how to machine an internal curve

I want to make some friction rings in stainless steel similar to the one in the picture ( the second picture shows it in a typical use instead of a pulley) .

I know I can buy them ( very expensive) but that is not the point of my question.

Can someone tell me how they would go about making one please. I have the stainless steel bar. the hole in the middle is no more than 15mm diam( prefer 10-12) so I can drill that. But how do I form the inner curve but what shape is the cutter to get the clearance? If the cutter is made as a full form tool I am pretty certain my Warco 250MV will have too much chatter.

Once I have the inner curve I suppose I can grip the item in a shaped mandrel & then deal with the outer internal curve but once again the curve does pose a problem as any form tool will be rather large. It has to be very smooth to allow ropes to pass through under high load without any damage

Any ideas please

Edited By Sam Longley 1 on 06/09/2018 08:16:10

Any odd shape is quite easy to machine if you draw it up and then produce a set of co-ordinates, machine in a series of steps to these co-ordinates and then blend them in by freehand turning and handwork

From this

To This

And another one showing teh stepped cuts

Blending

The external groove can be done with a narrower half round tool so you don't have a large area of contact to cause chatter use a bit of rod to gauge the shape, this was 3/8" cutter for 1/2" groove

Edited By JasonB on 06/09/2018 08:24:11

Do you have access to a mill ?...... Easy enough to do with an end mill and dividing head. You could probably do the same using the lathe, but it might take a bit of ingenuity to set up.

Have not tried this but looks like a ball turning attachment on a lathe could be adapted to cut the inside curves by moving the pivot centre to be in front of the axis. Cut one side of the inner curve and reverse the material to cut the second side. Then cut the outer groove with a shaped mandrel as you have described.

Paul.

Go with Jason's method, I spent 50 years as a mould and die toolmaker and that is how we did it before CNC.

Alternatively, mark out and make a template of the required shape, set up a follower and machine using "copying lathe" approach. Often seen in Andrew Johnstone's photos.

In order to acheive a similar result for one of my projects, I used a radius milling cutter held in a boring bar holder on the lathe. By carefully adjusting the cutter such that the cutting edge was tipped a fraction below horizontal and just at or below centre height, I managed to get a pretty reasonable finish. Probably not "accepted" engineering practice, but, hey, it worked for me. But then, I'm not an "accepted" engineer, either!

John

Sam what is the radius on the internal edge? As John says you may be able to use a roundover bit but my only worry is as you said about a form tool in that the width of cut becomes quite wide and you may get chatter or work hardening on the stainless, OK if it were ali or brass as even router cutters will work on that if held in the tool post.

Jason,

Unless Sam has really massive thumbs or that's his big toe in his first picture, I estimated that the internal radius was around 5mm. That and the pulley carries the figure "30" on it indicated to me that it was 30mm diameter. That's why I suggested what I did. It obviously negates the suggestion if it's a lot bigger or smaller.

John

The grade of stainless steel is important too. Grade 303 is lovely to machine, 316 is nice to machine too, but work hardens at the drop of a hat, don't even think about peck drilling! And 304 is horrible.

Andrew

Ideal job for a turnado

one of my issues is the clearance on the underside of the cutter. I would use HSS tool steel & can experiment with grinding but the bevels & angles have me a bit stumped.

& to answer query - yes I do have a warco 16 mill but a milling cutter with a round on it might be expensive.

the curve is essentially a full semicircle inside the piece.

I will look for some 303 SS to try it out or I could experiment with some aluminium first as i have a lot of that

I should have put the sizes of the one in the photo

Overall diam is 30mm ,Hole is 12 mm, thickness is 12mm. Outer groove is not a problem as it could possibly be square as the cordage is spliced to it & does not have much friction. The internal part has lots of friction though

Edited By Sam Longley 1 on 06/09/2018 10:57:18

jason

Thanks I have just followed the link, I had looked on other sites & was looking for the wrong thing as I came up with something very £££'s. I could go with that though, as it is not overly expensive & I can hold the part in the mill.

As an aside what RPM would you recommend as a start on 303 stainless using the one with 6mm radius cutter?

Thanks

Still would be interested to know how to grind a cutter for a one off though as one would not want to buy a milling cutter for a single job if one did not need to

Edited By Sam Longley 1 on 06/09/2018 11:10:34

No. Ball nose (aka bull nose) cutters from China these days are cheap as chips and quite reasonable quality for home use.

An addendum to Jason's method might be to use a hand graving tool (like the way woodworkers use a lathe) to do the final blending of the co-ordinate steps. I've never used one with stainless before so can't say how it would respond.

Personally, I like the idea of using a ball-turning tool. Outside groove would be easy enough with a slender pointed tool with the tip set in front of the pivot point by the radius of the groove. The inside part might be a bit trickier, requiring one half done then turn the job around in the chuck and do the other side.

Edited By Hopper on 06/09/2018 11:24:29

Hi Sam,

It might help to look at carbide router bits !

Edited By John Baron on 06/09/2018 12:05:34

That'd be my first resort, too, held in the tailstock drill chuck if that blends correctly with the centre hole, or in the toolpost if not.

Hi Guys,

Its taken me nearly an hour to post a picture !

Put your own expletives in !

Use about half the speed you would if cutting steel and treat it as a 20mm dia cutter.

I would have thought you want 316 stainless for marine use.

Hopper that is a hand graver being used to do the blending in my 4th photo (HSS woodturning scraper actually)

I use router cutters the same way - but often remove the end screw and bearing if it gets in the way.

Are router cutters really up to the task? In the past I found that a router cutter has only had to look at a piece of steel to shatter & stainless steel would surely be a step too far!!!

I do have a few but do not want to smash them needlessly