turning a large diameter

You mean before going to bed after they get back from clubbing?

BTW this 'long radius' thing is common model engineer practice making expansion links on steam engines.

Edited By Bazyle on 05/09/2023 11:08:23

But some of them might let their wives to sleep late

I'll get me coat...

Rob

Its 39" RAD chaps

39" makes no real difference to the method I showed, might just need to extend the pivot point out beyond the R?H end of the lathe but that is simple enough.

All the curves on this bench I made were done in a similar sort of way and radiate out from a single point, from memory the top of the rear rail is 6.5m radius

OK, it is not turning and it will not produce a circular arc, but you would get very close with a boring head in a mill and angling the head of the mill.

When the head is perpendicular to the table, it produces an infinite radius (flat surface). When it is parallel to the table, it produces a circular radius equal to the swept diameter of the cutter.

So at some angle between those two it will produce an elliptical arc which will be very close to the 39" circular radius needed.

Replace the single point tool of the boring head with an abrasive wheel and the ground finish becomes a possibility.

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There is also the method used in woodworking of calculating the sagitta of the arc, nailing two straight pieces together at an angle and swinging them between the two end points of the chord. The vertex sweeps out the arc.

Thanks for all the ideas. Perhaps you now understand the lack of dimensions. This is for a clock pendulum suspension, and needs to be accurate.

I am leaning towards making this of two identical pieces of, say, 2mm thick steel plate. The only critical dimension is the radius and the need for the rolling edge to be at 90 degrees to the faces. I am hoping to avoid hand working the rolling faces to preserve accuracy. If I had the choice of machines, Ithink I would be looking at a pantograph engaving machine.

dave8

Edited By david bennett 8 on 05/09/2023 13:29:58

Edited By david bennett 8 on 05/09/2023 13:44:12

It will be as accurate as you make it. As Bazyle says the method I describe is commonly used to make parts of the valve gear on locos and traction engines 0.025mm or 0.001" accuracy is quite possible.

Though you will need the equipment to measure 39" to the accuracy you feel you need.

Edited By JasonB on 05/09/2023 13:39:49

You could try trepanning it out of a piece of flat bar - that's what I did with the traverse truck for a model carronade I was making:-

But the radius was far smaller than the one you're trying to do.

I'd reckon you'd gotta work on the biggest lathe, and do some o' the baddest things...

I thought I'd try generating it in CamBam for CNC milling.

Assuming it would be milled from 2mm plate, the top shows the plan view and the bottom the toolpaths including 6 holding tabs, using a 3mm endmill. These not positioned on the "active" top left face as they have to be filed away after cutting. You can just about see that this face is slightly curved.

Though designed initially "flat", I rotated the design to avoid one of the axes having to reverse while cutting the curve which inevitably leaves a tiny mark due to even the smallest backlash.

Jason, thanks for that.

dave8

Thanks for the correction - I think I was suffering from a thick brain.

dave8

So what is it, 1m OD (500mm Radius) or the 39" (990.6mm) Radius or 1m radius (1000mm)

500mm radius.

Use DRO . radius is set by X = required radius. X=0 can be anywhere you want off the end of the table. Input Xrad Set cutter diameter and cut on outside, use small incremental steps for Y. When done merely smooth rouugh edge to take off peaks.

First post says 39" rad

.

And subsequent posts have corrected that ^^^

MichaelG.

DRO Is OK if you have a mill but the OP says he only has a Myford and he also says he does not want to hand finish.

Ditto CNC which would breeze through a simple part but again he does not have one.

But still no consensus on what radius/diameter is wanted.

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I think we can take Mr Haine’s statement as authoritative

MichaelG.

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see post by dave8 05/09/2023 20:52:52

Edited By Michael Gilligan on 06/09/2023 10:53:53

So is that the Approx 1m dia or the 500mm (no tolerance) radius for this accurate job

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With regard the manufacturing process [which was the essence of the opening question] … I suspect it matters something less than 2/3 of b***** all.

MichaelG.