turning a large diameter

[DC31kParticipant @dc31k]

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.

—

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.

[david bennett 8Participant @davidbennett8]

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

[JasonBModerator @jasonb]

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

[Mick B1Participant @mickb1]

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…

[John HaineParticipant @johnhaine32865]

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.

[david bennett 8Participant @davidbennett8]

Jason, thanks for that.

dave8

[david bennett 8Participant @davidbennett8]

Posted by John Haine on 05/09/2023 08:33:01:

Cnc mill the profile. Polish by hand. Actually 1 metre approx outside diameter.

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

dave8

[JasonBModerator @jasonb]

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

[John HaineParticipant @johnhaine32865]

500mm radius.

[KWILParticipant @kwil]

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.

[bernard towersParticipant @bernardtowers37738]

First post says 39" rad

[Michael GilliganParticipant @michaelgilligan61133]

Posted by bernard towers on 06/09/2023 09:06:29:

First post says 39" rad

.

And subsequent posts have corrected that ^^^

MichaelG.

[JasonBModerator @jasonb]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 06/09/2023 10:00:00:

[…]

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

.

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

MichaelG.

.

see post by dave8 05/09/2023 20:52:52

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

[JasonBModerator @jasonb]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 06/09/2023 12:18:05:

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

.

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.

[JasonBModerator @jasonb]

Ah, it's only when you get down to less two you need to worry

Edited By JasonB on 06/09/2023 13:06:02

[Michael GilliganParticipant @michaelgilligan61133]

As I am sure you know, Jason … That was not my point.

Presumably John will respond when/if he considers it appropriate.

MichaelG.

[david bennett 8Participant @davidbennett8]

Gentlemen, I originated this post, and the confusion. First , I should have said 39" diameter. Sorry. I neededd the best way to do this in principle.That has now been answered. Many of the quoted sizes are adjustable .Thanks.

dave8

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