Need help--how to remove Nikon focus block fine focus knob?

[Chris S.]

Oh, ye great microscope gurus, please help this neophyte!

Here are pictures of the Nikon microscope focus block I'm using for macro stacking. I want to a add a step motor, and if I understand correctly, the best approach is to remove either the right or left fine focus knob, find the drive shaft hidden beneath, and attach a "shaft coupler" to either connect the fine focus drive shaft with a step motor, or (again with a shaft coupler) to attach a gear and then use a timing belt to drive the fine focus from the step motor.

My immediate questions are, "How the heck to I remove the fine focus knob? And if I do, what will I find--will it be a nice, clean drive shaft that I can couple to? Or will it be more complicated, and I'll find myself buying myself problems?" And perhaps most important of all: "If I attempt to remove a fine focus knob, do I risk destroying this excellent, buttery-smooth focus block?"

I have a number of Nikon microscope service manuals, but none contain dissasembly instructions for this particular focus block--which appears quite different in design from the models the service manuals that I have cover. I believe it to be from a Nikon Optiphot (with no extra letters or numbers), but don't know for certain. The Optiphots with extra numbers that I have seen have a different sort of focusing block handle.

Any advice is very welcome--I suspect that for some members of this community, taking apart a focusing block like this child's play. Sorry to admit that it isn't so for me. I have some ideas, but don't want to risk destroying a really nice focusing block when advice from those who know could save me a lot of pain.

Thanks extremely, in advance, to those who respond.


Photo of both sides of focus block. On both left and right, coarse and fine focusing knobs are visible. Left side fine focus is labled with a scale. Please note "holes" in fine focus knobs on both sides--more later on that.



Right side (non labeled) knobs of focus block--coarse focus is the big, outside knob, fine focus is the small, inside knob. How the heck to do I remove the fine focus knob on this side, and what will I find if I do? Would it be a useful drive shaft for connecting a step motor via a shaft coupler?



Closeup of one of the "holes" in the fine focus knobs. Such a "hole" exists on the fine focus knobs on both sides of the focus block. Each hole appears to contain a brass construct of unknown porperties--I cannot discern if these brass constructs are threaded, hex-keyed, or whatever. Advice very strongly requested before I risk this nice focus block in disassembling it.

Many thanks,

--Chris

[Barry]

Hi Chris,

Personally I dont own such a focus block but am quite sure the two holes in the fine focus knob are for set screws (likely hexagonal socket, occasionally may be slotted) which fix the knob on a shaft.

Try to insert your allen keys and see if one easily gets hold of the screw, is my suggestion.
Have always wondered what is inside - I guess it works like a planetary gear set, where 3 small gears move the ring gear of the coarse focus knob, which is connected to the main shaft. Would be great if you could post images (probably I can rebuild one).

Good luck,
Barry

[rjlittlefield]

Comment #1: This is probably a silly question, but have you considered just coupling to the fine focus knob, instead of the shaft that it mounts on?

Comment #2: If you don't get an adequate response here, or maybe just to be sure the bases are covered, you might try asking on the Yahoo Microscope group. Just give them a URL that points to your posting here, and you won't even have to repost the pictures.

--Rik

[ChrisR]

ChrisS - I expect you've already looked here:
http://www.tofrainc.net/focus-drive-integrated.htm
Though not a lot of info, they all look smilar, and in most cases the instruction is "remove knob"!
There are come pics for Barry to muse over on the TE2000 (mounting instructions) page

[ChrisR]

I expect it might have a 4mm shaft so you'll need one of these:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... IF:GB:1123
Not too easy to find.

[Charles Krebs]

Chris,

I've taken apart several Labophots to replace the fine focus shaft... but it was about four or five years ago. I suspect this is pretty similar. In the interim I've disassembled three or four other brands/models so I don't remember each specifically. One thing about all of them was that you really can't mess things up by only removing the FF knobs.

On this one they are held in place with hexagonal set screws. Sometimes these are pretty small... 1.5 or 1 mm. (Keep notes as you remove the knobs about the "order" and position of any brass/plastic washers or spring washers so that they can be put back in the same order).

On the Labophot (and some Olympus models) the FF shaft is a single long shaft that has a pinion gear attached on one side. (BTW... in the Labophot this was a plastic gear and a source of trouble on that microscope... it would often crack if the lube in the gear train dried up and "excessive" force was used to try and move the fine focus). This pinion gear "nests" into a more complex gear train that is fastened down and will not "disassemble" unless you start unscrewing additional parts. You won't need to, or want to, mess with that.

On one stand I worked on there was a spring washer around the FF shaft that provided proper "tension". So that when re-assembling you needed to "push in" the FF knobs on each side to slightly compress that washer before tightening the set screws.

Charlie

[Chris S.]

Barry and Charlie, you were right--a 1.5mm hex key did the trick. It didn't look like a hex-driven set screw when I studied it through that small access hole, but when I inserted the hex key--of necessity blindly--and gave it a fair amount of persuasion, it turned. Having expert advice--not just on what to do but that I wasn't hurting anything--was mighty nice. Thanks especially, Charlie, for your detailed and very useful reply. I'd seen posts in which you've mentioned replacing that plastic gear--any advice on how to prevent the lube from drying up?

Rik, I've indeed been thinking about coupling to the fine focus knob instead of the drive shaft, and am still undecided about that. I did at least want to see the drive shaft before deciding--and with the advice given to me here, it really wasn't difficult to remove the knob. Attaching to the drive shaft would perhaps simplify things, as somewhere there is probably a 3mm (ff drive shaft diameter) to 6mm (step motor drive shaft diameter) coupling available off the shelf. Of if not, asking Don Wilson to whip one up for me would probably not tax him a bit. On the other hand, coupling to the knob has appeal--right now, the focus block can be removed in a few seconds for use on tripod. If I couple the motor to the knob in a way that quickly releases, I preserve that ability. But the knob is a bit oddly shaped for coupling--I've been pondering ways to do it, but it doesn't appear as simple as just coupling to a metal shaft.

Also, thanks for the suggestion to try the Yahoo Microscope group. Not necessary, as it happens, but a good idea.

ChrisR, I had indeed been studying those Tofra units and their supporting information. As you say, my question was neatly skipped over in their step: "Remove knob." But I've learned quite a bit from that Website.

Also, ChrisR, regarding your link to the flexible shaft coupling, let me ask what is probably a dumb question. Do I actually want a flexible coupling? Seems to me that flexibility is only needed if I don't line up the drive shafts properly. Or in practice, are the tolerances so critical that it's always better to use a flexible coupler? A Google search turns up a mind-blowing variety of flexible couplings, which appear to range from adding slight flexure to being a universal joint. Not sure how far to go. The one you linked to seems very similar to the ones that Tofra is using--which I appreciate. But my ff shaft is 3mm (which this unit would fit--I suspect your mentioning 4mm was a typo) and my motor is 6mm--whereas this unit is 2.39mm at the other end.

Thanks much, guys! Here are some followup pix for those interested.

--Chris



Here is the backside of the fine focus knob after removal. As Barry and Charlie assured me, there is a set screw there, and the hole in the ff knob was an access hole for inserting a 1.5mm hex key.




Fine focus knob held near the site from whence it came. The fine focus drive shaft, now exposed, measures 3mm diameter.




Some following this thread might be interested in seeing the step motor I'm hoping to couple with the fine focus. I'm uncertain whether I'd be better off coupling to the drive shaft or reattaching the knob and coupling to that. Am also thinking about coupling through a belt drive, as has been suggested in other threads.

[ChrisR]

No I'd measured a few scopes and they were all 4mm!
I believe you want rotational stiffness in the coupling, but with axial ("bend") flexibility, because it's hard to get things lined up perfectly.
The one on my desk (6mm in and out) bends easily 5 degrees or so either way, but through its 10 turns of helix, no noticeable elasticity torsionally.
These guys do a 3 to 6mm, for about $100
http://www.sdp-si.com/eStore/CoverPg/co ... +Couplings

[NikonUser]

Just a thought: perhaps it's worth waiting to get the Nikon block with the larger FF dial. That instrument has divisions of 1micron as opposed to 2 microns in your model; but, then again, it may not make any difference to a step motor.

[rjlittlefield]

I see that some of this has been covered while I've been fixing dinner and writing notes. But since they're already written...

Do I actually want a flexible coupling? Seems to me that flexibility is only needed if I don't line up the drive shafts properly.

See http://www.photomacrography.net/forum/v ... 6885#46885 and Charlie's earlier post in the same thread for some comments about this.

Part of whether you need a flexible coupler depends on how you plan to mount the motor. Both the fine focus shaft and the shaft of the motor have bearings that force them to stay precisely aligned with their respective enclosures. If you hard-mount everything, it's going to be pretty challenging to get everything lined up to within the tolerances allowed by the bearings. A compliant mount would take care of that, but then you might need to worry about the weight of the motor or other continuous side force against the focus shaft. It's not the sort of problem that would cause instantaneous failure, but long-term wear might be an issue. On the other hand, eventually everything wears out, for example the shutter on your camera. No sense over-engineering something relatively cheap like a focus block.

What I'd been thinking for flexible drive was a side-by-side arrangement with a ribbed timing belt. The https://sdp-si.com/eStore/ folks have a large collection of plastic timing pulleys that it seemed would not be difficult to bore out if necessary.

--Rik

[Chris S.]

ChrisR, thanks for the link to the vendor selling that particular coupler. While pricey, it may be what I need.

NU, could you be thinking about somebody else's focus block? Mine does have divisions of 1 micron, and I'm quite happy with it. The travel is a bit more than 2.5 inches, and it is silky smooth with no noticeable backlash or stiction. I consider it somewhat better than my Olympus BHMJ focus block, and far, far superior to my Newport linear stage model 462. (I'm aware that linear stages come in a wide variety of quality levels, and don't have enough experience to tell how this one compares with others--but my focus blocks far surpass it. Also, for translation stage fans, I'd point out that a mere 4.25 turns of the Nikon focus block's coarse focus dial returns the block to the starting point--undoing 100 turns of the fine focus dial, so returning to square one is nearly instantaneous.) Having had this block modified to include Arca Swiss clamps on top and bottom, I'm really happy with it, and not expecting to part with it any time soon.

Rik, thanks for making sure I saw that link, which I had in fact studied with considerable interest. While posters have opined about the need for flexible coupling, I hadn't actually seen this supposition questioned very deeply, and wanted to know if anyone had solid evidence that a flex coupling is really needed. At this point, I'm inclined to agree with you and ChrisR, and will assume a flex coupling reduces stress on the components, but will still try to get things lined up as well as possible to further reduce this stress. Building a bit of adjustability into a homemade motor mount would add compexity, but does not seen like the end of the world.

Not sure see a focus block as "cheap." It certainly is in the absolute sense--I paid $66 for mine. But I'd combed thousands of listings over more than a month to find what I wanted. If time has value, this nominally inexpensive focus block starts to look like more of an investment--especially after having it adapted to mount the Arca-Swiss-type clamps. But maybe I don't know the best sources for focus blocks (mostly used eBay), and could have done this with less fuss if better informed? I can say that this focus block, after adaptations, does exactly what I want it to, and I don't want to face replacing it any sooner than I have to.

The ribbed timing belt approach intrigues me, too, Rik. One big question for me is how much vibration or jarring will be imparted by the step motor. If insignificant, then direct flex coupling would be simpler. But if the motor jars the rig, a timing belt could help isolate that perturbation--or so it seems to me. Am planning on experimenting with the motor before making up my mind. Another benefit to a timing belt approach would be the ability to change gearing--and hence reduce focus steps to much less than a micron--but the considerable added complexity gives me pause. In my setup, the focus block can be moved 18 inches to accomodate varied working distances and bellows draws--so any step motor has to adjust in parallel with the focus block. Moving a belt-driven motor is certainly doable, but more complicated than moving a motor attached directly to the focusing block.

Here is what I mean about the focus block travel. The focus block can be positioned anywhere along the red line in the image below--so any stepping motor assembly has to accomodate this adjustability. (Some of the adjustability is accomplished by moving the focus block on the 12-inch dovetail, while the most extreme settings are accomplished by moving the dovetail. Am currently seeking to extend that dovetail to make extreme adjustments a bit quicker.)




Cheers,

--Chris

[DaveW]

If the focus spindle is of a small enough diameter, and the motor one large enough, couldn't you just bore out the motor spindle (which looks as if it already has a hole down the centre) to take the focus spindle and then secure it with a set screw as the drive knob originally was? That would mean the motor was mounted directly on the focus spindle with no need for a flexible drive and so give more precise control?

The other alternative, instead of a flexible drive, just a short piece of circular rod to make a collar with one end bored for the motor shaft diameter and the other for the focus spindle and just secured to both with set screws as the knob was originally.

DaveW

[NikonUser]

Chris S. wrote "NU, could you be thinking about somebody else's focus block? Mine does have divisions of 1 micron, and I'm quite happy with it."
If you are happy with it then there is nothing else to say from me.


However, I have what looks like an identical focus block where the divisions are 2 microns; on my block (and it looks like on yours also) there just 10 divisions between 0-20 and numbers up to 200 (180) on the dial.

On another Nikon focus block the FF dial is larger, there are 10 divisions between 0-10 and the dial numbers range from 0-100 (90). Each division represents a 1 micron movement.


NU09-234

[Barry]

Chris S,

A coupling from 3 to 6 mm is not usual. What I do in such cases is make a little sleeve for the 3mm so you can use a coupling for 6mm axis at both sides.
In case you use a rigid coupling, it is best that the motor is mounted flexible, for example on rubber dampers.

If you need anything I can help.

Cheers,
Barry

[Chris S.]

NikonUser is right and I'm wrong. NU, thanks for the correction!

My calculations were OK, but maybe it's time to get my eyes checked. (I don't need reading glasses yet--no, really--it's just that they keep making the type smaller.) Here was my approach:

A: 100 revolutions of the fine focus dial moves the stage 20mm (checked that again before last post).
B: 200 divisions per revolution (see the rub?)
C: Therefore 20,000 divisions per 20mm movement. So each division equals 1/1000 mm, aka 1 micron.

My mistake was that while there are numbers from 1-200 per revolution, those blasted tiny tick marks only occur at ten tick marks per 20-unit numerical increment, not twenty tick marks per increment as I had thought. See above comment about not yet needing reading glasses.

While this doesn't change the fact that my stage moves 200 microns per ff revolution, it highlights the fact that NU's moves 100 microns/ff rev. Used by hand, this would certainly give NU's better resolution. Ditto, I'd think, if directly coupled to a step motor.

But what if one uses gearing in the step motor connection? Either focus block could be geared using a toothed belt drive, and I'd think that other mechanical qualities of the block would affect how fine one could ultimately go. Which focus block, I wonder, could ultimately be used more finely? One thing I like about this older block is how solid and and silky it is--I have a subjective impression that with gearing, I could decrease the movement increment quite a lot before running into mechanical limitations. Are newer models more or less mechanically able? I have no idea--have not played with enough focus blocks to know.

While probably irrelevant, I have some experience with older Nikon camera lenses from the days when they had knobby, rather than finely-ridged, focusing rings. I have one that was already old when I purchased it 25 years ago, and despite years of use, it continues to have a smooth, solid mechanical feel that lenses of newer vintages lack. This experience, while probably inappropriate to apply here, may have colored my preference for an older, knobby-wheeled focus block.

NU, how much travel do you get with your 100-micron-per-turn block? I get a bit over 2.5 inches.

Cheers,

--Chris