Micron-level accuracy over a meter?

[zondar]

Hello Forum Members,

I have a project idea, but it's one that comes with a potentially difficult metrology issue. I need to measure a distance of about a meter to micro-meter accuracy. The actual length isn't very important (it doesn't have to actually be one meter), it just has to be measured accurately. The part to be measured would likely be a carbon-fiber or Invar rod (for temperature stability) with knife-edges at both ends. The distance between the knife edges is to be measured.

I imagine that laser interferometry would be the ticket here, but I'm not aware of such devices suitable for amateur-level budgets.

About the best idea I have at present is to construct a traveling microscope mounted along a meter+ glass scale. However, despite various claims, I don't think these scales (at least typical hobby-grade ones) are good to a micrometer over the span of a meter.

Using a large mill with high quality scales and a centering microscope could substitute for a custom-made solution, but the part may need to be measured repeatedly in situ if the temperature or orientation changes, and I don't have access to such a machine anyway.

Any brilliant ideas short of asking NIST for help?

Thank you.

 

[rwm]

.001 mm will be tough to be repeatable. .01 more easily achievable. No good idea here, but I am curious what others have to say. Many DRO scales boast accuracy to 1um but I don't know how that goes in practice with temperature changes and backlash.

 

[zondar]

Yes, six orders of magnitude is going to be tough!

Glass DRO's boast 1 um "resolution." That is not the same as accuracy. It just means that one unit of distance to the next (e.g. on your DRO display) is 1 um.

If I look at dropros.com for an example, their "1um" glass scales show +/- 12 um accuracy over about a meter. I interpret that as being 12um standard deviation, meaning that any given scale could be off a fair amount more than that (or not).

Repeatability is claimed to be 2-3 um, but repeatability doesn't confer accuracy either, it just means your measurement is wrong by about the same amount every time.

I know that scientists were able to do this a century ago using optical means, but I don't know how. It may be that the traveling-microscope idea using a glass scale is the best that I could do as a hobbyist, but losing more than an order of magnitude vs. what people did a century ago would feel a bit frustrating.

 

[rwm]

That's kind of what I was thinking.
I actually misinterpreted your original post. When you said the exact length didn't matter I assumed you just wanted precise measurements, not accurate measurements. (For those who are not familiar, precision is the repeatability of a measurement i.e how often it agrees with itself. Accuracy is the deviation of the measurement from a know standard.) That will be even harder!

 

[RJSakowski]

Many, many years ago, I measured distance to a quarter wavelength or better (around 0.1 micron) using interferometry. You say that the actual distance between the knife edges isn't important but I assume that an accurate measure of that distance is. If a +/- 1 micron will work, then one of the 1 micron magnetic or glass scales will work. I have a 300x microscope setup for my Tormach CNC that is capable of visually resolving .0001" or around 2 microns. With a little effort, it could be extended down to 1 micron. A mill with a 1 meter travel equipped with a 1 micron scale and such a microscope would offer the best low cost chance of achieving your objective.

You wouldn't need a mill but some means of making micro-adjustments to position would be highly desirable. I would consider a movable stage for rough travel and a stepper driven sub stage for fine adjust. With the right lead screw and stepper, you should be able to get controllable nanometer level adjustments.

As to material for the bar, Invar would be the choice. I doubt that carbon fiber would have the thermal stability you require. Even at that, temperature control would be mandated. Ideally, you would be controlling ambient temperature to within a degree.

 

[rwm]

RJ- How would the setup work with the microscope? I can't envision this.
R

 

[zondar]

Yes, the distance is arbitrary (except for being as long a baseline as practical - a meter is good), but its length must be known accurately.

I haven't found glass scales specified to yield 1 um accuracy over a meter. They are generally worse than 10 um over a meter (e.g. claimed +/-12 um in the above example).

The information I have on lead screws is that most are horrible by comparison; probably 10 times worse, and I wouldn't rely on them for more than motion control.

I imagine a microscope mounted to a meter+ carriage, e.g. linear bearings on a pair of rods, with the scope attached to a glass scale. A lead-screw and stepper could be used for positioning (only, not measurement except maybe as a sanity check). It would be a fun project in itself.

How did you do the interferometry?

 

[zondar]

By the way, I was thinking about the materials involved.

My guess is that good glass scales use borosilicate glass, which (wikipedia) has a temperature coefficient of expansion that is at least twice that of good quality Invar. So really, the Invar would be sort-of "measuring" the glass scale. ;-)

Fun fact: The guy that developed Invar won a Nobel Prize for it!

 

[RJSakowski]

RJ- How would the setup work with the microscope? I can't envision this.
R

A robust stage would be set up to travel the required distance. A sub-stage would be mounted for fine adjustment. Something similar to a lathe cross slide with the compound set to travel along the x axis. There would be no need for a lead screw mechanism for coarse travel as you are measuring with the linear scale. Sub-stage travel would be accomplished with the stepper. The microscope would be mounted on the sub-stage so you can image your two knife edges.

Here is a link to my thread on the mill mounted microscope for your reference.

[Metrology] - Mill Spindle Mounted Microscope (aka Cheap Optical Comparator)

I had thought about building a spindle mounted microscope for the Tormach for some time. The idea was to cannibalize and old phone or adapt a webcam camera but the optics was rather daunting some the project was shelved. About a month ago, I saw an ad for a USB 50 - 500X microscope on eBay for...

www.hobby-machinist.com

 

[zondar]

Perhaps a manually-operated micrometer could be used for the short distances needed to bring the knife edges into view. Probably would need to lock down the carriage first. But it's likely simpler than going to the trouble of a stepper.

Another concern about glass scales is the possibility of missing steps, for which a stepper-controlled lead-screw could help by delivering slow, consistent, motion, but maybe that's not a big problem if you are careful about moving the microscope stage.