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THE KEY QUESTIONS

G J M

Registered User
Mar 2, 2018
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What is the purpose of the winding arbor on fusee movements being tapered? I have done loads of reading and haven't seen anything suggesting that they require a special type key so will any key that fits tight on part of the arbor work? Wouldn't it only be gripping a small amount of the arbor and have room for a lot of end play.
Thank you. Gary
 

Alex K

Registered User
Jun 20, 2020
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I think answer is very simple. This is bacause of making square hole procedure. So if it does not go through then the hole in the key will be of tappered form. Thats why corresponding arbor form should repeat this conus to fit better. In short words - this is technology limitation at that time (not only fusee clocks had that but all others as well).

Broaching-A-Square-Hole.png
 

Christopher Lloyd Owen

Registered User
Aug 27, 2020
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I've read about this somewhere, but cannot recall where. The taper prevents the key going too far onto the arbor. Key fit is essential as you say, because too small is ineffective and too large is useless and can damage the square. When you measure for a key you must of course measure the outermost point (with a micrometer preferably) and buy a size up. I seem to remember reading that a key should go onto the arbor a length at least twice the width of the key hole. I use my spider keys or pocket watch key to check. You just know when it's right don't you?!
 

G J M

Registered User
Mar 2, 2018
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Thank you both for your response.
I didn't realize that the key didn't start out with a through hole.
After doing more research I have answered my own question so it appears a key specifically for use on a fusee has to be used.
Tank you. Take care. G