63. The position
of the guard pin should be as close to the bottom of the slot of the
fork as possible and be safe. As to the size of the guard pin, it is
usual to make it about one-third or half the diameter of the jewel pin.
The size and position of the guard pin decided on and the small circle
_N_ drawn, to define the size and position of the roller we set our
dividers so that a circle drawn from the center _A_ will just touch the
edge of the small circle _N_, and thus define the outer boundary of our
roller, or roller table, as it is frequently called.
[Illustration: Fig. 61]
[Illustration: Fig. 62]
For deciding the angular extent of the passing hollow we have no fixed
rule, but if we make it to occupy about half more angular extent on the
circle _y_ than will coincide with the angular extent of the jewel pin,
it will be perfectly safe and effectual. We previously stated that the
jewel pin should occupy about twelve degrees of angular extent on the
circle _c_, and if we make the passing hollow occupy eighteen degrees
(which is one and a half the angular extent of the jewel pin) it will do
nicely.
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