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Automating and characterizing an Olympus CHT focus block
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rjlittlefield
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PostPosted: Wed May 13, 2015 1:35 pm    Post subject: Automating and characterizing an Olympus CHT focus block Reply with quote

This post is a follow-on to http://www.photomacrography.net/forum/viewtopic.php?t=26997, where I described how to harvest an Olympus CHT focus block and adapt it for Arca-Swiss mounts.

Here, I have added a stepper motor drive.



The mechanical mount in this case consists of two pieces of aluminum angle stock, carved to shape, glued to each other with Loctite Epoxy Weld Bonding Compound, and glued to the focus block with Devcon Plastic Welder (HERE). I used the two different glues because of their handling characteristics. It was easier to make a clean joint between the two pieces of aluminum using the less viscous Weldbond, but I opted for four dots of cyanocrylate Plastic Welder for the attachment to the focus block just on the theory that I might be able to clean it off easier if I should ever want to knock this thing apart.

The pulleys are chosen to give exactly 2:1 reduction ratio, 25 teeth on the motor and 50 teeth on the focus shaft, using a belt with 1 mm pitch. Finding pulleys to do this was much more difficult than I had expected, mainly because the two shaft diameters are different and that affects which families and sizes of pulleys are available. After writing to Stock Drive Products and failing to get an answer, I finally just opted to get two pulleys intended for 4 mm shafts, and drill one of them out to 5 mm to fit the motor. That worked OK, but to be honest I took that route only because I got stubborn about exactly what I wanted. I think it would have been a better solution, and certainly much cheaper, to use the MakeBlock 90:32 components described by JGVilla at http://www.photomacrography.net/forum/viewtopic.php?t=27093.

Of course once I had this thing assembled I couldn't resist measuring its performance.

First, let's talk about how this focus block works on the inside.

Here is a "partially transparent" composite photo showing most of the components of the gear train:


In brief, there's a 14-tooth driving gear fastened to the fine focus knob, followed by a train of three reduction stages, each having a 45-tooth driven gear fastened to a 12-tooth driving gear, followed by a main drive gear that has a 47-tooth driven end and a 26-tooth drive end that engages with the rack that is screwed to the back of the focus slider. The arrangement is sometimes called "planetary", but in normal operation it's really not because all of the gear shafts stay in fixed positions and the gears just rotate. In the special case that the focus slider runs up against a stop, then the main drive gear stops rotating and rotation of the fine focus knob simply rotates the whole reduction gear assembly instead. In that situation the system does become planetary, but again, that's a special case.

Anyway, so there's this chain of reduction gears that ends up being about 177:1 (to be exact it's 158625:896, according to Wolfram Alpha). Backlash is removed by gravity or spring pressure on the slider, so that turning the fine focus knob either positively drives the slider in one direction, or allows the slider to move back in the other direction, always keeping the same faces of the gears engaged.

If the gears were mathematically perfect, then rotation of the fine focus knob would translate to perfectly smooth rotation of all the gears and eventually to perfectly smooth linear motion of the focus slider. But of course the gears are not perfect, and the result of that imperfection is that the rotation is a little bit uneven. As each tooth engages or disengages, the driven gear alternately gets a little ahead or a little behind where it would be if things were perfect.

We can see this behavior in the following graph, which shows measured movement of the slider as the fine focus knob is rotated uniformly. The straight line fitted to the blue measured positions represents the average linear movement, but note that the measured positions drift a little above and below the straight line.



We can see the behavior of the errors more clearly by looking at the deviations between expected and measured positions:



The two big humps shown above correspond to teeth between the last reduction gear and the main drive gear.

Of course the same sort of effect occurs at each pair of gears.

Here's the picture at the finest end of the scale, where I'm driving the system at 1/32 micron per step (actually a "microstep"), 1024 steps for a total of 32 microns. In this case the humps are due to about 2 teeth on the fine focus knob gear, and none of the other gears turns enough to matter.



The above graph also shows some even finer scale structure that is due to some combination of friction and nonlinearity in the stepper motor drive, coming from one of the first version Cognisys StackShot controllers. Here's a closer look at that pattern, shown here as absolute movement instead of deviations from expected. The repetition every 16 data points is because there are 16 "microsteps" per full step, with each of the microsteps accomplished by analog modulation of the drive currents in the stepper motor coils.



Getting back to the deviations, things get really amusing over distances where the effects of multiple gear stages can be seen at one time. Here are a couple of my favorites:





To be sure, all of these effects are small, on the order of 1% or less. They are of no significance for focus stacking.

In the main, I'm very pleased to see that the system seems to be usable even down in the range of 0.1 micron. That's no surprise, given the results described earlier at Tiny focus steps: how to make them, how to measure them.

Speaking of which, I should mention that all the graphs shown above were generated using the method described at "Tiny focus steps...", looking at focus slider movement from the side using magnifying optics and then analyzing the results using Zerene Stacker's alignment process.

As with earlier work, I had assumed that the focus block was designed to be accurate, that is, one rotation of the fine focus knob would produce (on average) 200 microns of physical movement.

But this time, I decided to check that assumption with a couple of direct measurements, one of them involving a digital caliper clamped to the focus block, and the other involving a precision leadscrew on the table holding the camera. Here's the physical setup showing the caliper; the table is out of sight on the left side of the frame.



The two direct measurements (caliper and leadscrew) gave the same surprising result: in fact the focus block is not accurate, even on average. While 5*25 turns of the "200 microns per rotation" fine focus knob should produce 25 mm of movement, give or take a little for the gear tooth effects described above, in fact it produces about 25.9 mm of physical movement. Look here:



I find this puzzling. It seems like no great trick to choose gear ratios and rack pitch so that the final result would be dead on. But it sure looks like Olympus didn't do that, because I don't see any way that my measurements could be off by far enough to explain the discrepancies. Again, this doesn't matter for focus stacking, but in the words of my former employer this device is clearly "UNCALIBRATED: FOR INDICATION ONLY". Wink

I hope you find this interesting also.

--Rik
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ChrisR
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PostPosted: Wed May 13, 2015 2:50 pm    Post subject: Reply with quote

A nice piece of work. Applause
I'd previously noticed Olympus focus blocks being a little off.
I just checked a BHMJ over 100 turns, "by hand" with a vernier caliper; I get 208µm per turn. Within error.

[block designation corrected]


Last edited by ChrisR on Thu May 14, 2015 1:40 am; edited 1 time in total
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BugEZ



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PostPosted: Wed May 13, 2015 7:21 pm    Post subject: Reply with quote

I find myself smiling when I look at Rik's first two plots labeled "deviation from linear distance". In mechanical engineering lingo, we would call that transmission error. These plots are very similar in shape to the transmission error predictions created by our gearing analysis programs.

Gears are generally designed to produce "conjugate" action. That means when the input gear turns at constant speed, the output gear also turns at constant speed. If the gear teeth are not perfect (they never are...) the output gear speeds up and slows down slightly relative to the input gear. This normally repeats at the gear meshing frequency. This is transmission error and it normally follows the bumpy shape of Rick's curves. Gear teeth and shafts deflect slightly under load slightly changing the gear's shape. Thus the torque being transmitted will alter the transmission error. The gears we design (I design power transmissions for a living) are optimized in shape to minimize transmission error at the normal operating load. Gears last longer when they run smoothly without the variations in speed caused by transmission error so we work hard to keep it small.

The rack and pinion in a microscope focusing mechanism does not drive a lot of torque, however the gears are narrow, plastic, and the pressure angle is small (certainly compared to power transmissions!) which makes the teeth susceptible to bending. I would be willing to bet that the errors would go down (though this does not particularly matter) if you were to reduce the bias load (from the surgical tubing spring) on the mechanism.

I doubt that the subtleties of transmission error were of much importance in the original design. The position control feedback loop was intended to be closed by the scientist using the microscope so the tiny transmission errors would not matter. The thing of greatest importance was to move the microscope tube smoothly and to hold it securely. Things the mechanism no doubt did quite well.

Thanks for sharing the photos!

Keith
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ChrisR
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PostPosted: Thu May 14, 2015 1:46 am    Post subject: Reply with quote

Rik - about the 2:1 ratio?
I understand it's a compromise of resolution for fine steps against speed for larger movements, but wouldn't have thought it was critical.
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PostPosted: Thu May 14, 2015 10:15 am    Post subject: Reply with quote

ChrisR wrote:
I just checked a BHMJ over 100 turns, "by hand" with a vernier caliper; I get 208µm per turn.

Thanks for the data point. I went back and rechecked the Amscope B490/T490 block that I reported on last year. That one works out to about 215 µm per turn, versus the 200 that I had assumed. My curiosity is now aroused to know whether Nikon blocks have the same issue.

Quote:
about the 2:1 ratio?
I understand it's a compromise of resolution for fine steps against speed for larger movements, but wouldn't have thought it was critical.

Speed is not an issue for me. I keep the controller cranked down well below maximum all the time anyway. If I want to make a quick movement, that's done by turning the coarse focus knob by hand (overcoming the clutch friction).

My obsession with exactly 2:1 is a matter of psychology, not engineering. Because I know about the "scallops" in the movement curves due to microstepping, I have an emotional fondness for setups that make it simple to specify full-step movements of the stepper motor. With 2:1 gearing, this is accomplished by any multiple of 0.5 µm per focus step (assuming 200 µm and 200 full steps per rotation). With 32:90 gearing, the corresponding number would be 0.3555... (repeating). At this point in my life, anything that reduces my cognitive load is a very welcome thing, so 2:1 wins big in the category of conceptual simplicity.

Other than that, the 32:90 gearing has three advantages (finer steps, lower cost, and no mods required), and no disadvantages that I can think of.

BugEZ wrote:
I find myself smiling when I look at Rik's first two plots labeled "deviation from linear distance". In mechanical engineering lingo, we would call that transmission error.

What can I say? I'm a software guy. In my world, "transmission error" is what happens when the message that is received is not what was sent. (Hhmm... That's not such a bad analogy, come to think of it, but as always the devil is in the details.)

Quote:
I design power transmissions for a living

I bow in honor! Devices that can operate at high speed and high load for thousands of hours get my utmost admiration. The CVT in my latest van strikes me as some sort of alien technology. As far as I can tell from watching YouTube (HERE, HERE), the beast works by running a segmented metal belt between a couple of tapered pulleys, with power transmitted by pushing the belt, not pulling it. If I had seen that design presented as a proposal, I would have first checked the date to see if perhaps it was April 1. But it works astonishingly well, at least from my standpoint as an ordinary user.

Quote:
The rack and pinion in a microscope focusing mechanism does not drive a lot of torque, however the gears are narrow, plastic, and the pressure angle is small

One minor correction: in this block and the few others that I've looked at, all the gears are metal except for the one mounted on the fine focus knob and the first reduction gear next to that.

Quote:
...which makes the teeth susceptible to bending

True. It's hard for me to get a grip on how much this might matter. In the end, each of those 1/32 µm movements of the rack is accomplished by roughly 8.6E-07 of a full rotation of the main drive spur. If I've worked the sums correctly, that's about the width of three fingers viewed at a distance of 10 kilometers. With such small rotations and displacements involved, I'm surprised that the curves are even monotonic, let alone pretty smooth!

--Rik

Edit: removed erroneous comment about another gear being plastic on some Nikon blocks.


Last edited by rjlittlefield on Fri Jun 26, 2015 12:07 pm; edited 1 time in total
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TheLostVertex



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PostPosted: Thu May 14, 2015 2:24 pm    Post subject: Reply with quote

Very informative thread. I am currently in the process of doing this and several of these points had come up for me (gearing, and measuring actual movement). Hopefully I will be able to do some testing on my nikon block soon.

I have one question regarding your direct mounting method. Have you noticed that this produces any visible high frequency vibration with the motor producing constant torque? Some stepper motors can be quite loud, and loud means movement. Though it may be too little to get transferred to the camera.
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PostPosted: Thu May 14, 2015 6:26 pm    Post subject: Reply with quote

TheLostVertex wrote:
Have you noticed that this produces any visible high frequency vibration with the motor producing constant torque?

I have the same worry as you. Surely sound means movement and movement must mean blur.

But I haven't actually seen a problem.

I just now ran a not-so-quick test to confirm my earlier impressions.

What I did was to mount a moth wing where the laser printer pattern is shown in earlier in this thread, and I swapped out the 10X Mitutoyo for a 50X Mitutoyo. Then I set my StackShot controller on maximum torque and high precision mode, which gives the noisiest behavior I know, and I ran a sequence of 64 frames, stepping by 1 microstep. That gave me two complete cycles through the 32-step pattern that I've seen before, with an impressive variety of sounds ranging from barely audible to a pretty annoying whine to one that sounded almost like something was loose and rattling. At each position, after 10 seconds settling delay, I shot a single frame at 0.2 seconds exposure with Jansjo illumination using a Canon T1i in live view mode, so shooting with EFSC. That gave me 64 frames, two at each of 32 different whines, again ranging from barely audible to pretty annoying

Finally I pulled the resulting 64 images into an image viewer, zoomed in to actual pixels, and scrolled back and forth through the array of images looking for softness that might correlate with the whines.

I didn't find any.

That said, the test is not simple to perform at the level of precision that I'm attempting. My first three attempts simply failed to produce good data due to focal plane shifts, apparently because the subject and/or its mounting platform were still warming up after I turned the light on. Even in the last run, there was a slight shift in focus between the first and last frames, about 12 minutes apart. That's not surprising, considering that the DOF of that objective is about 2 microns, while the coupling distance from subject to objective is more than 1 meter by the time all the spans get added up. However, it happens that the barely audible / annoying / rattling frames are pretty close together in the sequence, so I don't think that the drift is impacting my ability to spot any blurring.

On the other hand, the test that I did probably isn't exactly the test that we'd like to do. If you look again at the test setup, next to last image in the first post, you'll be reminded that what I'm photographing captures movement along the focus block axis and vertical. It misses the lateral component that in practice would be across the width of a landscape frame. To catch that I would need to photograph along a different axis, and then there would surely be a significant amount of focus shift because that's what's supposed to be happening along that axis.

I suppose I could remove the drive belt so that the motor could still be run through its sequence of whines without altering focus. That would be an interesting experiment, but unfortunately it will have to wait for another day. This one has run out of time.

--Rik
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PostPosted: Fri May 15, 2015 1:22 am    Post subject: Reply with quote

Do we know what Cognisys DO which makes the racket in High Precicion mode?

Readers might be thinking that it would be possible to do a comparison with the motor power turned off, or flash used instead. In principle that would be true of course, but I can't think of a way which would make a direct comparison possible. Important things would change at the same time.
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PostPosted: Fri May 15, 2015 7:39 am    Post subject: Reply with quote

ChrisR wrote:
Do we know what Cognisys DO which makes the racket in High Precicion mode?


I haven't used one of their controllers so I can not speak from first hand experience. It would be my assumption though that it forces it to operate in mixed decay mode, instead of slow decay(or a mixed mode which does not work adequately for their motor's microstepping). In mixed decay mode for an H-bridge, slow and fast decays are switched during certain points of the motors charging and discharging in order to create a more accurate movement pattern. One of the draw backs of switching between fast decay, is a lot more EMI and noise.

rjlittlefield wrote:

In Rik's image, the peaks for each step is where the decay mode would ideally switch, roughly 1/4 way between windings. My understanding is that once the rotor reach a certain position, the motors inductance is too high for the driver to produce a smooth current transition while in slow decay.

Another cause is the holding torque and high current for keeping the rotor in the right place between windings. This further increases the amount of EMI and audible noise in the system, as well as out ability or perceive it.


Rik, I was thinking of a more simple test of just watching a zoomed in live view image with the motor in operation, and not in operation. Maybe one with the motor on and no belt connected as well. At least on the cameras I own, vibration appears very readily in live view for some reason. Even when it is less visible in the final image.

Never the less, it is good to hear that there isn't any easily noticeable effect on image quality that you can tell.
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PostPosted: Sat May 16, 2015 12:55 am    Post subject: Reply with quote

Way to go, Rik!

You’ve provided a path that can be followed by most do-it-yourselfers with basic hand tools, and will give them a well-implemented motorized focus block. Much needed post.

I particularly like your elegantly simple motor-mount, which (crucially) allows for adjusting the timing belt’s tension.

A few thoughts: First is that rather than using a 2:1 reducing ratio between the timing pulleys, you might instead have used a stepping motor with 400 steps/revolution, rather than 200 steps/revolution. Not that there is anything wrong with the way you did it—except the frustrating search for parts. On the converse, simply ordering a 400 step/revolution (aka “0.9 degrees per step,” rather than 1.8 degrees per step) stepping motor would have accomplished the same result without frustration. (It’s the approach I use in the motorized Bratcam.) I’m sure you considered this, but am pointing it out for those who will model their own projects on your thread.

Second thought is also for folks who implement this approach. In my experience driving a microscope focus block with a motor and timing pulley, I get the most consistent results when the timing belt has a bit of slack in it—not so much slack that the belt slips, but enough slack that the belt pulls as little as possible on the fine focus shaft. Rik’s timing belt is probably not overtight, but from my experience, it is worth pointing out the value of adjusting timing belt tension "just so."

Third: With my timing-belt/motor/focus block setup, I find it easy and preferable to avoid the StackShot controller’s precision mode, which increases heat and noise from the stepping motor. I also set the torque and speed of the motor very low (these are easily adjustable parameters on the StackShot controller, and likely easy, too, with a homebuilt controller). As a result, my stepping motor is quiet and not overly warm. Though Rik’s testing suggests that a noisy stepping motor may not translate to image problems, I like a quiet rig and a low-temperature motor.

Rik, a few questions:
    1) If you had it to do again, would you repeat your use of epoxy to join the two pieces of L-stock (which appears both simple and effective), or would you consider joining these two parts with screws in tracks, which would add adjustability along the axis of the focus shaft/ motor shaft, to permit adjusting alignment of the timing belt?

    2) Your motor mount has two curved edges, both of which appear admirably smooth and regular. Did you cut these with a rod blade in hack saw, then dress them with a rod-shaped metal file, and then, perhaps, emery paper? Or did you use some other method?

    3) How did you fashion the (very important) belt tensioning groove in your motor mount? My first thought is to drill two holes in the aluminum, then connect them by cutting a groove between the two holes with a rod blade on a hacksaw, then evening out this cut with a small file. Is this how did you do it?

rjlittlefield wrote:
My curiosity is now aroused to know whether Nikon blocks have the same issue.

No, no, no--I did not read this. And I solemnly swear (with fingers crossed behind my back) that I don't have a few dozen Nikon focus blocks--Labophot and Labophot 2—in my basement that I’ve been intending to cut out and motorize, very similarly to the way you've shown here, then make available to other photomacrographers. So I cannot possibly perform such a test, and most particularly, cannot do so across multiple specimens of each model, in order to assess not just brand and model variance, but sample variance. I will stick my fingers in my ears and repeat "No, no, no," until I turn blue, or until next winter, when natural subjects are not calling me outside in loud voices.

Also, having read Keith’s fascinating post, I now wonder how much variance comes from bias load. So I’d feel compelled to test a few blocks under a range of bias loads. But it’s the spring of the year, and after a long winter, a young man’s fancy turns to thoughts of field macro, not the testing of focus blocks. Wink

---Chris


Last edited by Chris S. on Mon May 18, 2015 4:29 pm; edited 1 time in total
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rjlittlefield
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PostPosted: Sun May 17, 2015 8:50 am    Post subject: Reply with quote

Chris S. wrote:
1) If you had it to do again, would you repeat your use of epoxy to join the two pieces of L-stock (which appears both simple and effective), or would you consider joining these two parts with screws in tracks, which would add adjustability along the axis of the focus shaft/ motor shaft, to permit adjusting alignment of the timing belt?

Screws and tracks would have worked fine, but in this case I decided to take advantage of an opportunity to save myself some machining time.

With this focus block and this motor, there's no point in adjusting the bracket because the length of the motor shaft provides ample adjustability. Because this particular motor is almost too large for this application, my major concern was only to be sure that the back of the motor was mounted very close to the focus block, just enough to allow free movement for belt tensioning. To accomplish that, I did all the machining first, then glued the L-stock by assembling those two pieces and the motor on a flat surface, with a couple of index cards placed under the motor to provide the required clearance. You can't tell from the image as posted, but there's only about 0.025" of clearance between the motor and the focus block.

Quote:
2) Your motor mount has two curved edges, both of which appear admirably smooth and regular. Did you cut these with a rod blade in hack saw, then dress them with a rod-shaped metal file, and then, perhaps, emery paper? Or did you use some other method?

The large curves were cut with ordinary hole saws mounted in a drill press. In this case the fact that I was working with L-stock was very helpful because it was simple to clamp the stock against the side of a wooden backing block to resist the large torque of the hole saw.

Quote:
3) How did you fashion the (very important) belt tensioning groove in your motor mount? My first thought is to drill two holes in the aluminum, then connect them by cutting a groove between the two holes with a rod blade on a hacksaw, then evening out this cut with a small file. Is this how did you do it?

That would have worked fine, but actually I milled them out using an 1/8" mill bit again mounted in the drill press.

For this part of the project, I limited myself to a design that could be implemented with hand tools, but granted myself permission to save some time by using other equipment that I had at hand.

--Rik
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PostPosted: Sun May 17, 2015 9:08 am    Post subject: Reply with quote

When I was a lad making electronics things which needed "D" shaped holes for connectors and so on, the cheap/simple tool of choice was a Jeweller's Piercing Saw.
This is a somewhat like a Fret or Coping Saw, but used more typically by jewellers and watch and clock makers. Extremely fine blades are available, but for most purposes stronger ones are better. There's one type which is tightly twisted, so it cuts in any direction, but the whole saw is usually easy to turn.
An example:
https://www.cousinsuk.com/category/piercing-or-fret-saws-blades

They're a little slow, but get the job done.
They do leave a fairly rough edge, but soft metal is easy to smooth with a small file or abrasive paper wrapped around a blade, drill, jar or other curved object.


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PostPosted: Sun May 17, 2015 10:15 am    Post subject: Reply with quote

Chris S. wrote:
A few thoughts: First is that rather than using a 2:1 reducing ratio between the timing pulleys, you might instead have used a stepping motor with 400 steps/revolution, rather than 200 steps/revolution. Not that there is anything wrong with the way you did it—except the frustrating search for parts. On the converse, simply ordering a 400 step/revolution (aka “0.9 degrees per step,” rather than 1.8 degrees per step) stepping motor would have accomplished the same result without frustration. (It’s the approach I use in the motorized Bratcam.) I’m sure you considered this, but am pointing it out for those who will model their own projects on your thread.

All good points. Part of my motivation in going the route I've described here was simply that I had never done a timing belt assembly before and I wanted to know how the details worked out. Initially I had expected that it would be trivial to find appropriate pulleys from the large assortment of parts available from Stock Drive Products. When I discovered that it was not, I became intrigued -- "got stubborn" if you will -- and pushed the effort through to completion more or less as originally planned but at higher cost and more effort. As mentioned earlier, in the end I opted to buy a small bore pulley and drill it larger because I couldn't find one that would naturally fit the motor shaft and still have the size ratio I wanted. There's always a risk that the hole will drift slightly off center when drilling it out, and indeed that happened in this case. I was concerned that this might degrade the quality of movement, and I suppose it does at some level, but when I took the measurements I discovered that against the background of interesting deviations due to gearing inside the focus block, I can't see whatever minor effect there is from the slightly off center bore in the motor pulley.

One of the big reasons that I now look fondly on the 90:32 gearing used by JGVilla is that it comes off-the-shelf able to handle the discrepancy in shaft diameters: 5 mm for the stepper motor versus 4 mm for this focus block.

In practice, it seems to me that just about whatever we do with a microscope focus block will work out fine for focus stacking. Even my initial hack with the vinyl tubing coupler, using a 200 steps per turn motor and a 200 microns per turn focus block, enabled steps in the range of 0.1 micron. With finer blocks, and finer motors, and gearing down, I think that mostly what we're doing is playing with various degrees of overkill.

I'm not opposed to overkill, by the way. One of my tongue-in-cheek mottos is that "Anything worth doing is worth over-doing."

But with these focus blocks, I'm coming to think that what's "best" may have more to do with ease of integration than with ultimate performance. The ultimate performance seems to be fine in every case.

--Rik
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PostPosted: Sun May 17, 2015 5:49 pm    Post subject: Reply with quote

Chris S. wrote:
In my experience driving a microscope focus block with a motor and timing pulley, I get the most consistent results when the timing belt has a bit of slack in it—not so much slack that the belt slips, but enough slack that the belt pulls as little as possible on the fine focus shaft. Rik’s timing belt is probably not overtight, but from my experience, this it is worth pointing out the value of adjusting timing belt tension "just so."

Revisiting this point, I'm wondering what "a bit of slack" means. One reservation I've always had about screw adjusters like HERE, image #5, is that it's very easy to build up large forces due to mechanical advantage of the screw. On the build here, I deliberately opted to not include any such device, relying instead on fingertip sensitivity to hold the belt tight whilst tightening the screws. When I rechecked just now, I seem to be using around 10 ounces total belt tension, that conveniently also being the weight of the motor so it was easy to compare in a vertical configuration.

Can you say a few more words about what "most consistent results" means, and maybe what you think causes the inconsistency and what you've found the optimum tension to be?

--Rik
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Location: Ohio, USA

PostPosted: Sun May 17, 2015 9:19 pm    Post subject: Reply with quote

rjlittlefield wrote:
One reservation I've always had about screw adjusters . . . is that it's very easy to build up large forces due to mechanical advantage of the screw. On the build here, I deliberately opted to not include any such device, relying instead on fingertip sensitivity to hold the belt tight whilst tightening the screws.

The nice thing about screw adjusters is that they make it easy to increase or reduce belt tension by very small amounts. The adjustment screw I use has reasonably fine threads, and a fraction of a turn of the screw makes the difference between the belt’s slipping and not slipping.

As with any tool, one could abuse the screw adjuster and mess things up, but I’d argue that this is not all that easy to do, with just a little bit of knowledge. The proper tension is not at all close to anything that would cause immediate damage.

rjlittlefield wrote:
Revisiting this point, I'm wondering what "a bit of slack" means.

Let me preface a response by talking a bit about timing belts, for readers who have not had one in hand. Timing belts are not stretchy. Unlike tension belts, such as the V-belts you may see under the hood in your car, they engage not by friction, but with tooth-like ridges that fit into grooves in the timing pulleys. In this way, they operate more like flexible cogs rather than the stretchy belts we tend to be familiar with. (Many automobiles do also use a timing belt, but it's not readily visible.)

My macro rig's timing belt is made by a company called “Bando,” and Rik’s is made by “Fenner.” In both cases, the belt contains fiberglass cords that keep it from stretching. Both catalog excerpts give formulae for calculating appropriate belt tension (a process that includes parameters that include the distance being spanned and the size of the pulleys). But importantly, they (Bando, with particular clarity) tell you to use less tension if you can get away with it.
    Quoting Fenner, page 110 (aka page 24 of the pdf excerpt): “Synchronous belt drives operate by positive meshing and do not require high installed belt tensions.”

    Quoting Bando, page 133: “The proper tension is the lowest tension at which the belt(s) won’t slip or “squeal” under peak load.”
So what I suggest is starting with the belt loose enough that it slips, and adding tension bit by tiny bit until the slipping stops. At this point, lock your adjustment in place.

Effectively, my timing belt is not under tension at all. The two sides of the belt bulge outward slightly between the two pulleys—to the extent of a millimeter or two. (Since the belts have a degree of stiffness, they would spring outward into something more resembling a circle if not constrained by the pulleys.)

It may be worth mentioning that the tension adjustment capability—however it is achieved—has an additional, and very important function in a timing-belt driven rig: shortening the distance between the pulleys enough to initially place the belt on the pulleys without attempting to stretch it. Stretching it to fit over the edges of the pulleys would damage the belt.

rjlittlefield wrote:
Can you say a few more words about what "most consistent results" means, and maybe what you think causes the inconsistency. . .?

Like any right-thinking tinkerer, I didn’t bother reading the instructions at first, but got parts in hand and started playing. I knew that timing belts shouldn’t be stretched, so initially tensioned my belt fairly loosely, though not so loosely as now. (It now seems almost embarrassingly loose, as if I forgot to tension it—but works perfectly.) But I noticed something that bothered me. In my imperfect memory, I think it was a slight wobble observable at high magnification—likely 100x. Whatever the problem actually was, it went away when I relaxed the tension on the belt as described above, and has never returned.

I think what I had going on was a very slight pressure on the fine focus shaft, which was directed onto the gearing and creating the wobble—if I correctly recall the issue—or other inconsistency, if my memory is off. Even if one does not observe a wobble, eliminating even a slight sideways pressure on the fine focus shaft is probably a good thing, in terms of minimizing wear and tear on the focus block.

--Chris
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