Is that REALLY the size?

[Doozer]

BCR- Jerry's brain can't compute anything without units.
You can't teach him anything. He enjoys fuking with people
for fun until he wears them out, and when they give up and
go away, he gets some twisted satisfaction for achieving his
goal. Maybe we can secretly get Chat GTP registered here
as an undercover forum member, and debate Jerry to death,
using the collective bullshlt of the internet, to debate Jerry to
over 50,000 posts. Maybe GTP can get Jerry to 100,000 if
he lives long enough. It's only a $15 subscription fee.
It might be worth it for our entertainment.

-Doozer

[darryl]

Let's magnify this work that is within a tenth- what does that look like? Then compare that to grinding your own telescope mirror, for instance. What is the tolerance for a lens, say, in tenths? And what about your glasses, precision formed and shaped in the little shop down the street in an hour- heck, I want one of those machines at home. I'll fit it in the corner in the bathroom-

[BCRider]

Yes, a tenth looks like a mile when we start talking about stuff like optics where angstroms count. But hey, the home telescope constructor's groups have their ways for even working to this sort of accuracy. Why can't we?

Here's a close up shot that I posted of the end button that slips into the saw blade and then into the arbor. The hole in the arbor looks pretty much like this too. This was done with stoning as described in my other post about the R8 arbor. The surface smoothness is on par with most of my ground tooling.



In effect it's probably comparable to lapping. But without the laps. It's slow but that's good because it encourages frequent checking for measured size or fit. But best of all it's not hard to do.

The end collar on the arbor that probably started all this? Yesterday was the first day that I fitted it to the mill. The size that ended up a tenths grade match to the example needed a light back and forth twist to fit into the spindle. Just like the arbors I chose as the pattern to go by.

I'm no miracle machinist. I make more than my share of mistakes. The finish I get on my lathe is acceptable at best. It has bearing issues. But if we have the right method, just like the home telescope constructors, then we CAN work to get great surfaces and get size matches to example items that can be a size match to within a tenth. And we can do it without a tool post or surface grinder.

[Paul Alciatore]

The purpose of standards is to allow shops (home or professional) to make parts TODAY that have a well controlled FIT with parts that were made TEN YEARS AGO and that are not at hand. This was accomplished around 200 years ago when people (Eli Whitney) started making rifles with interchangeable parts. I believe he made his own standards. He probably made them as close to the accepted size of the inch in that day, but I am fairly sure he made his own, in house, standards. I don't think he had very good temperature control by today's standards. Open and close windows, put more wood in the fireplace, stuff like that. But he made things work. He made parts that worked together.

Others observed his success and followed his example. They also needed standards. Soon there was a real need for parts that were made in one factory to fit with parts made in another. So people compared standards - as best as they could. Soon they saw the need for country wide and even world wide standards and places like the National Bureau of Standards (US) were created. And world wide bodies to facilitate world wide trade.

So now the world and national organizations maintain the standards. And they distribute and re-calibrate the standards used in commercial labs that service the factories of the nation and world. Things like calipers and micrometers and gauge blocks are among the ways that standards reach down to commercial shops. And, yes even to HOME shops, like yours and mine.

It is not difficult to imagine that the caliper or micrometer sitting on your or my shelf is only about four steps from the world wide standard meter.

World standard meter to National standard,
National standard to Regional lab standard,
Regional lab standard to Factory standard,
Factory standard to Micrometer.

Today the world standard is defined by the speed of light. That seems fixed, but it also involves the second which we also define to great accuracy. From NIST: "The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s⁻¹, where the second is defined in terms of ∆ν_Cs." So the accuracy is around 0.0000033mm. Since it is defined by physical constants that can be measured, that level of accuracy would be at the National level and perhaps even down at the Regional lab level in some cases. Multiply by a generous factor of ten for the next two steps and you are still at 0.00033mm (around 0.000013") for a well made micrometer.

Perhaps there is another step in there, but it illustrates the idea. Everyone in that chain is concerned with making things match as close as they can. The result is that two factories, half a world apart and fifty years apart can make parts to the same standard. OK, things at the FACTORY level may differ by hundred millionths of a meter (0.00001mm). Nothing is perfect. But the system works pretty well.

How close can a home shop come to the best or perhaps to the average industrial shop? Well it is going to be hard to split thousandths of a mm in our garages, even with temperature control. It could probably be done, but it would require a sizeable investment in improvements. More insulation and a far better AC/heater would only be the starting place. People put these labs below ground you know. And with things like entrances with double doors. Etc. Could it all be done at a home shop? Sure. Has anyone with a strictly home shop done it? I doubt it.

So what level is the home shop able to reach. Frankly I see no reason why a good home shop couldn't do 0.0001" with confidence. It will cost some money for things like the best grade of surface plate, the best grade of gauge blocks, probably the best DIs and micrometers or other, electronic instruments. Etc. Such a shop would be comparing parts to a stack of gauge blocks on a surface plate with a DI or other instrument that can detect differences of 0.00001". And doing it under controlled conditions. The gauge blocks would need to be sent to a lab for calibration on a regular basis: more $$$. Can't just open the garage door to take out the garbage like I do. But, with dollars and care, it can be done.

The average home shop may be closer to the 0.001" level or even above it in some cases. One of my micrometers reads down to 0.00005" Please note that I said "reads to", not "is accurate to". And I only have a set of shop blocks which have not been checked for the past ten years or more. They do match. But just how much confidence does that give me? Perhaps I can be confident down to 0.0005". Perhaps. I definitely would not claim better than that. And if I had a real need for better accuracy I would need to take appropriate measures ($$$$). If you asked me for 0.001" (0.025mm) accuracy I would not hesitate to say I can achieve it. 0.0005" accuracy? I might pause for a few seconds. 0.0001" accuracy? I would need to take some measures first.

Perhaps some of my numbers are off. In all likelihood they are on the large side. NIST may not agree with my estimate on the accuracy with which they can reproduce the meter. They are probably a lot better than my guess. But I tried to speak to a final result that I can be confident of, not one that could be easily disputed.

[Paul Alciatore]

Spring calipers are used to make comparative measurements. You MUST have something to compare your part to. This, I believe is one of the reasons why they sell sets of "shop blocks".

One can make excellent fits with nothing more than spring calipers. If you can show what units those are calibrated in, I'd love to know.

[Paul Alciatore]

Actually it is not the camera so much as the lens system. And you will be depending a lot on the accuracy of all the optical elements for the accuracy of the result. One tenth of a percent or one part in 1000 may be close to what you can achieve with any confidence.

Actually, you can measure to microns using a dslr camera and stitching software.

[tomato coupe]

Today the world standard is defined by the speed of light. That seems fixed, but it also involves the second which we also define to great accuracy. From NIST: "The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s⁻¹, where the second is defined in terms of ∆ν_Cs." So the accuracy is around 0.0000033mm. Since it is defined by physical constants that can be measured, that level of accuracy would be at the National level and perhaps even down at the Regional lab level in some cases. Multiply by a generous factor of ten for the next two steps and you are still at 0.00033mm (around 0.000013") for a well made micrometer.

This statement is incorrect, and it doesn't make any sense.

[Paul Alciatore]

He is measuring FLATNESS, not a dimension. The laser beam travels above the surface he wants to check. The camera's image chip is mounted without a lens so the beam just strikes it. The software interpolates over a number of pixels that catch the laser light and produces a number.

It is clever. It uses a laser level so the beam crosses the image pickup from side to side, but this path is not necessarily aligned with the row of pixels. It can go wrong. It does depend on the surface being measured being level side to side. He talks about this around 15:15. Also his mount did not use three point suspension. That would be a great improvement.

This could be a good starting place for a device to check surface plates. It does need work. The proper lens on the image pickup may improve the accuracy. And perhaps a better laser (level?). The one he was using seemed to have a very wide beam. And a way to align the rows of pixels with the surface being checked.

It is interesting and I have saved it for later reference.

I stumbled on this video yesterday. Measuring flatness to microns with laser and webcam.

https://youtu.be/hnHjrz_inQU

[darryl]

I can interpret a couple tenths on a half thou per mark gauge easily, but that doesn't mean the measurement is absolute. I have more than one gauge which is drunken basically, off by about 3 thou over certain portions of its range, and matching well with a couple micrometers at other points. One of these is a digital gauge, one is a dial gauge. Where lead screw based, you don't know the actual accuracy either, so you could work to a couple tenths, and be off by 20 thou. Do we know our home shop lead screws are on the money?

I tend to look at accuracy as how well parts fit. A good example would be something I just went through- boring some holes to be a very close fit to a shaft. I did use the calipers to get close, but then I did the trial and error thing to get the fit. I keep learning about the difference between the caliper reading and the actual size.

I might call this kind of thing a close-range accuracy, and it follows into the next step, which is reducing surface roughness. We can of course use methods in our home shops to achieve a very respectable surface finish- and accuracy as it applies to optical elements. You can see a good example if you look through a dollar store lens- it's all smooth and clear, but parts of the image are distorted. I don't know how many of us could afford a lens grinding machine, but I'd bet a good many of us could. Desktop machines, obviously capable of grinding lenses that we look through everyday- we would not accept visual distortions in our eyeglasses.

Maybe we ( I ) should confine our discussions to machine work using the typical cutters- hss, carbide, etc. We'll have to include the effects of burnishing in our calculations and measurements, and the phenomenon associated with making spring passes. I find that the more experience I get turning, the better I get at being able to hold a close tolerance- and again I mean at close range, not an absolute. And I have to say the machines we're using are going to set a reasonable limit to how close we can work. It's so nice to have a rigid and smooth spindle, lack of play in the slides- in other words full control over where the cutting edge is at all times, and full control over the positioning of the workpiece. I'm sure a lot of our home shop machines are simply not that tight.

[Paul Alciatore]

I have done the telescope mirror grinding. There are different tolerances for the shape of the curve and other things like the exact focal length and the thickness of optical lenses.

The shape of telescope mirrors is controlled in millionths of an inch. This can be achieved using a very simple apparatus called the knife edge tester. I made a very accurate version of that with an actual micrometer screw to measure the differences in the focal length for various zones of the mirror. But that micrometer screw was shop made and probably only +/-0.002" or so. Yet, overall that knife edge tester allowed me to see differences of millionths of an inch. This was in the SHAPE of the mirror's surface. Many amateur telescope makers use knife edge testers that use less accurate scales to measure the differences in focal length.

The actual focal length was not as important and the tolerance there was measured in, perhaps whole inches. It mattered little if the focal length was 47", 48", or 49". All would have performed in a like manner in the telescope. I measured the actual focal length with a tape measure (perhaps +/- 1/8").

Different tolerances: shape of mirror in millionths of an inch and focal length in whole inches.

Other optical elements may have similar differences in the tolerances. Lens elements frequently have that same millonths tolerance for their shape. Things are a little looser for the for alignment of their axis. But the thickness of a lens element can vary by much larger amounts, perhaps thousandths.

Very high quality telescope mirrors and lenses have been made in home shops with very primitive measuring equipment.

The lenses in glasses do not require the same level of precision as telescope optics. But modern lenses are made to high precision levels anyway. Many years ago when actual glass was used I got a new lens that had a defect. I immediately saw an optical effect. The actual deviation of that part of the lens was probably under a tenth. It doesn't take much to bend light.

While the glass lenses were made in local optical shops back in that day, I believe the opticians of today just stock a number of plastic lenses for common prescriptions. All they need to do is cut the outline to fit the frames that you choose and pop them in. They don't actually make the lens surfaces locally. And the factories that do form them probably use precision molding.

https://www.google.com/search?q=how+are+eyeglass+lenses+made&client=firefox-b-1-d&sxsrf=APwXEdf-URpzndoklBtOT7f2QTiZzPNkpQ%3A1680235127194&ei=d1omZNO9C4C7qtsPxJKGwAs&oq=how+are+eyeglass&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQARgAMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDoKCAAQRxDWBBCwAzoICAAQigUQkQI6CwguEIAEELEDEIMBOhEILhCABBCxAxCDARDHARDRAzoLCAAQgAQQsQMQgwE6DgguEIAEELEDEMcBENEDOggIABCABBCxAzoECCMQJzoHCAAQigUQQzoOCC4QigUQxwEQrwEQkQI6CwguEIAEELEDENQCSgQIQRgAUMcVWLdPYMJjaARwAXgAgAHbAogBmRGSAQg0LjEwLjEuMZgBAKABAcgBCMABAQ&sclient=gws-wiz-serp#kpvalbx=_vXYmZKb2L8GvqtsP2tqf8AE_35

Let's magnify this work that is within a tenth- what does that look like? Then compare that to grinding your own telescope mirror, for instance. What is the tolerance for a lens, say, in tenths? And what about your glasses, precision formed and shaped in the little shop down the street in an hour- heck, I want one of those machines at home. I'll fit it in the corner in the bathroom-

[Paul Alciatore]

Well, don't stop there. What is incorrect and what is the correction?

This statement is incorrect, and it doesn't make any sense.

[tomato coupe]

Well, don't stop there. What is incorrect and what is the correction?

Maybe you should explain how you came up with that uncertainty and what it is supposed to mean, because it makes no sense in a discussion of primary length standards.

(It appears that you took the inverse of the speed of light, gave it units of [m], multiplied by 1000 to give it units of [mm], and then called it an uncertainty.)

[BCRider]

Darryl, for my posts I never claimed that the values I got were absolutes. In fact I posted that I took sample readings with my mic off three other commercial R8 arbors and that they were X size. And that I managed, with a smile of surprise and self satisfaction that I didn't mention, to match the values from my mic of those samples to what sure looked to me to be a match or at most within a tenth either way on my part using a technique that I thought might work. Then along comes this thread.

I thought it was pretty much that way for everyone. And now we have this thread. I start to think I'm imagining things and I go back and measure stuff again paying attention to possible smaller differences between readings. But three, four or more readings keep coming back as the same value within a range that might be at most two index lines worth of total variation. And the width of two index lines worth of variation is roughly a tenth or at most two tenths on my mic. So how do I call it if I'm not doing it right?

Color me puzzled....

Do others out there typically get consecutive readings from their mics of the same part that vary by a quarter or so of a division? And that's OK?

[JRouche]

Its funny, you folks are craze. So am I.

Talking about measuments and such. Dozz might have it again. Its a number, compared to what? My standard, I have a few top notch standards, according to who? The makers and the Cert guys that certified my blocks with theirs? Arbitrarily at best.

At some point you have to take a stand, as in standard. Where do you say this is good enough? I know mine, just around the .001" point. JR

[J Tiers]

Woe, you folks are getting way out there. We started out talking about tenths of a thousandth of an inch (.00254 mm). You have now moved onward to accuracies of 1000 times less error.

TLDR, did someone claim to be making measurements down there? 🙄🙄

Maybe you should explain how you came up with that uncertainty and what it is supposed to mean, because it makes no sense in a discussion of primary length standards.

(It appears that you took the inverse of the speed of light, gave it units of [m], divided by 1000 to give it units of [mm], and then called it an uncertainty.)

It appears that he may have been trying to get to a +- 1 wavelength error.