Spurious Accuracy

[not done it yetParticipant @notdoneityet]

Or "nearenuff", "closenuff" or eqwivalent of a gnat's sumthing….

[richardandtracyParticipant @richardandtracy]

Not only is there a problem of spurious accuracy, but I think there is a problem with the perceived need for spuriously good fits.

I'm currently designing a bit of kit with a 5ft diameter slew ring using a certain German made plastic bearing material. I wanted to know the 'best' fit for good slewing and reduced friction under a large overturning moment. I was thinking that .005 mm would be about right, as that seems to be the sort of clearance for a bush bearing.

Not a bit of it. 0.25mm was to be the absolute tightest I should go to, according to the bearing manufacturers advice. And preferably with more clearance than that. It's something I will have to look at elsewhere, too.

Regards

Richard

[john carruthersParticipant @johncarruthers46255]

As you say, temp is important, especially for optics. My 10" telescope mirror is better than 1/20 wave of sodium light, +/- aprox 1 millionth of an inch (0.0000254mm). I would figure for 5 minutes, then cool in a bucket of water at room temp for 15, then test, then repeat as necessary.
We have much to thank Léon Foucault for.

[Neil WyattModerator @neilwyatt]

I think it's amazing that mirror grinders at home in their workshops can produce surfaces more accurate than teh world's roundest object with little more than pitch and grit

[vintagengineerParticipant @vintagengineer]

I was commissioned to make a gear change selector linkage for an Edwardian racing car. The plan was to base it on a Bugatti design. Basically it was a shaft sliding inside a tube which slide inside another tube all bolted to the chassis.

It worked really well on the bench, smooth as silk with nice close tolerances. Totally failed on the car. So managed to strip down a Bugatti one and found it was as sloppy as hell with 1/16th clearances.

Taught me that you can be too accurate!

[Mick B1Participant @mickb1]

At a woodworking course I was once on, one of the trainees (a highly qualified instrument engineer) was faffing about trying to get a dimension more precise than the instructor said was necessary.

"But I've worked to microns!" protested the trainee.

Quick as a flash the instructor replied: "Ah, but I've worked with morons!"

You couldn't've scripted it better….

Edited By Mick B1 on 05/11/2017 10:01:28

[John HaineParticipant @johnhaine32865]

Posted by Neil Wyatt on 05/11/2017 09:13:58:

I think it's amazing that mirror grinders at home in their workshops can produce surfaces more accurate than teh world's roundest object with little more than pitch and grit

More accurate (close to a standard value), or more precise? When you grind a mirror, are you aiming to get a specific focal length (which would need a curve of a certain radius), or get very close to a uniform curvature, or indeed both? Presumably you need pitch in both senses of the word too!

[Charles PParticipant @charlesp60333]

I remember a whole module at university about accuracy and precision – and the importance to understand when you need one, the other or both.

One pet hate is the idea that skateboarders willl actually benefit from ABEC 7 grade bearings. Your average Bridgeport probably spends most of its productive life with bearings less accurate than that!

[Howard LewisParticipant @howardlewis46836]

I am suspicious of those who claim to be able to work to microns other than in temperature and humidity controlled conditions after a good long "soak". (As already said, a Calibration lab).

Hold your micrometer long enough, or breathe hard on the work, and you may well get the reading that you desire.

In my book that is "Delusions of Accuracy" rather than "Spurious"

Hands up anyone manufacturing a space ship!

Howard

[John HaineParticipant @johnhaine32865]

Posted by Charles P on 05/11/2017 21:02:28:

I remember a whole module at university about accuracy and precision – and the importance to understand when you need one, the other or both.

One pet hate is the idea that skateboarders willl actually benefit from ABEC 7 grade bearings. Your average Bridgeport probably spends most of its productive life with bearings less accurate than that!

When my son was skateboard mad he insisted in buying these. They might have been precision when new but by after a couple of hours skating in the dust they weren't any longer! A triumph of marketing…

[John HaineParticipant @johnhaine32865]

More accurate (close to a standard value), or more precise? When you grind a mirror, are you aiming to get a specific focal length (which would need a curve of a certain radius), or get very close to a uniform curvature, or indeed both? Presumably you need pitch in both senses of the word too!

That should have said grit, not pitch!

[Mark RandParticipant @markrand96270]

Well, I don't know about you lot, but my shed's built with 5" of PU foam in the walls and a split unit AC/heater. It maintains +/- 1 degree C from week to week and only changes by 3°C per day if I turn the AC off. Yes, I do work to tenths when I can.

When I've finished reorganizing the shed after converting my gantry crane into a bridge crane, the next task is to re-lap my 36"x48" surface table to the best quality I can achieve prior to using it in the re-scraping of my surface grinder. The precision of the H&W autocollimator I'm using is equivalent to 0.00005" over the diagonal of the table and the repeat meter I'll be using for short disance checking has a resolution of 2.5 micro inches… If I can keep the temperature constant and not introduce other perturbations. I'll be doing my best to achieve that. It's part of the fun of having the workshop.

 

When I want less precise work, there's the vegetable patch outside.

Edited By Mark Rand on 05/11/2017 23:19:31

[vintagengineerParticipant @vintagengineer]

I once had to make a large bronze bush for a Bugatti gearbox, the customer supplied the Ali gearbox casing. When the customer got it all back, he phoned me up to say the bush wouldn't fit!

When I got to his workshop the temperature was about 5C! Put the gearbox in front of a fan heater for 1/2 hour and the slide in nicely!

[Martin KyteParticipant @martinkyte99762]

Posted by John Haine on 05/11/2017 21:49:10:

More accurate (close to a standard value), or more precise? When you grind a mirror, are you aiming to get a specific focal length (which would need a curve of a certain radius), or get very close to a uniform curvature, or indeed both? Presumably you need pitch in both senses of the word too!

That should have said grit, not pitch!

No Pitch is fine. Thats whats used as the polishing lap with cerium oxide or rouge if you are being old fashioned.

The aim is to figure for a parabolic surface. Asolute focal length is not critical.

regards Martin

[Neil WyattModerator @neilwyatt]

Posted by John Haine on 05/11/2017 10:16:55:

Posted by Neil Wyatt on 05/11/2017 09:13:58:

I think it's amazing that mirror grinders at home in their workshops can produce surfaces more accurate than teh world's roundest object with little more than pitch and grit

More accurate (close to a standard value), or more precise? When you grind a mirror, are you aiming to get a specific focal length (which would need a curve of a certain radius), or get very close to a uniform curvature, or indeed both? Presumably you need pitch in both senses of the word too!

Actually you are aiming for a parabola (for a quality mirror), basically you create diffraction patterns using a knife edge (focault test) and the shapes you see show you the errors in the mirror. The aim is NOT to hit a specific focal length, but to get a near-perfect parabola close to your intended focal length.

Neil

[Martin KyteParticipant @martinkyte99762]

I kinda just said that.

:0)

 

Edited By Martin Kyte on 06/11/2017 11:01:54

[Mick B1Participant @mickb1]

Posted by Neil Wyatt on 06/11/2017 10:59:27:

The aim is NOT to hit a specific focal length, but to get a near-perfect parabola close to your intended focal length.

Neil

That's why you have an adjustable focusser. And the closer you get to a uniform focal length, the more of your field of view will share a common focus (?).

[John HaineParticipant @johnhaine32865]

Thanks for the explanation. Precision then?

[Neil WyattModerator @neilwyatt]

Posted by Mick B1 on 06/11/2017 12:01:32:

Posted by Neil Wyatt on 06/11/2017 10:59:27:

The aim is NOT to hit a specific focal length, but to get a near-perfect parabola close to your intended focal length.

Neil

That's why you have an adjustable focusser. And the closer you get to a uniform focal length, the more of your field of view will share a common focus (?).

And because people have different eyes, cameras and eyepieces!

To get a sharp image across the field, reflectors of about F5 and faster need a 'coma corrector' as stars near the edge get 'stretched'

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