Wild m420 focus mount problem [fixed!]

[Scarodactyl]

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).

[MichaelG.]

A lovely instrument ... definitely an 'object of desire'

I don't think I have seen an exploded view of the focus mechanism, so it's probably a matter of exploration.

Meanwhile, this may be of interest : http://www.savazzi.net/download/manuals ... manual.pdf

MichaelG.
.
Update: http://www.savazzi.net/download/manuals ... Manual.pdf

[Hobbyst46]

http://www.savazzi.net/download/manuals ... Manual.pdf

An impressive manual, in four languages, wow.

[75RR]

Nice manuals! Hard to think of a better way to answer OP's question.
Thanks for the upload.

[Scarodactyl]

Thanks! I'd seen the basic manual but didn't realize there was a service manual available as well.
I will have a go at it myself, and try to document it properly too. It looks like it won't be nearly as bad as I'd feared.

[Scarodactyl]

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 it is indeed a broken gear. 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.

[PeteM]

It looks like just the central portion of the pinion gear is stripped?

If so it might be possible to pull the gear off it's shaft and cut/move the bad part to the side and a move the largest good part to the center??

Otherwise you may have to have a new one purchased or made to order.

As for the stuck screw, make sure you have a perfectly fitting removal tool (metric hex?). Let a drop of penetrating oil sit in the thread area overnight -- and give it a try. If that doesn't do it, add a bit of heat to the housing right next to the screw and another drop of oil. Depending on what lenses etc. are nearby I'll use either a heat gun with a focus attachment or a tiny butane torch quickly and carefully moved about to get the female threaded part of the housing around 200F and the male thread somewhat cooler. You could also try heating just the screw head to let it expand and contract.

Threads are sometimes locked with a heat-sensitive compound . . . and just expanding the metal around the screw can help the solvent do its work. Sometimes it helps to try to break the thread loose both clockwise and counterclockwise to get just the slightest bit of initial movement. Take your time.

[MichaelG.]

An excellent start, albeit I'm sure very frustrating.

There are multiple versions of this drive, so it's perhaps worth documenting a few features for ease of reference:

• The [first] troublesome screw is (B) on p20

• Double coarse and fine knobs ... so p21 is relevant

• Ball Slide or Crossed Rollers [?] sorry I can't tell from the photo

• < etc. >

In a reasonably detailed manual, I would expect to see specific reference to Loctite or similar products, if they were used. ... I suspect, therefore, that the screw is simply very tight, or held by corrosion.
Gentle mechanical persuasion [applying pressure alternately in each direction of rotation] using a very close-fitting Allen key, would my starting point. If that fails, then try local heat, or the application of PlusGas.

MichaelG.

[Hobbyst46]

and just expanding the metal around the screw can help the solvent do its work

Likewise, cooling an inner part can cause it to shrink. Although the initial temperature difference upon cooling is much less than upon heating, direct contact of the stuck screw with an ice crystal (dry ice is certainly better, if available) may sometimes help.

[Scarodactyl]

OK, I did try putting a little bit of liquid wrench in the thread and leaving it overnight. 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, this may be normal wear and tear; 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.)

So, I guess once I figure out how to get it loose I'll see if I can have a replacement made. I have had Al Meekins at gearsmade.com recommended, so I will probably try there first.

[MichaelG.]

Great progresss ... and very useful photos for anyone that follows in your footsteps.

Nothing broken there that can't be remade.

It looks rather like Wild was an victim of the deteriorating quality of available hex-socket screws.
The black ones from the likes of Allen, and Unbrako, were [are?] 'toolmaker' quality, but the shiny modern ones are often 'general purpose' items, of lower spec. ... It may be worth replacing all the screws when you rebuild it.

MichaelG.

[MichaelG.]

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:
[...]
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.

May I suggest very careful inspection of that pinion? ... I don't think it's hand-cut, but the quality doesn't look great. If you're getting a replacement made, some improvements could probably be incorporated; to make the repair better-than-new.

MichaelG.
.
P.S. Is it feasible that the pinion has torsionally twisted ??
... when some monkey worked the left & right knobs, trying to 'unlock' the drive ??

[MichaelG.]

I have just tried a quick&dirty crop and straighten on your image, to get some idea of the pinion angle:

Pinion: angled cut, or is it twisted ?

IMG_2012.PNG (474.75 KiB) Viewed 13014 times

To me, it does look abused rather than made that way.

MichaelG

[Scarodactyl]

The screw that stripped kind of smeared around the hex wrench in a way that made me think it wasn't performing to spec. Maybe the same guy who used the loctite replaced the screws. Do you have a line on where I could get some better ones? I suspect I am going to get the same grade of Chinese steel at my local home depot or ace.

It's entirely possible it's been twisted, especially given that the rack has straight teeth. I was thinking hand-cut more because the ends of the cuts didn't look completely uniform in the hand, but looking closer at the pictures it looks more like variable buildup of gunk at the ends.

I got a quote back quickly on the pinion--375 dollars, more than half of what I paid for the entire microscope. I have mixed feelings--I imagine that's reasonable for the work involved, but for that price I will probably just wait for a broken m420 with an intact focus housing to come up for sale--I have seen two on eBay in the past month that sold for a bit less than that.

[MichaelG.]

The screw that stripped kind of smeared around the hex wrench in a way that made me think it wasn't performing to spec. Maybe the same guy who used the loctite replaced the screws. Do you have a line on where I could get some better ones? I suspect I am going to get the same grade of Chinese steel at my local home depot or ace.

Apologies if I have missed it, but I don't know where in the world you are .. [I'm in England].

Happy to investigate suppliers for you; but meanwhile, here's the Unbrako website:
http://www.unbrako.com/socket-screws

MichaelG.

[MichaelG.]

I got a quote back quickly on the pinion--375 dollars, more than half of what I paid for the entire microscope. I have mixed feelings--I imagine that's reasonable for the work involved, but for that price I will probably just wait for a broken m420 with an intact focus housing to come up for sale--I have seen two on eBay in the past month that sold for a bit less than that.

Yes, that is quite possibly reasonable for the work in making a one-off replacement to a higher standard than that original: But it does seem an unpleasantly large number.

When you succeed in removing the basic item, please photograph and measure it ... Between us all, someone might have a bright idea for a substitute. [Meanwhile, some detail of the rack would be useful for reference]

MichaelG.
.

Edit: The original looks like it was made on an automatic machine, which would [once it had been set] produce such items "like shelling peas".
The automatic machines use a 'cold forming' process to make pinions, as illustrated here:
https://www.ernst-grob.com/application/ ... skizze.jpg

[Hobbyst46]

Quoting MichaelG:

P.S. Is it feasible that the pinion has torsionally twisted ??
... when some monkey worked the left & right knobs, trying to 'unlock' the drive ??

Curiousity+fun calculation, with a rod-twisting formula ("Torsion of Circular Section", from Colorado.edu):
Assuming that the pinion shaft is 4" long and of 0.25" diameter, is made of steel, and the focusing knob's diameter is 2",
and the microscopist exerts a moment of 100 lb-in (bare hands or vice-grip) the twist angle would be a ridiculously high 5 degrees.
So, the looks of the pinion can be attributed to such twist.
Please correct if I am wrong...

[MichaelG.]

... the twist angle would be a ridiculously high 5 degrees.
So, the looks of the pinion can be attributed to such twist.
Please correct if I am wrong...

Thanks for adding the calculation. That seems a realistic first-estimate; but I would add that the torsional stiffness of the 'pinion portion' of the shaft will be somewhat lower than that of the full diameter, and also that stresses will be concentrated into that area.

MichaelG.

[Scarodactyl]

A couple weeks back, GearsMade mailed me the knob back, asking me to remove the pinion gear. I had thought it was simply taken apart by unscrewing it with the pin wrench holes, but how wrong I was. I got it mostly taken apart, but the last step has completely defeated me:

I don't know if this is threaded, epoxied or perhaps pressed together, but it's a mess.

So I've had to cancel having it remade (which is a real shame, because I was pretty excited to find out how good they are), and instead I found a focus drive designed for a different microscope entirely on eBay (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.

[75RR]

Glad you have it cobbled it together sufficiently to play with.

looking very fancy in a 70s paneling sort of way.

not sure that is possible ;)

[MicroBob]

Often the causes for broken gears are gummed up guides or stuck mechanisms. So it is always good to adress these problems as soon as they arise.
Nice new stand!

Bob

[Scarodactyl]

I have been having lots and lots of fun with my M420 since I got the basic setup completed, mostly taking photographs of faceted stones which I'd previously had trouble getting consistent results on:

(Color changing tourmaline under halogen and LED lighting)
However, the wooden pole (in spite of its charming appearance) did prevent any work at magnifications much over 1x. I figured out the focus mount was indeed off an Olympus, and as luck would have it I found a used Olympus SZX-ILLD100, one of their brightfield/darkfield stereo stages. Darkfield illumination is pretty vital for examining gem inclusions so I was pretty happy to find one that would work with my scope! I just got it in, and I think the whole setup looks pretty snazzy:

This has resulted in a huge increase in stability and I can go up to max mag without issue now. Unfortunately it only takes a 30w bulb, and I think even that has some sort of problem because the brightfield setting is more of a dull glow, with the darkfield hardly having any visible illumination at all. 30w probably won't cut it even if it were working, though, so I may take this chance to upgrade to LED or fiber optic illumination. There's been some skepticism of using LED lighting for colored stones in the past, but it is still tempting. This ebay user offers a simple retrofit that's ready to go for this particular base: https://www.ebay.com/itm/LED-lamp-house ... 2357353805
Maybe a bit pricier than I want to do, but I'll need to figure something out one way or another.
I may or may not miss the X-Y table that the other base came with, though. I have used it once for photographing a mineral thumbnail. I guess a removeable x-y table is another thing to look for in case I need it.
The other potential downside is that the lens is over the light well but not perfectly centered. I might have to replace the focus mount/head coupler with one that's slightly thicker if that ends up being a problem.
Overall though I'm pretty darned excited. It's a nice heavy base and it compensates in looks for the sad departure of the baseball bat stand.

[75RR]

This ebay user offers a simple retrofit that's ready to go for this particular base: https://www.ebay.com/itm/LED-lamp-house ... 2357353805
Maybe a bit pricier than I want to do, but I'll need to figure something out one way or another.

Amazing what some ebay sellers will try and get away with.

Here is a link to an article on a 'home made' conversion.

http://www.microscopy-uk.org.uk/mag/art ... ersion.pdf

The important takeaways are:
Use a known LED manufacturer that provides documentation
Use a heat sink
Match original filament and LED size*
Precise placing is important.
Use of a constant current driver avoids the flickering of pulse-width modulation controllers.

* Perhaps not as critical in a Stereoscope

[Hobbyst46]

Amazing saga... two notes:

1. Wood in general is undesirable not only because it is edible to termites, but because it expands or shrinks or bends as weather changes, namely when ambient temperature and humidity change. After a day of extreme dry heat, for example, most cabinet doors and wooden drawers at my home get stuck and virtually impossible to open. Some types of wood are probably better in this respect, but for microscopy, the requirements of mechanical stability are even stricter than for drawers...

2. just to add to the important comments by 75RR on the LED conversion (previous response), I would check if the LED is "cold white", neutral, warm white etc. In general, the blue part of the LED light is relatively accentuated, compared to halogen or incandescent lamp light. Filters can modify it, at the expense of intensity. I tried but failed to know the wavelength specification in that listed eBay LED (cf the posted link).

[Scarodactyl]

Sequel!
I picked up an M410 for a decent price with the same issue. Long story short, while it has exactly the same symptom it has a different cause. When I took it apart I found the pinion is actually bent:

I... how?
No idea if that can be fixed, but I'll probably give it a try. If not I have an SMZU focus mount I can put on it no problem.
The M410 is only binocular, and under the hood is a prism with a blacked-off top, not the beamsplitter you'd have in the other M400 series entries. Now to ponder whether it would be feasible to modify it to be trinocular......
.........
.....probably not? But it was cheap enough I might be willing to try.
If I decide to take it on it'll have its own thread.