A lot of the corrosion problem with the zinc-aluminum alloys is because people add lead to them to make them flow better when casting. Then you get intergranular corrosion, and the stuff disintegrates... Especially where it gets damp.
HTH!
Bill in OKC
--- In gingery_machines@yahoogroups.com, Matthew Tinker <mattinker@...> wrote:
>
> David,
>
> A couple of things I don't agree with. The only problem with the Zinc based alloys is corrosion. Unfortunately, this excellent range of alloys has a bad press because it is frequently under- designed to minimise costs and therefore breaks. The ZA 12 is great, 11% Al, 1% Cu, and the remaining 88% Zn, percentages are by weight. This alloy has characteristics similar to cast iron, it is an excellent structural material with good vibration damping. This alloy is also very good for bearing surfaces. To cap it all, it is a "dream to machine"!
>
> Aluminium extrusions are a good source of "pure" Al for making the ZA alloys or to make a harder Al alloy, add 5% of copper. It is surprising, but molten Al is sufficiently corrosive to dissolve the Cu without having to reach it's melting point.
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> WARNING, if you take Zn, Zinc over 700°C, it will "smoke" ie. it reaches it's boiling point and gives off zinc in the form of vapour, this is not to be inhaled! Poison!
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> If you do anything nasty to yourself, I accept no responsibility!
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> Regards, Matthew
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> Matthew TINKER
>
> CNC conversion 1944 Colchester Lathe build-up log
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> http://www.cnczone.com/forums/showthread.php?t=35519
>
> --- On Mon, 28/2/11, David Frantz <websterindustro@...> wrote:
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> From: David Frantz <websterindustro@...>
> Subject: Re: [gingery_machines] Re: New Here
> To: "gingery_machines@yahoogroups.com" <gingery_machines@yahoogroups.com>
> Cc: "gingery_machines@yahoogroups.com" <gingery_machines@yahoogroups.com>
> Date: Monday, 28 February, 2011, 9:32
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> Â
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> Extruded aluminum NO. Look for originally cast Aluminum as you will already have an alloy suitable for casting.
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> As to zinc it is suitable for many machine parts so don't loose any stash you may have. What you should avoid is using it for structural parts of the lathe. Especially parts where vibration or ringing might be a problem. Further don't mix tools used for Aluminum, such as crucibles, with those used for zinc.
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> Zinc alloys should be much easier to cast and frankly easier to find. I would not avoid using it, it is just a matter of selecting the proper places to implement zinc parts. In some places I'd even go so far as to suggest that zinc would be a better choice, for example knobs and handles which could remain bare. It would avoid that nasty aluminum stain on the hands.
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> One other way to use zinc would be to cast a frame or structure and then fill it with epoxy concrete. For a lathe bed this would deaden the frame yet give you metal to bolt things like the ways to. So there is more than one way to skin a cat here. Adaptation is really the word here; Gingery designed a lathe that is ideal for the material, Aluminum in this case, going to zinc just requires a bit of thinking or design effort.
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> David A Frantz
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> websterindustro4at4mac.com
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> Sent from my iPhone.
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> On Feb 27, 2011, at 7:57 PM, james <james122964@...> wrote:
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> > So, it would be better to use the extruded aluminum? or should I hunt around for actual cast aluminum?
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> > I sure wish the die cast stuff would have been worth it, as it melts a lot easier and is heavier, guess I could use it for pulleys.
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> > Jim
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> > --- In gingery_machines@yahoogroups.com, David Frantz <websterindustro@> wrote:
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> >>
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> >> Long ago I worked in the die cast industry so I have some experience with this. First die cast alloys are not the same as zinc based casting alloys designed for sand casting. Second die cast alloys age significantly of time. Third die cast alloys are sensitive to the ratios of metals which means it is easy to get fining results if the alloy varies from the accepted ratios much.
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> >>
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> >> Given that you wouldn't have any problems pouring a Zinc die cast alloy. The low melting points mean that you can get by with a cheaper furnace. Sources for the alloys where at one time cheap and plentiful (carburetor and other auto parts). Zinc die castings can have initial strengths equal to similar cast iron parts. So there are good points to consider with respect to Zinc castings.
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> >>
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> >> The bad points are also worth considering. One in this context is that a Zinc casting might not be as dead as a cast-iron or aluminum one. That is you may see much more in the way of ringing and vibration transmission. As mentioned above the alloys do age over time which might result in a need to re scrape parts overtime. Actually all cast metals have an aging factor it is just that some Zinc alloys can age more than others, plus this is in the context of die casting.
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> >>
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> >> In the end I would NOT recommend zinc die cast alloys for most parts of a lathe. I just think you would get far better results from other materials for most of the structure.
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> >> On the granite:
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> >> Granite counter top material can pass as A surface plate. You should check the flatness your self and make sure it is well supported to prevent bending under use. Simply due to the thickness it ought to be a better starting point for a surface plate than ceramic tile or other things that are often suggested. In any event realize that a surface plate requires maintenance just like every other tool.
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> >> David A Frantz
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> >> websterindustro4at4mac.com
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