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

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

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

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

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

pgk

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.

.

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

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

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

.

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

Ref. ^^^

Just resurrecting this thread to mention that the video I linked this morning includes an interesting piece about reflective diffraction gratings; commencing a little after 22minutes duration.

MichaelG.

.

Ref. https://www.model-engineer.co.uk/forums/postings.asp?th=169694&p=1

Edited By Michael Gilligan on 25/08/2021 13:25:02

This might help, Robin

Light Waves – Finding Slits per mm for a Diffraction Grating: **LINK**

MichaelG.