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Wild M420 focus mount problem [fixed!]

 
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Thu Jun 14, 2018 4:34 pm    Post subject: Wild M420 focus mount problem [fixed!] Reply with quote

I got a little caught up in some eBay bidding this week and ended up buying a Wild M420. It just came in, and has a couple of problems. There are some minor issues with the base (a few loose rollers and a sheared off thumbscrew), but they aren't likely to affect my usage too much. The big problem is the focus mount. Turning the knob adjusts the focus fine until it hits a specific point in the rotation, where it loses all grip and the head falls down to the bottom of the track. Hopefully this is a fixable problem, because the image through the eyepieces looks really crisp.

It is an older Wild-only one (before the Leica takeover).


Last edited by Scarodactyl on Mon Sep 10, 2018 6:15 pm; edited 1 time in total
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Pitufo



Joined: 21 Jun 2015
Posts: 206
Location: United Kingdom

PostPosted: Fri Jun 15, 2018 8:44 am    Post subject: Reply with quote

Nice find.

I don't know the internal focus mechanism of this microscope but it sounds very much like a problem I had with an Olympus CH2 recently due to a metal gear with a damaged/missing tooth (photo here http://www.photomacrography.net/forum/viewtopic.php?t=37255).

When the focus gears rotate and engage the damaged tooth, they can spin freely and the focus drops.

I hope this helps.
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Fri Jun 15, 2018 6:13 pm    Post subject: Reply with quote

That makes sense--I figured it must be something like that. I'm going to take it apart, hopefully sometime in the next couple days, and see what's what. I'll try and document it along the way.
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Mon Jun 18, 2018 5:06 pm    Post subject: Reply with quote

OK, I had a look through the service manual and started to get it apart.
Step 1: remove the makrozoom objective. Very easy to do, it's a dovetail connection held on with a single screw.

Step 2: unscrew this hex screw (then two others) to get the focus drive housing off of the head

That's as far as I got, because this screw is STUCK on. Is there a product that would help loosen it that would be safe to use?

On the plus side, removing the objective did give me a chance to get eyes on the problem:

Unsurprisingly you were dead on. The head uses a rack and pinion style gear mechanism and one of the teeth is just straight up gone. The whole thing is also fairly worn, it must have seen a lot of hard use.

So at this point I am not 100% sure how to proceed. While it appears to only be a single part that's broken I doubt I'll be able to find a replacement just for the pinion? This may mean I'll need to replace the entire focus drive by cannibalizing one off a (more) broken scope. It looks like the M5 and M8 stereoscopes might have compatible focus drives in addition to other 400 series makroscopes. And of course I'll have to get this screw unscrewed somehow to access it.
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viktor j nilsson



Joined: 01 Mar 2013
Posts: 63
Location: Lund, Sweden

PostPosted: Tue Jun 19, 2018 12:55 am    Post subject: Reply with quote

Nice scope!

In this thread: http://www.photomacrography.net/forum/viewtopic.php?t=37255
https://www.gearsmade.com was recommended.

If you can't find the original part, it could be worth making a new part to keep such a fine instrument alive.
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Thu Jun 21, 2018 6:42 pm    Post subject: Reply with quote

Thanks for the recommendation! I will definitely drop him a line and see if he's willing to take on a weird pinion gear like this one.

OK, unfortunately as events unfolded I couldn't go easy on this screw:

Turns out you can strip a hex screw even with a perfectly sized wrench (3mm). We finally got the screw extractor seated, and after the application of some tremendous force it suddenly popped.

Yup, whatever son of a fish last maintained this did use loctite or some other epoxy.
Once I knew that, though, I was able to get the other two out by exerting a lot of force and pressure. Both popped free and came quietly thereafter.

This allowed me to move the rack down and get a look at the collar that holds the focus mechanisms together:

thankfully these screws were much easier to get out, since if I stripped one of them I'd definitely be up a creek.

With the collar off, the knobs on the right side unscrewed:

Allowing me to get two separate parts, the right knob with a small threaded section that goes into the collar, and the left knob which has the pinion gear attached:


I think the next thing I'll have to do is get one of those two-toothed key things to get this unscrewed. This part is officially past where the service manual goes, so I am getting into uncharted territory here.


Not only is the pinion badly worn, you can also see it appears to have been hand-cut, and the teeth are diagonal relative to the long axis of the pinion. I am not sure why.

The rack also shows some wear, but nothing catastrophic. Hopefully replacing the pinion will be an adequate long-term solution (especially since I don't plan to abuse it; I don't know that that's what happened here, but based on the state of that gear I think there is an outside chance that the lab it was in was staffed by overexcited howler monkies.)
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19337
Location: Richland, Washington State, USA

PostPosted: Thu Jun 21, 2018 7:31 pm    Post subject: Reply with quote

Scarodactyl wrote:
...teeth are diagonal relative to the long axis of the pinion. I am not sure why.

I find this design quite puzzling.

The shaft part looks looks like half of a "helical rack and pinion" with an unusually shallow pitch.

In general the helical tooth design is used to minimize changes in the way the teeth fit as the pinion is rotated. In most helical racks, the pitch is steep enough that no matter where you are in the rotation, part of one tooth is completely engaged, part of one tooth is just coming into contact, part of one tooth is just coming out of contact, and so on. Those points move across the rack as the shaft is turned, but there's always some of each. The advantage is smoother operation. Helical racks are commonly used in things like bellows and macro rails. All except my cheapest bellows use them.

The puzzling part here is that the pitch is so shallow. It's much shallower than all the other examples I know, and I'm not sure that it's even steep enough to guarantee all the advantages I mentioned above. I don't know why this design would bother to use helical teeth but then go with such a shallow pitch.

--Rik
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Thu Jun 21, 2018 8:23 pm    Post subject: Reply with quote

Someone on another forum pointed out that, given the level of abuse, it's possible it has actually been twisted rather than cut that way.

I got a quote back very quickly--375 to make the gear (though he'd handle the knob and also make sure it matched the rack, which would be great). Unfortunately that's more than half of what I paid for the entire microscope, so I am not sure I want to go that route. I have seen two broken m420s with apparently intact focus housings sell for less than that over the last month on eBay (though none before then that I remember) so I am not sure if I should just bit the bullet or wait in the hopes that a used replacement to become available.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19337
Location: Richland, Washington State, USA

PostPosted: Thu Jun 21, 2018 10:05 pm    Post subject: Reply with quote

Good point about the abuse and possible twisting! I did not think of that possibility.

Thanks for info about pricing for the gear. To me that seems quite reasonable as price for a custom piece, but at the same time it's high enough to spend some time looking for a "junkyard part" replacement.

--Rik
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Sat Aug 11, 2018 8:32 pm    Post subject: Reply with quote

It's been a while since my last update. Al Meekins requested I send the entire assembly, knob and all, probably assuming as I did that all it needed was a pin wrench to dissassemble. He ended up sending it back to me to try and take apart.
I ordered a pin wrench which (with a bit of modification) fit the pins and let me start to dissassemble the entire thing. Most of it came apart with little difficulty, and I even have pictures of each step which I'll add here eventually to make reassembly easier. However, I have hit another wall, hopefully the last:

I can't get the actual pinion out of this bearing. It seems to extend all the way through to the hole at the middle there.

It looks to me like the pinion is held into a collar (which has the holes for the pin wrench) which is in turn threaded into the bearing.

I had a go at using the hole to my advantage to get it unscrewed (using wood to avoid damaging the metal), but...

Yeah, that didn't work. I also tried a steel drill wit wrapped in a latex glove to avoid damage, and it would not budge. I am concerned it may be epoxied or pressed together or something. At this point I am not sure I'll be able to get it apart, or what I should try next: maybe soaking it in some aggressive solvent, or trying to heat one of the components to get it loose?
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Scarodactyl



Joined: 14 Apr 2018
Posts: 62

PostPosted: Mon Aug 27, 2018 5:23 pm    Post subject: Reply with quote

Well, that last step has defeated me. I couldn't get it to budge. I don't know if it is threaded, epoxied or perhaps pressed together, but it's a mess and I gave up.

So I've had to cancel having it remade, and instead I found a focus drive designed for a different microscope entirely (I am not sure which) which was semi-compatible. By this I mean it is a rack and pinion with a fine and coarse focus knob which has a flat front with screws to attach a head mount, rather than just having the head mount directly attached to the rack. This meant I could make an adapter with holes to match the M420 head and the focus drive and attach the whole thing together.

Here's a picture of the focus drive with the adapter attached and drilled. Rather than making it from aluminum, as might have been sensible, I went to our local woodworking shop and got some Bolivian ironwood. This is cool stuff, with a specific gravity slightly over 1 so it actually sinks in water when dry. It should work similarly to aluminum while a) being a bit easier for me to work with, and b) looking very fancy in a 70s paneling sort of way. I had to do a few iterations of adjustments on this to get everything lined up nicely, but it worked surprisingly well, and the first one I drilled ended up working with modifications. If I did it again there are things I'd probably do differently, but I'm pretty happy with it.
The other down side was that the new focus drive takes a 1 1/4 inch diameter shaft, much thicker than the one the m420 was originally attached to. So I made a temporary replacement with an oak dowel, drilled down the center with the threaded section from a carriage bolt inserted:

This is not a permanent solution unfortunately, as the wood does allow too many vibrations at max mag. It is also susceptible to termites. So I will eventually replace it with a metal one. But nevertheless the m420 is now functional!

This is a quick stack I shot at low mag. I am currently moving so the table the m420 is on does not have a power outlet nearby and I thus fiber optic lighting isn't available yet, so I used cheap 'UV' LED flashlight to illuminate this petroleum-included quartz. It would need more pictures to really turn out, but I am nevertheless pretty happy with it. It's a really nice piece of equipment, and it's really, really nice for it to not be sitting in my parent's living room in pieces anymore.
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