone actually market 7.5mm radius cutters?
19/64" is close, but I'd have thought even that would fit in between normal preferred size cutters.

Bill

If you said you wanted an accurate 15mm why have you bought 8mm which will give 16mm