DIY-Macro-Rail: step size?

[DeepB]

Hello,

I am building a diy macro rail.

It will be movable on 3 axis. (2 axis specimen movement, for macro-Panorama, 1 axis for focus stacking).

For the Z axis (focus stacking axis) I got a linear axis that was used in some kind of optical machinery. One turn of the axis equals 0,5mm movement of the slide.
For the X/Y axis (specimen movement) I got a linear rail with 4mm/movement per turn of the axis.

Now I need to decide which stepper motor to use on the axis. The "standard" stepper motors have either 200 or 400 steps pre revolution (so they can move in 1,8° or 0,9°). Or I can use a stepper motor with a gearbox. Standards would be approx 1:5, 1:10 ...

So for the axis I have the following possibilities:

Z (focus Stacking): 0,0025mm (200 steps), 0,00125mm (400 steps), 0,0005mm(1000 steps, 200 steps motor + 5:1 gear)

x/y: 0,02mm (200 steps), 0,01mm (400 steps), 0,004mm (1000 steps).


I do realize that depending on the quality of the linear rails the finer steps might not be completely achievable.

What step size would be desirable?

thanks
Daniel

[Chris S.]

Daniel,

For the Z-axis, the answer partly depends on the highest numerical aperture (NA) you envision using on your rig. As you likely know, depth of field goes down as NA goes up. With smaller DOF, you want finer steps.

Here are the default step increments I use at various numerical apertures:

5x/0.14, 18.4 microns
10x/0.28, 4.6 microns.
20x/0.42, 1.8 microns
50x/0.55, 0.8 microns
100x/0.70, 0.5 microns

So to work up to 100/0.70 on your rig, I would think about the 5:1 gear, with its theoretical resolution of 0.0005mm, or 0.5 microns.

Of course, this rests on a bunch of assumptions. In your other posts, you've discussed 10x and (I think) 20x. At typical NAs for these magnifications, you would be fine with the theoretical resolution of the 400 step/revolution stepper motor, ungeared. I don't know if you contemplate working at higher magnifications.

Secondly, there is a good deal of "season to taste" in choosing step sizes. On my rig, steps of double the size listed above produce images almost as good; with a quickly-wilting subject, for example, I go with a larger step size. So I wouldn't argue with someone who preferred a larger step size by default.

And as you brought up, it's difficult to predict the actual, repeatable minimum step your rail will produce, no matter what sort of motor you put on it. I suspect you'll have to determine this by testing. (On my rig, I perform stepping movement with a motorized microscope focus block, which produces very fine, repeatable steps. For coarse movement of this focus block, I have it mounted on a screw-driven linear stage.

I've liked the 400-step and 200-step, ungeared stepper motors I've mounted on things. I have not had occasion to try one with gearing. Do remember that your stepper motor driver can micro-step your motor to produce finer increments than full steps. (Though I have a personal preference for working in full steps).

For your X/Y axes, I doubt your motor choice is very critical, as you'll want lots of overlap for stitching.

--Chris S.

[Macro_Cosmos]

Do you have a digital gauge indicator? That is far better than any numbers crunching.
As for the desirable step sizes, aim for 0.5um, that will cover most objectives people use here. It will depend on the objectives you have access to, as Chris S said above.

[Chris S.]

Do you have a digital gauge indicator? That is far better than any numbers crunching.

Good idea, Macro_Cosmos! Decent low-end digital gauge indicators don't cost much, but should be capable of measuring a screw-drive's smallest repeatable increments quickly and conveniently.

I've been meaning to pick up a gauge indicator for my growing tool room. Using this tool to characterize a linear stage seems obvious, once you've mentioned it. Edit: The foregoing is probably wrong: These tools are probably not good for characterizing our linear stage movements--see below.

--Chris S.

[rjlittlefield]

Decent low-end digital gauge indicators don't cost much, but should be capable of measuring a screw-drive's smallest repeatable increments quickly and conveniently.

This seems overly optimistic to me. 0.5 microns is roughly 0.00002 inches, which is something like 10X smaller than anything I've run across in low end indicators.

Can you explain your thoughts in more detail?

--Rik

[Adalbert]

Hi Daniel,

I calculate the depth of field as follows: DOF = 0,00055 / (NA*NA)
Theoretically one shot a DOF should be good enough, but I usually take 3 photos a DOF,
so in my case the step-size=DOF/3

BTW, you can measure the step-size of your rail using Zerene.

Best, ADi

[Chris S.]

This seems overly optimistic to me. 0.5 microns is roughly 0.00002 inches, which is something like 10X smaller than anything I've run across in low end indicators.

You're right, Rik. I should not attempt imperial-to-metric conversions while tired--misplaced a decimal point. For the record, the indicators I've looked at for my tool room come in varying levels of fineness. The finer-measuring ones I've considered (analog, not digital) have tick marks at about 2.5 microns. Yes, an order of magnitude coarser than the 0.25 microns I had in my head when typing. Oof!

Thanks for the correction. I've edited my earlier comment to direct readers here.

--Chris S.

[Macro_Cosmos]

Decent low-end digital gauge indicators don't cost much, but should be capable of measuring a screw-drive's smallest repeatable increments quickly and conveniently.

This seems overly optimistic to me. 0.5 microns is roughly 0.00002 inches, which is something like 10X smaller than anything I've run across in low end indicators.

Can you explain your thoughts in more detail?

--Rik

Agree, 0.5um is pushing it. 1um is completely possible. Still not the most accurate as it is a contact method.
Low-end ones, the ~$30 new types come with a resolution of 0.01mm.

A cheap one won't do, of course, a used Mitutoyo can be purchased for $75. That's how much I paid for mine. Likewise, a workshop will have good ones too.
500nm digital indicators do exist, Fowler Sylvac offers them. Likewise, using ZS to measure the step size would do wonders.

[rjlittlefield]

DeepB, I'm pretty sure that the mentions of using Zerene Stacker to measure step size refer to the second aspect of my post titled Tiny focus steps: how to make them, how to measure them. (Good grief, was that really over 10 years ago?!)

It is a very useful method for characterizing a rail; see for example HERE. But it is not intended to be used routinely as any part of photographic operations, and offhand I do not see any reasonable method to use it that way.

In contrast, coupling a digital indicator to the stage could provide position feedback continuously, albeit at lower resolution within your budget. That said, I do not know anybody who has actually done it, and I'm not sure what the value would be given that skipping steps is not a problem in these setups.

--Rik