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Wild M420 focus mount problem

 
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Scarodactyl



Joined: 14 Apr 2018
Posts: 15

PostPosted: Thu Jun 14, 2018 4:34 pm    Post subject: Wild M420 focus mount problem 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).
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Pitufo



Joined: 21 Jun 2015
Posts: 194
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: 15

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: 15

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: 31
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: 15

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
Site Admin


Joined: 01 Aug 2006
Posts: 18917
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: 15

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
Site Admin


Joined: 01 Aug 2006
Posts: 18917
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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