Speeds/feeds for 2mm mill in cast iron

[Andy_GParticipant @andy_g]

I doubt that you’ll polish out anything but the very finest of scratches without losing the flatness. (So fine that they wouldn’t be a problem for most uses of a flat.)

You would really need to go back to grinding to get rid of scratches.

[jaCK HobsonParticipant @jackhobson50760]

Thanks, and yes – I agree! Surface under a microscope looks bad! I will go all the way back to medium then fine grits before try and polish again. I spent a lot of time working out how to grind fast but consistent – and that is where the grooves in cast iron lap didn’t work for me.  i’m going to try regrind on flat cast iron (no grooves) before final polish on pitch lap. I have some slight hope that the middle of the flat might get flatish enough for some interference patterns of any shape. If I can get those then I have some sort of feedback. I’m using green laser through a ping pong ball as light source. I’m not in any rush – well, I think I have time for maybe a decade of learning still.

I’ll measure the cast iron flats to see how good they are – I should be able to check to less than 0.01mm fairly easy.

[Andy_GParticipant @andy_g]

I think you should be able to see fringes with that setup – one fringe from a green LASER is about 0.25µm (0.00025mm) deviation.

You could probably find a piece of window glass that is quite flat if you had enough pieces (3+) to compare against each other using interference fringes.

I would encourage you to keep going, as it is possible – this is a small (~30mm x 50mm) octagonal flat that I made for a Newtonian telescope a long time ago. The fringes are from a neon lamp, so are about 0.3µm.

(Apologies for the image quality, but they are cropped from larger images, captured at low light at a time when 1.3 megapixels was the cutting edge of digital photography!)

 

 

 

Good luck!

[Michael GilliganParticipant @michaelgilligan61133]

jaCK

… I have only watched a few minutes of it so far, but this looks like a useful set of videos: https://youtu.be/Z0jCycLpXSM?feature=shared

 

MichaelG.

[jaCK HobsonParticipant @jackhobson50760]

Thanks! I discovered a big problem with my process. A case of stupidity! I used some pitch I made for workholding. Looks nice but I added plaster of paris for bulking up and somehow thought that wouldn’t be a problem. The result is that the pitch is quite a coarse abrasive itself!

I will try again with different pitch but without added abrasive. Meanwhile I have assembled a comprehensive collection of abrasive powders.

[jaCK HobsonParticipant @jackhobson50760]

To finish off the story…

Today I got a polish on both sides of the ‘flat’ good enough to get interference lines on the end of a gauge block. I can’t get it to work over a large surface- but I have some polishing to go. I estimate it’s within about 150 nanometers per centimeter or a radius greater than 40 meters. So rubbish in terms of optical flat but flippin amazing for me, a plonker, and so much better than I ever hoped! And I have three good laps, proper optical pitch, a huge selection of lapping abrasives, and quite a bit less in my bank account.

[jaCK HobsonParticipant @jackhobson50760]

Other people might not see em… but I see fringes!

[Michael GilliganParticipant @michaelgilligan61133]

I see fringes too

… Well done, you !

MichaelG.

[Author]

Posts