More Q's about surface finish.

This is a bit of an academic question really, but I am intrigued.

Facing off a 2" brass bar this evening I noticed this pattern:

It's not a great photo (best I can do with my 'phone) but you will perhaps be able to see five distinct 'zones' going in from the circumference to the centre. The boundaries are very well defined - it's not at all like the effect one sees with tool chatter or that sort of thing.

I did wonder if it might be purely an optical effect - at certain angles there are diffraction patterns:

which seem to coincide, roughly at least, with the 'zones'

I'd be interested to hear any opinions.

The question is academic because the part will eventually be polished, but I'd like to know what's going on.

Robin.

I believe this may explain: The Whitaker ring(bottom paragraph)

Source: Workshop Technology Part three, W.A.J. Chapman 1965 reprint.

Very interesting reference, William … Thanks

MichaelG.

Thats something I didn't know.

So Why 5 zones instead of just one. The book reference is for steel and this is brass?

The other thing that changes apart from surface speed is the approach curve of the material ahead of the tool tip. Much tighter in the centre and wider at the outside. Purely from an observational point of view surface finish has a lot to do with the way the material curls off the tip. I have watched steel chips when straight turning flip from being thrown out behind the tool tip to ahead with a direct change in surface finish. You would expect that finish would be affected by the way the chip is sheared from the stock and also any subsequent scuffing of the swarf against the finished surface. With Brass as the chip comes of in a sucsession of small 'flakes' for there to be some high speed variation in the force on the cutting tip which would vary with the chip size, again a function of the changing speed and dynamic motion of the surface as the radius is reduced.

All of the above is purely my speculation so I'm happy to be "put right' by more informed metallurgists.

regards Martin

Vibration...or, more accurately, the various stages of resonance of the system.

When using the Smart & Brown model A at the museum years ago when it had a very worn cross slide screw and nut, everything which was faced off had pronounced ripples. Since replacing the leadscrew with a new one and having two nuts set up for antibacklash which is set at 0.001" the ripples have dissapeared. I doubt whether that lathe could produce the quality of facing that yours can to reveal that rare effect.

A modern cnc lathe would have some compensation for tool speed, cutting radius and feed rate.

I wonder if the effect would still be there if a little WD40 was used during the facing?

Edited By old mart on 25/07/2021 17:26:03

Edited By old mart on 25/07/2021 17:27:09

Edited By old mart on 25/07/2021 17:31:00

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That second photo is particularly disconcerting, because it looks like a rainbow … with the sequence of colours inverted.

MichaelG.

It's what happens using a very long lens - doppler effect

pgk

It may well be an optical effect, relating to the wavelength of the colour visible in each band and the physical sizes of the grooves on the surface of the material. A result of the light being reflected minutely different distances, to cause interference patterns..

Howard

There is no real reason to assume that the groove size varies, Howard

A diffraction grating will produce the full visible spectrum from white light.

MichaelG.

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Ref: http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/phy217_inst_grating.html

The photo at the bottom of that page ^^^ is an excellent illustration.

Edited By Michael Gilligan on 25/07/2021 23:01:57

I suppose this is vaguely related, apologies if it isn't. Where I used to work they took great pride in their optical manufacturing capabilities. The prize piece of equipment was a diamond turning machine. It was used to generate mirrors, concave, flat and convex from aluminium. The surface finish was optically superb (apparently "cock on" is a term used by optics dudes.........much used in regards to the DTM). Usually machined al. alloy is very prone to micro damage from fingerprints and similar, but the DTM generated surface was not.....so I was told.

cheers

Bill

Michael,

Unless power cross feed is used, even with a skilled operator, the groove spacing is likely to vary slightly, and that assumes that the power cross feed is absolutely uniform in its operation, as is the speed of rotation of the workpiece.

Since we are discussing wavelengths of light, we can, I think, afford to be a little pedantic in those respects.

As you say a matter of interference and diffraction causing an optical effect. (note the partial mspectrum visible in places.

Howard

I’m bewildered by your response, Howard … but never mind.

All I pointed out was “There is no real reason to assume that the groove size varies”

Not that it definitely does not vary.

In what particular sense do you wish to be pedantic ?

MichaelG.

Michael. I think that we both were cast in similar moulds!

But you will agree that once we get into the wavelengths of light, the units are physically VERY small.

Have you never been troubled by Newton's Rings in glass mounted slides?

Not even sure if we are actually differing, let alone disagreeing.

Howard

From the photo the reflected light seems to come from a direction roughly along the lines of the groves rather than perpendicular so my take on this is that the diffreaction is occuring on the chip boundries along each each grove. Think old fasioned Vinyl LP's. As the colours are seen from the same angle it would apper that the chip spacing is varying with diameter which would be consistant with decreasing surface speed towards the centre of the bar.

n λ = d sin .

For constant the equation above shows that the wavelength seen varies in proportion to spacing d so the red sectors have larger chip sizes than the blue.

If this is correct then the feed or groove spacing has nothing to do with it.

regards Martin

Edited By Martin Kyte on 26/07/2021 09:44:51

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I’ve been ‘troubled’ by ‘Newton’s Rings in many places, Howard … and have also used them to gauge surfaces.

I didn’t intend to differ or disagree … merely to observe that the colours in Robin’s photo are not necessarily an indication of variations in pitch. The rendering is not good enough to perform quantitative analysis.

Peace !

MichaelG.

Edited By Michael Gilligan on 26/07/2021 09:47:42

Thanks for replies, and apologies for not getting back to this sooner - my wife has taken on a foster dog so I've been in woodworking mode making stair gates. C'est la vie!

William - many thanks for the info about Whitaker rings. I'm not convinced that's what going on here though - brass, very light cuts (~10 thou) slow feed. But maybe - and valuable info even if it isn't.

Bob Stevenson - I was initially sceptical about 'stages of resonance', but changing the spindle speed from 550 to 700 rpm gave different 'zones':

and returning to 550 gave results broadly similar to my first photo:

So it's mechanical. I think I understand what's going on now.

On the optical effects, I found, unsurprisingly in retrospect, that the 'spectrum' was very much dependent on camera and lighting angles. Thanks Martin.

I doubt that I can get much further in understanding optical effects without specialist kit - a trichroic beam splitter would be an absolute minimum requirement I expect. Sadly , I haven't got one.

I should just be happy that I can get this finish on a mass-market hobby lathe.

Robin.

Edited By Robin Graham on 30/07/2021 01:21:13

Edited By Robin Graham on 30/07/2021 01:28:35

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For anyone who might be interested, but didn’t follow that link … here is the referenced photo:

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The ‘double rainbow’ group at lower-right of the picture bears [shall we say] a striking similarity to the pattern that Robin photographed.

MichaelG.

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Sorry, I can’t be bothered distorting the picture to get the curvature … but it doesn’t take much imagination

Ref. ^^^