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Source for very small platinum points material?

Cismontguy

Aluminum
Joined
Feb 28, 2007
Location
Earlysville, VA
We are rebuilding an 1918 hand cranked magneto. One of the parts that is missing is a single platinum point, contact that is 3/32" in diameter and probably only 3/32" tall, or deep. The tip of the screw driver is pointing to the hole where the points should go. Very small!

Does anyone have any ideas of a suitable source for a new point that we can make? (there is only one missing) I have never machined platinum so I have no idea? Can you buy a short bit of platinum rod somewhere? Please note, this is a one off repair and so needs to be reasonable in price and not involve manufacturing all new points for 10,000 magnetos ;).

Thanks,

Cismontguy
 

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"...hand cranked magneto..." Old telephone, physics lab demo or medical "treatment" device?

Even if the 1918 part used platinum, does the repair part have to be platinum? Silver is likely to look and function as well, assuming the device is more for display and demonstration than everyday use. Silver is a lot easier to get, either from old coins or from a jewelry maker. Silver is easy to cast and sterling wire is easy to get and machine. Platinum is difficult to melt. Dental labs that make porcelain crowns will probably be able to work platinum.

I once needed an ignition contact point and just went to the auto supply store and bought a replacement point set for some automobile and transplanted the points. I suppose the points were tungsten, which obviously used to last for thousands of miles in cars, even if they are now obsolete for new vehicles.

Larry
 
There not normally platinum, but tungsten or silver depending on age. Platinum is just too damn expensive!!
 
Platinum points are usually not pure platinum. It is often an alloy that can contain one or more of Gold, Indium, Nickel, Palladium, Platinum, Ruthenium, Silver,Tungsten or Iridium. One can buy those, the term is "Electrical Contact Rivet". Many years ago one could replace just the contacts on the car ignition points - this is before the introduction of the magnetic pickup in the distributor.
 
Platinum points are usually not pure platinum. It is often an alloy that can contain one or more of Gold, Indium, Nickel, Palladium, Platinum, Ruthenium, Silver,Tungsten or Iridium. One can buy those, the term is "Electrical Contact Rivet". Many years ago one could replace just the contacts on the car ignition points - this is before the introduction of the magnetic pickup in the distributor.


Thanks! that is the search term I did not know! "Electrical Contact Rivets". I will see what we can find? Hopefully someone out there still sells them?

Cismontguy
 
Platinum points are usually not pure platinum. It is often an alloy...
As a correction to this, "Pt" points are never pure Pt. The pure element is too soft (as is Ag, suggested by someone else) and quickly would be hammered out of shape in operation in anything but an essentially static application. As for contact rivets, by far the most common use of points these days is in relays that operate infrequently and have low contact pressure so most of these use materials that wouldn't stand up to the hammering they would receive in an automotive application.

The reason early magnetos used "Pt" points was the belief that it had lower resistance and thus the magneto would work better. This turns out not to be the case and only in humid environments and where the magneto would sit unused for months or years would it have a possible advantage over W in not slightly oxidizing. However, for automotive applications this is more than balanced by the fact that Pt readily forms a brittle carbide in the presence of hydrocarbons which then results in rapid erosion of the points in operation.

TIG electrodes were mentioned in an earlier post. They're what I use to make replacement points for uncommon magnetos, but be sure to use the ones marked with green, i.e. pure W. Small amounts of Ce, Th, etc. are alloyed in the other ones in order to lower the work function to make it easier to emit electrons, i.e. to create an arc. With magneto points you want it difficult, not easy, for the points to arc.
 
As a correction to this, "Pt" points are never pure Pt. The pure element is too soft (as is Ag, suggested by someone else) and quickly would be hammered out of shape in operation in anything but an essentially static application. As for contact rivets, by far the most common use of points these days is in relays that operate infrequently and have low contact pressure so most of these use materials that wouldn't stand up to the hammering they would receive in an automotive application.

The reason early magnetos used "Pt" points was the belief that it had lower resistance and thus the magneto would work better. This turns out not to be the case and only in humid environments and where the magneto would sit unused for months or years would it have a possible advantage over W in not slightly oxidizing. However, for automotive applications this is more than balanced by the fact that Pt readily forms a brittle carbide in the presence of hydrocarbons which then results in rapid erosion of the points in operation.

TIG electrodes were mentioned in an earlier post. They're what I use to make replacement points for uncommon magnetos, but be sure to use the ones marked with green, i.e. pure W. Small amounts of Ce, Th, etc. are alloyed in the other ones in order to lower the work function to make it easier to emit electrons, i.e. to create an arc. With magneto points you want it difficult, not easy, for the points to arc.

The only problem with tungsten is a rather higher electrical resistance. For electrical application I prefer to use a tungsten-copper alloy (really a pseudo-alloy). there is a wide selection of sizes available in this material.
 
The only problem with tungsten is a rather higher electrical resistance. For electrical application I prefer to use a tungsten-copper alloy (really a pseudo-alloy). there is a wide selection of sizes available in this material.
While the resistivity of tungsten is ~3x higher than copper, the total resistance of a set of points made of either material would be negligible (assuming pure copper was hard enough to stand up to the beating, which it isn't). If the points were, say, 1/8" dia. x 1/8" long the total resistance of tungsten points would be less than 50 micro-Ohms while pure copper would be 1/3 of that (W-Cu alloys would be between those two). However, since the total resistance of the primary of the magneto is at least 1000x larger, i.e. 0.5-1 Ohm, it would make no difference even if pure copper were used for the points.
 








 
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