Wild M20 Repair of Microscope Stage with Plastic Gear Deutsch

subtitle: handicraft

Tolerances 0.1 mm or better.

Spare parts are not available for years.
Plastic gets brittle with time. So gears tend to loose teeth. Anyway, the combination of brass with thermoplastic needs some explanation. After some time, cogs break off and and get stuck behind the wheel, blocking it completely. Cleaning and grease does not help.
The clearance between the head of the cogs and the aluminum case is 0.57 mm, where as the cogs are 0.78 mm.
Another cause may a wrong conclusion. As several axles can be loosened or tightened by holding one end and turning the other end (turning the internal screw), such a try with this axle will break the cogs off, because it is not an axle, but a double gearing.

Preparation:

Get spare parts : e.g. MG 717N, 4 pieces, at www.nozag.ch. (MG stands for "Messing" = brass)
The brass cog wheel (module 0.7, 17 cogs) of Nozag has a length of 10 mm (cogs 4mm and the neck 6 mm).
Take the stage apart. Turn all screws out and remove 3 pins. Two pins are fixing the plastic wheels. If the plastic breaks don’t mind, it is no longer needed. The point of the inner axle is used as a point bearing, so take care. Friction is enhanced in the leadscrew, so it is a good idea, to rinse it with lamp oil. Re-greasing is suspended until reassembling. Unpin the fixed hand wheel for sagittal movement. The hand wheel for lateral movement (the handle with the blank brass cog wheel) is fixed with a squeezable nut. Loosen the tiny screw and take the big round nut off. Take care of the washers.

Changes on the leadscrew assembly (driven wheel):

Leitspindelrad
Original plastic wheelmodified with brass cog wheel
Fix the wheel in a electric drill and remove the plastic cog wheel with the teeth of combination cutting pliers. (This part is easy, because plastic melts down.) Turn the remaining steel tube to a diameter of 7 mm with a file and polish it. The diameter of the hole in the brass wheel has to be the same 7 mm. The widening is done by hand drill, small needle-files and grinding (and patience). The use of a lathe is recommended. Correct cutting angle for brass is about 90°. Now shorten the wheel at the hub end to a length of 8 mm (2mm for Nozag wheel) to match the cog wheels lateraly.
This is a compensation of new to old, because new cogs have a width of 4 mm instead old 6 mm.
This cog wheel is fixed with a pin on the axle. Work is easier if the hole for the bolt is first drilled only on one side. Then put the cog wheel on the hollow axle and use the holes as guide.
If the helical cogs are replaced by normal cogs, the lateral pressure of the leading spindle is not compensated. So the friction in the point bearing is enhanced. This compensation is needed for fast moving parts and is almost useless within slow moving gears with the play of static friction and kinetic friction together with lubricants.

Driving axle

driving axle
Original hand wheelmodified hand wheel
Original cog wheel: Brass, chromium plated, 33 teeth, helix angle 15°, Module 0.3486.
Take off the cog wheel with a fret saw, smoothen the cut with a file and grinding paper until the cut is vertical to the axis. The Nozag cog wheel has to have a hole of 8 mm diameter. Use a lathe. As I have none, I used drills, router, files, and scraper (and lot of patience). Now the hub is cut so, that the old length is almost reached again (leave some space for solder).
The new cog wheel should have a length of 6 mm + thickness of fret saw blade and reshaping losses - thickness of solder (for Nozag wheel cut off some 3 mm).
The new wheel is fastened with soft soldering. Tricky is the exact alignment of axis. A stem of dry reed of 8 mm diameter helps. Both parts are stuck onto the reed, some electronic soldering is melted on the cut and the assembly is heated (with an industrial dryer) until the cog wheel is falling on the hand wheel. Solder has about 0.1 mm thickness. After cooling clean the hole and grind it on the axle until it turns easy. Fine valve grinding paste (grease with carborundum) was used.
Reed was chosen because of: a) there were left over’s of a bee’s hotel. (Bamboo would do also) b) material needed must resist for some time to the temperature of liquid solder, further it has to be flexible and straight. Bad heat conductance helps also and it has to be easy to remove.

Pins

The pins (bolts) are so soft, that they were not fit for reuse. They were replaced by wire nails of 1.6 mm diameter. Cut the edge, grind it, conify the nail, put the pin in, cut it and flatten the top. The edge is cut, because there should be a chance to take it out again.

Modified gear after correction

modified gear

Sketch of new gear

sketch of modified gear
The grey shaded area shows the tolerances.
The bearing of the concentric double axle is fixed with two screws with spring washers. It can be adjusted horizontally for the axle distance of the new cog wheels and vertically for the grip into the rack. So adjust it after assemblage and before adding the brass plates, which are fixed with 4 tiny screws.

Centric drilling of holes is easy with a lathe. If done manually, wheels tend to wobble, because the center was lost. So work stepwise and use milling cutters instead of simple drills. Electric drills are designed for wood with high speed. Brass needs low speed and another cutting angle. (Some 90°, i.e. scrapping instead of cutting). Brass is used, because of soldering and it is strong enough.
Further cog wheels which would match with the M20 stage (brass, ≥4 mm wide cogs and total length ≥8mm)
Module 1 is not applicable, see table
Module 0.5 24 or 25 cogs (teeth)
Module 0.4: 30 or 31 cogs
Module 0.3: 39, 40 or 41 cogs

Used tools

Beside of a lathe, all tools are already in a hobby work shop. vise, fret-saw with fine blade (any blade will work, because the chromium surface is very thin), file, small hammer, wire nails 1, 1.5 and 1.6 mm diameter, drill, combination pliers (all purpose wrench), grinding paper, soldering iron, solder, screwdrivers (watch maker set), 7 mm and 8 mm drill (milling cutter, if no lathe available) electric drill (with low gear), needle-files, valve grinding grease, industrial heater (300°C, 1kW), petroleum (lamp oil) and grease, some reed shafts (left over’s of a bee’s hotel)

Remarks

The easiest would have been to replace the plastic by brass. But if you really want to work properly, you have to start in making the cutters for the helical brass wheel, because even those module 0.3486 cutters are not available. (I guess, that this was some easy available cog combination on Wild's lathe.)
As point bearing was not changed, which is crucial to the tolerances, the precision is still the same. Full functionality is re-established for years. If the new gear starts to squeeze, clean it, rinse it and grease it (most friction is caused by dust in the leadscrew).
Nozag was choosen, because it's for me just around the corner and because they have a catalogue in the internet. The choice of cog wheels is restricted to those, which are not common, so they are not cheap.
 

Possible spare cog wheels

Following cog wheels meet:
Distance between axles ≥ 11.7 and ≤12.61 millimeter (diameter of dividing circle) and head radius ≤ 6.93 mm and foot diameter ≥ 10mm.
Cogs width ≥ 4 mm, length of wheel (inclusive hub) ≥8 mm, hub (neck) diameter ≥9.5 mm (best 10.4 mm).
spare
part
Nozag
Original
Wild
wrong
Module 0.3 0.3 0.3 0.4 0.4 0.5 0.5 0.7 .3486 0.8 1 Modul
Cogs 39 40 41 30 31 24 25 17 33 15 11 Zähne
dividing Ø 11.7 12.0 12.3 12.0 12.4 12.0 12.5 11.9 11.5 12.0 11.0 Teilkreis Durchm. Ø mm
head Ø 12.3 12.6 12.9 12.8 13.2 13.0 13.5 13.3 12.63 13.6 13.0 Kopfkreis Ø mm
foot Ø ≈ 10.9 11.2 11.5 11.0 11.4 10.7 11.2 10.0 10.62 10.0 8.5 Fusskreis Ø mm
thickness cog 0.459 0.459 0.459 0.613 0.613 0.766 0.766 1.072 .553 1.225 1.532 Zahndicke mm
between cogs 0.483 0.483 0.483 0.644 0.644 0.805 0.805 1.127 .581 1.288 1.610 Zahnlücke mm
helix angle° 0 0 0 0 0 0 0 0 15 0 0 Schrägungswinkel °

left overs (600dpi scan)

old helical cog wheel
 

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