[Metrology] Autocollimator Question

Moderatemixed

Moderatemixed

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Hi all.... further to my last post.

Can anyone tell me if an Autocollimator can be used to produce a reference flat surface. If I wanted to scrape in a piece of cast iron, how would I use the autocollimator to assist me in making it flat if I didn’t have a surface plate to “blue up”?

If so, where would I look for the how to information?

Thanks.


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Morning @Moderatemixed. I read your previous post and I am also seeing this one here and man you are definitely working with some equipment most of us will never touch in our lifetime. With that being said you may not find anyone here with that particular knowledge ad experience - although I'd love to be proved wrong.

There are other places which people geek out over metrology. When I worked in the industry, I spent a lot of time there getting up to speed. You may find information in those places as well.

General Metrology Forum

www.qimtonline.com www.qimtonline.com

General Measurement Device and Calibration Topics

Metrology - Calibration, Gage and 'How to Measure' (and related) questions.
elsmar.com elsmar.com

In addition, other machinist forums (not to be named) have pretty active metrology subforums. You may find help there.

So here is my analysis. An autocollimator detects angular misalignment in yaw and pitch relative to the target, but not roll. You could develop an instrument to mount the autocollimator on two round feet, maybe 1" in diameter, 6" apart. These each would bridge any topography under them, somewhat like a scraping flat. The would need to be ground and lapped extremely flat.

Then manipulate the unit to take measurements, rotating about the table surface to find a measurement minima (removing the yaw component of the measurement, leaving only pitch, the topography of the surface). This, compared to a reference zero, gives the relative elevation between the two pads. You would need to walk this instrument across the surface, taking angular misalignment (over a known span = vertical misalignment) measurements. Then move the setup and repeat in the other axis. Eventually you should be able to build a mesh of the surface with incremental displacement measurements from your zero point.

But I've never done this, so just speculating.
 
Thanks for the links! My experience with “the forum that shall not be named” has been poor at best. Seems those who do this for fun, enjoy doing so, but those who do it for a living don’t. I know that I’m generalizing. I will check out what you’ve suggested. I understand what you are talking about/suggesting and I think that once I complete the Repeat O Meter it will do much of what you are suggesting.

My other thought was that if I can isolate a portion of one of my plates that is reasonably (A to B grade) flat I could use that portion to mark and scrape in a straight edge that could then be used as my shop reference to move forward.

I have several books, Machine Tool Reconditioning by Conoly and Holes Contours and Surfaces by Moore (on it’s way). MT Recon talks about using a straight and flat reference to then make a surface plate flat.

I have a Starrett Master Precision Level which is the only item (at present) in my shop that is scraped and presumed “flat”. As I recall it is 15” long. Maybe that level becomes my reference surface onto which I build. Thoughts?


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Some years ago I attempted a similar project on an 18” diameter cast iron platen. Originally it was used in the vinyl record industry for pressing the discs so had indeed been a good surface at some point but then subsequently abused as a hammering anvil. Anyway, my idea was to use an 18” straightedge combined with a level to regain the surface flatness, and to a fair degree it did work but was an arduous task.

Essentially I marked out a square such that the points came just shy of the maximum diameter, scraped a trench for lack of a better description along one side of my square getting it as flat as I could along it’s length. I established that flattened trench as level, then scraped the opposite side of the square coplanar to the first. Now I had two trenches, both level, on opposite sides of my square (say the 12 o’clock and 6 o’clock positions).

After that it was just a case of taking a print using the edge of my straightedge between the two trenches and removing all the high points until the ends straightedge just touched the bottom of each trench without being hollow in between. I should confess here that in the absence of real HiSpot I was using an especially greasy lipstick procured from the drugstore for such purpose — it worked, kind of, sorta...

Anyhow, I kept at it for days and eventually resorted to a small angle grinder to speed things up. My elevations were in the orders of tens of thousandths to remove, and I didn’t have a very good scraper either really so it was a mountain of a task. But, I got it pretty flat. Not surface plate quality, but flat, and I still use it for a welding setup.

I don’t know enough (ya think? ) about autocollimators and the like to say yeah or nay, but I suspect that while the theory may be solid the practicality my not bear out to the degree of accuracy desired. That’s just my guessing though. I have seen demonstrations of obtaining flat reference surfaces using the three plate method but that may be outside of where you want to go with this venture.

Good luck with the experiments though, you’re bound to learn a ton!

-frank
 
Thanks all. I broke down and bought a Camelback straightedge that was scraped flat to match an A grade surface plate. I’ll use a procedure in Conely’s book to mark the Brown and Sharpe plate and scrape it flat. From there I’ll do the same with the lapping plate which will set me up for the granite plates.

A whole bunch of effort when it is unlikely that I’ll ever achieve more than .0005 accuracy in anything that I machine..... That said, aim high right!

Cheers all,

Thanks for your patience.

Derek


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Moderatemixed said:
Thanks all. I broke down and bought a Camelback straightedge that was scraped flat to match an A grade surface plate. I’ll use a procedure in Conely’s book to mark the Brown and Sharpe plate and scrape it flat. From there I’ll do the same with the lapping plate which will set me up for the granite plates.

A whole bunch of effort when it is unlikely that I’ll ever achieve more than .0005 accuracy in anything that I machine..... That said, aim high right!

Cheers all,

Thanks for your patience.

Derek


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Click to expand...
Nice! Where'd you get the straightedge and how much? I've looked at 24" ones a few times.

Hope I didn't force your hand one way or another.
 
I thought the autocollimator worked through an optical flat to measure.

In any case, it measures in wavelengths of light. You've got to be crazy flat before it will measure anything useful. You have to be able to count the bands or look at patterns.

You might want to start with the three plate method.

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Weldingrod1 said:
I thought the autocollimator worked through an optical flat to measure.

In any case, it measures in wavelengths of light. You've got to be crazy flat before it will measure anything useful. You have to be able to count the bands or look at patterns.

You might want to start with the three plate method.

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Click to expand...

I don't think you need the optical flat, rather 2 mirrors. The interference patterns in the wavelengths of light are seen at the autocollimator.

But yes, you have to be crazy close before it is a useful instrument.
 
...... this will add to how ridiculous I am. I have 4 optical flats. 8”, 4”, 2” and 1 inch along with the monochromatic light source. Neither is of much help here. I have a copy of “Foundations of Mechanical Accuracy, and will check the link you suggested. The straightedge is 24” x 2”. Has been scraped in by a scraping pro using an A grade plate. Keep in mind that is is in very nice condition. I paid $500 CAD for it. To answer your question, no you didn’t push me over the edge. Curiosity and a profound interest in this particular “rabbit hole” did.

Cheers,

Derek


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