I never heard of one, and do not understand why Jacobs would make it that way. After a Google search, I found 2 pics. I guess the spiral could pull the key in, making it take less pressure to tighten the chuck. Just a guess on my part.
Generally speaking, spiral gearing is used to lessen the pressure on any one tooth in the gear, the spiral allows more than one tooth to share the load. I have never seen an unworn high quality straight tooth chuck key tend to disengage while tightening it.
Those keys aren't available from Jacobs & unfortunately they are very hard to find. The 12S & 14S chucks pop up on ebay from time to time but seem to rarely sell if they are missing the keys.
got it moving haven't tried to drill yet,motor is faster than what came off,leather belt broke as shortly after start up,made a belt from car serpentine looks like it works pretty good
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