Archived posting to the Leica Users Group, 1999/07/22
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The M6 (and later M4-P) top-plate made from zinc was an economy-move by Midland and later by Wetzlar/Solms. By using centrifugal casting processes they could make a lightweight and strong top-plate that needed very little "post" casting machining and polishing. Zinc also takes fairly well (note the "fairly" wording) to black chrome and chrome plating. I saw some of the first zinc top-plates and also some zinc cast baseplates in 1983 at Midland. The engineer that showed them to me exclaimed, "Ah, Zinc iz wunderbar!" - There were problems with the early tops for the R-series, the zinc would react with the chrome and start bubbling and they changed the composition of the zinc and also changed the process for plating. The old process of making brass top-plates required a very complex set of dies, 6 of them, and each costing more than a top of the line Mercedes (their price-guide!). The process they used, and still use, is a vacuum draw die stamping. The brass is heated to a high temperature and thus made more 'malleable" and then sucked into the appropriate die and shaped, then moved down the line to the next die and the process repeated until the top-plate was finished. Time consuming and costly. The zinc casting is an automated process, molten zinc alloy is poured into the die, spun at high speed and the centrifugal force exerted removes bubbles and uneven spots. It is not a bad system and I looked into it when I started making the Rapidwinder housing. The cost of the die is very expensive and the process is not cheap either. To give you an ides of the cost involved, I wanted to make the plastic roller on the winder from cast alloy, the cost for the die was quoted at $22,000 and the cost per unit at $5 (and on top of that comes cleaning off casting edges and anodising). Today Leica could make this top-plate by CNC and in more suitable alloys, even in alloyed titanium or a stainless steel compound. The cost per unit would be reasonable; probably less than $100/piece in the volumes they produce cameras. The Zinc is not bad; it absorbs and dissipates impact fairly well. If you hit it really hard it can crack, but a brass housing hit with the same force, would have collapsed and crushed the finder assembly and metering circuitry. The biggest risk with using zinc is if you manage to scratch through the coating (chrome/black chrome. nickel and acid copper sealer) into the zinc itself. If you expose the "wound" to very salty air (sea spray, East Coast salted road slush) you can create a galvanic effect and the zinc will turn into a white powder and the plating will lift off. This being said, I have never had that happen to any of my M6's or late M4-P's, and they have not been babied over the years either and I do live less than 150ft/50meters from the Pacific Ocean. Evidently the M6 TTL will have a brass top-plate again and in spite of the zinc alloy's improved impact features I like that. Somehow having a white/grey surface show up after a couple of 1000 rolls in your camera doesn't look right. Well-worn Leicas show brass on the edges! Oh, they abandoned the zinc baseplate idea. It was too difficult to make the "hinge" where the plate attaches to the camera strong enough and after use it started to crack. Brass is more "flexible" and could take the on/off and twisting motion. Another reason why I abandoned zinc for the Rapidwinder housing. The alloy I use is much more flexible and also much stronger than the zinc alloy (and considerably more expensive too!) and can be CNC-machined to very tight tolerances. Of course when Leica goes back to the brass top-plate, we can then strip the chrome plating off and paint them black and we will then have our brassy M's again, only this time with meters in them! Tom A