How To Break the Famous Labophot-2 Plastic Focus Gear

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rjlittlefield
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How To Break the Famous Labophot-2 Plastic Focus Gear

Post by rjlittlefield »

Summary:
CAUTION --- When disassembling the Nikon Labophot 2 and similar focus blocks, you MUST remove the fine focus knobs by pulling the knob STRAIGHT ALONG THE SHAFT.
DO NOT APPLY TORQUE UNDER ANY CIRCUMSTANCES, no matter how much axial pull is required to remove the knob!

------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Here's why...

Certain models of Nikon microscopes are famous for having an easily cracked plastic gear on the fine focus shaft. I've read about it for years.

But I now have first-hand knowledge of the problem.

Indeed, the short story is that I received a Labophot-2 arm in good condition, started to disassemble it just far enough to measure the focus shaft diameter, and in the process of disassembly managed to crack that famous plastic gear.

The damage was annoying, of course, but that lasted only until I started to understand what all went wrong. Then it became quite fascinating, and I'm glad I had the experience. I will enjoy sharing it with you!

To begin with the cracked gear, this is what that looks like:

Image

Cracking like this is traditionally attributed to "old sticky grease", which is supposed to gum up the works so badly that forcing the fine focus knob will cause the gear to split.

However, it turns out that the gear is easily broken even when the grease is in perfect condition. :shock:

The problem is due to an intrinsic weakness in the design, which allows the gear to be cracked in an instant by simply twisting the two fine focus knobs in opposite directions.

Let me show you how that works...

This is a Labophot-2 fine focus shaft, with all parts attached. The assembly consists of two knobs, a retaining collar, and a plastic gear. The two knobs are retained on the shaft by what the repair manual calls "handle center nuts", which are slightly recessed into the ends of the knobs and are not visible in this image.

Image

Nothing in the above photo would suggest that there's anything to complain about.

The design weakness appears only when the parts are disassembled. Consider now these photographs of components at the left and right ends of the shaft.

Left:

Image

Right:

Image

In words, the chain of physical connections in the device goes like this:
  • Left fine focus knob is locked to a collar using pins in slots.
  • Collar is set-screwed to the focus shaft.
  • Focus shaft is locked to the plastic gear by a D-shaped cross-section cut into the shaft, around and into which the plastic gear is formed.
  • Plastic gear is locked to the right fine focus knob using pins in slots.
  • Right fine focus knob is retained by a threaded nut, but only friction prevents it from turning on the shaft.
As a result, any torque in excess of friction, applied between the two fine focus knobs, also gets applied to the D-shaped section between the shaft and the plastic gear.

Apply very much torque and the shaft turns inside the gear, the D-section acts like a cam, and the gear splits. No involvement of other gears or their lubrication is required.

The trick for successful disassembly is to realize that the left focus knob must be pulled straight off -- no twisting permitted!

This would be no problem, except that on the unit I received, there was a thin ring of plastic inside the bore of each fine focus knob, which interfered with the threads of the shaft so severely that the fine focus knob apparently could not be pulled straight off.

Failing in my attempt to pull it straight off, I tried twisting it, and that's when the gear split.

Now, the standard explanation for split gears is that the grease dried up and somebody forced the fine focus knob. After first-hand experience with the mechanism, I now suspect a different explanation. It looks to me like the gear train, with an intact plastic gear, could apply more force than needed to overcome all but the most egregious cases of dried lubricant.

Instead, I'd place a modest bet that many (most?) split gears actually happen like mine did -- somebody goes to disassemble the mechanism, makes the same mistake of applying torque between the two knobs, and there goes the gear. Dried lubricant is still important in the scenario, but only as motivation to disassemble.

After I did get things fully disassembled and figured out what had happened, I went back to see what more I could learn about the details. The result leaves me better informed but still quite puzzled.

Here is a closeup stereo view of the plastic ring inside one of the knobs. The ring is that section of plastic that looks like it has threads torn in it, sitting at the bottom of the hole.

Image

The reason for the threaded appearance is that the way I finally removed the knobs was to "unscrew" them by forcing them to turn on the shaft. Pulling them off axially seemed impossible.

After removing and reinstalling the knobs in this incorrect way several times, I finally decided to measure the force that would have been necessary to pull them off axially, by simply applying enough force to drag the threaded part of the shaft through the ring.

That measurement -- after the ring had been degraded by repeated manipulation -- still took 9 pounds for one knob and 10 pounds for the other. I suspect that in its initial condition, even more force would have been required. That's a lot of force, particularly because there's no obvious way to grab hold of the tapered knob when it is fully assembled on the scope. (See image below, shown with the retaining nut still in place.)

Image

Stepping back and thinking about the whole experience, there are two aspects that puzzle me.

1. I don't understand why that little ring of plastic was inside the knobs in the first place. It serves no useful purpose that I can see, so being an intended part of the design seems unlikely. It seems much too thick to be flashing around the parting line of a mold. I considered the possibility that it formed during or after assembly, by deforming the surrounding plastic. But that makes no sense given the shapes of other mating parts and the fact that the hole is clean for the rest of its depth. So I am mystified. In any case, requiring 10 pounds (or more) to pull the knobs off axially seems far beyond what the designer would have intended, particularly given the fragility of the rest of the design.

2. As for "fragility of the rest of the design", I don't understand that either. It would make perfect sense for the gear to be locked to the shaft, if both knobs were then locked only to the shaft and not directly to the gear. Or it would make perfect sense for the gear to be locked to one knob, if the knob were locked to the shaft and the gear was not. But to have the gear be locked to both the shaft and the knob, thus creating a chain of connections in which the left knob is locked to the right knob only through this fragile plastic sleeve that also serves as a gear, that just seems like a bad idea that somehow slipped through design review.

If anybody knows more about the history and design of this mechanism, I would be very interested to hear.

In the meantime, I'll draw comfort from the thought that perhaps this posting will keep somebody else from having the same difficulty. :)

I hope at the very least that you find this interesting!

--Rik

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Post by Charles Krebs »

Nice presentation!

What makes this design more puzzling is that they had a problem with the fine focus plastic gear breaking in the Labophot-1 as well... so this was a re-designed mechanism. Their "solution" apparently created a similar problem. (The the Labophot-1 is different, and on inspection it would seem that the likely cause there was dried up lube and excessive force).

On a fair number of microscope focusing mechanisms (granted... usually stereo stands), if the focus tension was too loose and it "creeped" downward, the way to tighten it was to turn the focus knobs in opposite directions. If someone tried this on the block you show here, that would result in a trip to the repair facility! Despite the fact that the tensioning control is quite obvious, it wouldn't surprise me if that misguided approach cracked a few of these gears over the years.

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Post by Pau »

Very nice (and sad) case.
Charles Krebs wrote: On a fair number of microscope focusing mechanisms (granted... usually stereo stands), if the focus tension was too loose and it "creeped" downward, the way to tighten it was to turn the focus knobs in opposite directions. If someone tried this on the block you show here, that would result in a trip to the repair facility!....
With the Oly CH clones I have at work turning the fine focus knobs in oposite directions loose them and I need to pierce the knobs to put two screwdrivers to tighten them again. This is one of my more frequent repairs (teens! :twisted: )
Now I have a BHMJ focus block, Is the same than with the CH?

When I bought a nice Wild Heerburg stereo the focus was a bit hard to move, and I was a bit hesitant when Gene g4lab told me to turn the knobs in opposite direction to regulate...of course he was right.
Pau

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Post by Choronzon »

Heat gun. If you want to be in the band, you've gotta put your hat on.
I am not young enough to know everything.

soldevilla
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Post by soldevilla »

obviously, this has the appearance of having been designed to not be removed or repaired. I am not able to see how plastic gear mounted on the shaft, but in my experience, there not have no good solution. If the plastic is molded directly on the shaft, when the plastic cools after moulding, it suffers a contraction that leaves the piece full of stress. The natural aging of the plastic makes the rest of the work.I think it was called "planned obsolescence" :D . If the piece was mechanically mounted on the shaft, it must be heated for mounting in Section D, and the problem is similar. The metal axles and plastic gears are never good friends.

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Post by RobertOToole »

Very interesting Rik, thanks a lot for sharing.

I think you are right about the cause of the damage.

My Labophot-1 style block came to me with a tiny bump or skip in the fine focus travel. I pulled the block apart to find a cracked, split gear. I have a replacement metal gear on the way thanks to the Lothman, and will report back once I get the new gear installed.

Good work Rik.

Robert

TheLostVertex
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Post by TheLostVertex »

Thanks for the detailed write up Rik.

After seeing the images, I can safely confirm that the Optiphot 2 that I have is much different in design and removal of the shaft. I haven't fully disassembled it, but the knobs are very easily removed with a set screw, and the shaft runs free once the knobs are off. So I do not see this happening on my model.

But since Charles said the Labphot 1 design was different and likely broke due to excess force, that makes me wonder what the fatal flaw the Optiphot I have is. :lol: Im sure Ill find out in time.

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Post by ChrisR »

Labophot User manual:
Image

So, you've been told! DISORDER!
The natural aging of the plastic makes the rest of the work. I think it was called "planned obsolescence"
It's often that the plasticizers evaporate over time, making the plastic brittle.
More about entropy than scheming mankind.

I wonder if these parts could be 3D printed.

TheLostVertex
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Post by TheLostVertex »

ChrisR wrote: So, you've been told! DISORDER!
:lol:
ChrisR wrote: I wonder if these parts could be 3D printed.


There are a lot of different types of 3d printers.

Most home 3d printers are extrusion based, and I would not trust them for making any sort of precision gearing.

Commercial places that offer services you send a model to often use laser melting or laser sintering for plastics, and wax casting for metals. Printed wax castings do not have fine enough tolerances for this application I imagine. SLM for plastics I am unsure about, it might work but I am some what skeptical of using rough parts for precise motion transfer.

There are some new stereo lithography 3d printers that look like that might be up to this task. They use a light cured resin in order to create the object.

To think of it another way, 3d printers ultimately have a fixed amount of surface noise they produce in each dimension. So as the size of the target detail gets smaller, the ratio of signal to noise gets larger. So if your tooth size is only 1mm, and the printer produces 0.1mm lines, there is going to be a fair amount of variability in the tooth's meshing, especially for a properly profiled tooth.

Then again, it would be interesting if these gears didn't have to be precise at all for reasonable functionality.

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Post by Charles Krebs »

Steven said:
, that makes me wonder what the fatal flaw the Optiphot I have is. Laughing Im sure Ill find out in time.
I thought the Optiphot-2 and Labophot-2 had the same fine focus shaft:

http://tinyurl.com/qeoruod

The gear/shaft on my Labophot-1 looked like this:
http://www.prc68.com/I/Images/MicroSolFnFcb.jpg

The aftermarket metal replacement was also used in Optophot-1, but was a different length. The replacement shaft was "scored" so that it could be cut to the appropriate length for other models.

To see the Labophot-1 arrangement, look a little ways down on this page:
http://www.prc68.com/I/Labophot.html

Also see Ebay 191375567487 and 190760877609

TheLostVertex
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Post by TheLostVertex »

Charles Krebs wrote: I thought the Optiphot-2 and Labophot-2 had the same fine focus shaft:
It seems I was mistaken. What I have appeared to be an Optiphot 1, not 2. Good catch.

So since this mechanism is like the Labphot 1, I should watch for the problems you outlined before I guess.

phil m
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Post by phil m »

The odd thing to me, since I am mostly used to working on microscopes 30 years old and over is: what was the ultimate benefit of making this part out of plastic? a couple of dollars at most, in savings and those were very orderly dollars! Sure it cracked due to disorderly conduct and they anticipated that. They knew there were disorderly microscopists out there, otherwise they wouldn't have warned them in the manual. We need a Real Gear movement, then disorderly microscopists can feel at ease.

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Post by rjlittlefield »

I am pleased to see that my little story has prompted such good discussion!
Pau wrote:Very nice (and sad) case.
For me this case is not sad at all. I got the block for a very good price, I have learned much, and in the end my gear was easily restored to working condition by simply banding it to close the crack and boring it out a little to fix the shrinkage problems mentioned by soldevilla. So I have spent some hours on education, which almost always turns out to be a good use of time.
Choronzon wrote:Heat gun.
I assume this means to heat the knob before attempting to pull it off. I'll salt that away as a useful piece of local knowledge that I presume is commonplace within the repair community. For those of us who are not part of that community, it's not so obvious. In the repair manual for the Labophot-2, I can't find any mention of heat gun in the list of tools. There is also no mention of potential difficulties in removing the knob. The process seemed like it should be quite straightforward:

Image
TheLostVertex wrote:Then again, it would be interesting if these gears didn't have to be precise at all for reasonable functionality.
I imagine this depends on the definition of "reasonable functionality". My cracked gear has 16 teeth, for a movement of 100 microns, giving 6.25 microns per tooth. Each tooth is a little under 1 mm wide, so if the printer resolution were 0.1 mm, that's about 0.6 microns of stage movement per printer step. However, assuming that only the one gear were printed (leaving its mate with the original smooth tooth shape), then I think that the effect of printer steps would be only to introduce a little ripple into the curves of "transmission error". I expect those would be easily visible with careful measurement, but I'd be surprised if it matters for use in automatic stacking.

If anybody has occasion to try this, I'd be very interested to hear the results.
phil m wrote:what was the ultimate benefit of making this part out of plastic?
In my limited experience it's universal for the first fine focus gear to be made of plastic. Certainly that's the case in the Olympus and Amscope blocks that I've disassembled. In this Nikon block, actually the first two gears are plastic -- the white one shown here and its mate, which is black. The manual is explicit on this point: there are three small gears, one each in chrome plated steel, brass, and black plastic, plus the white plastic one that was formed around the shaft. Why there are four different materials, I could only guess. They may be matched to the progressively lower torques found as you move from the main drive gear out to the fine focus knobs.

Personally I'm not bothered by the use of plastic in this application. The problem is only the exact way that the plastic is used: formed as a thin sleeve around a metal shaft (apparently in both the Labophot-1 and Labophot-2), and in the Labophot-2 serving as a critical link between the left and right focus knobs. Even so, a good argument could be made that something is going to break if the mechanism is forced, so you might as well design in some specific "sacrificial" part to make the repair simple to diagnose and fix.

--Rik

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Post by lothman »

phil m wrote: what was the ultimate benefit of making this part out of plastic? a couple of dollars at most, in savings and those were very orderly dollars! Sure it cracked due to disorderly conduct and they anticipated that. They knew there were disorderly microscopists out there, otherwise they wouldn't have warned them in the manual. We need a Real Gear movement, then disorderly microscopists can feel at ease.

I had the same Problem like Rik on a Labophot 1, see here my repair solution:

http://www.photomacrography.net/forum/v ... t=labophot

ALL BAD? No IMO not, the Nikon design acts like a predetermined breaking point and saves all the internal gears for further damage. If you are on the mechanical dead end of the focus block you can create at lot of torque due to the high gear transmission ratio of the fine focus. If an "inexperienced" user at the dead end (or another mechanical block like the lens contacting the specimen) would apply force to the fine focus even some of the metal teeth would be sheared off. Probably this would require a complete disassembly of the focus mechanism and some $$$ for spare parts.

But if only the plastic gear breaks it can be easily replaced by changing the axle together with the plastic gear (rather low repair time and low costs). The only what we can blame Nikon for is, that they no longer have spare parts. Or that they did not use a safety clutch instead of a braking plastic gear for preventing further damage of the focus mechanism ;-)

As a repair solution I don't think 3D printing is the way to go due to the lack of precision and stability of the materials. On my Labophot 1 Nikon used standard gears with module 0,3 and 18 teeth. You can get metal gears, drill them with 3mm for the axle and glue them on the axle. But then (hear me Robert) stop turning the fine focus knob if it feels going hard, otherwise something else might fail due to high torque.


@Rik
I think Nikon also uses module 0.3 on your focus block, then you could get for e few $ a 16 teeth pinion with part number RBS0323 from Lemo Solar. I did not search for trader in USA, but I can help on getting these pinions and drill them to 3mm.
Charles Krebs wrote:
Also see Ebay 191375567487 and 190760877609
a good but very expensive solution, material costs are under 10$.

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Post by ChrisR »

That's good information, Lothar, thanks for posting.

I think I should get a couple for the day when I break mine :roll:
(I am always puzzled by the "module" idea. I realise it's a Standard, but it seems to be a way to guarantee, that gears of different diameters will not work together :smt102 )

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