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Wild M5 focus mechanism service advice

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viktor j nilsson

Joined: 01 Mar 2013
Posts: 14
Location: Gothenburg, Sweden

PostPosted: Sat Dec 16, 2017 3:20 pm    Post subject: Wild M5 focus mechanism service advice Reply with quote

Long time since I posted on the forums.

I hope to attract the attention from g4lab, Ichthyophthirius and others who service stereo microscopes.

I have a nice old Wild M5 that I got for free from a University. Optically it is perfect, but the focus mechanism is not. It's not stuck, but the friction changes as you change the focus. It feels good at first, but as I shift the assembly downwards, it suddenly becomes much looser. The grease has definitely started to age.

I really hope to get the focus mechanism buttery smooth again. I've read everything I could find online about greases and focus blocks, but I'd love some feedback before I start.

First, am I correct that it should be safe to take apart the M5's focus mechanism before cleaning and re-greasing it? I've read that g4lab prefers to keep focus mechanisms intact whenever possible due to problems with "factory calibration" of some focus blocks, but I believe that this only applies to more complicated designs? Or could the teeth of the rack and pinion somehow become 'mismatched' if I take it apart?

Second, is it correct that I should only re-apply grease to the flat, sliding surfaces, not the rack and pinion? Should I apply any type of lubricant to the rack and pinion, or should they be kept completely clean and dry?

Third, do you think that the Nye 362HB general purpose grease would do a good job for the sliding surfaces? I've also considered damping greases such as Nye 868 and Nye 795A, but after reading lots of different opinions it seems as if that these might be a bit too sticky. I really like a smooth, buttery focus mechanism. Could it be worth trying the very light or ultra light grades of Nye's damping grease?

That's it I think. Many thanks in advance!

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viktor j nilsson

Joined: 01 Mar 2013
Posts: 14
Location: Gothenburg, Sweden

PostPosted: Sun Jan 14, 2018 5:02 am    Post subject: Reply with quote

I've went ahead and ordered some Nye 795A lubricant. It was quite cheap at Newgate Simms (£18.75 for 50 grams) compared to the alternatives, and seems to have a good reputation online for similar applications. I'll try it out to see how it feels, and take it from there.

I would still appreciate input on whether I should put any grease or oil in the rack and pinion.

I've also realized that I have a "new old stock" spare set of rack and pinion for the M5 in my drawer. I'm guessing that those parts are almost worth their weight in gold...
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Joined: 23 May 2008
Posts: 1426

PostPosted: Fri Jan 19, 2018 5:00 pm    Post subject: Reply with quote

The M5 is the last model with completely hand fitted focusing racks. The university where I work bought a bunch of them in about 1967 and many of them are still in service.

I have on occasion attempted to consolidate broken pieces into working scopes, and this is almost impossible on the M5. The two pieces of the sliding dovetail have to be kept together. Also the rack and pinions like to be kept together. for either part if you just replace one side it won't fit. Each rack and pinion and also the dovetails were hand fitted by very skilled feinmechanikbau assemblers. I have had many people ask for rack and pinion pieces, but I had no spares. I have had to take focusers into the machine shop to fit rack and pinions together even on much lesser instruments than the Wild M5. The amount of force the pinion gear exerts on the rack very much affects the feel of the focus and the amount of time it will last. Sometimes the rack has some relief underneath to allow it to flex.

One thing to watch out for is that if you should disassemble either the rack and pinion or the dovetail make certain you watch out for very thin and delicate sheet metal shims they used to make the fit perfect. Also if a scope is tight get it loosened with penetrating oil as soon as possible. Then use an atmospherically stable lubricant such as a teflon grease if you can. Many of the M5s that ended up going to surplus had the lubricants locked up and then someone tried to force it to focus and used the pinion as a milling tool on the focusing rack. (Freeing one of them involved clamping one side in a very good vise with smooth jaws and I don't remember what to protect the paint, and then whacking the other side with a piece wood driven by a hammer. It worked. Penetrating oil had not.)

Getting the pinion shaft out and then putting it back or replacing it with a new one is difficult almost to the point of impossibility. Also the slip clutches in the focuser knobs are very difficult to set correctly.
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