The
hole for this must be drilled quite centrally in the wheel; otherwise the
top will be badly balanced, and vibrate at high speeds. For the same
reason, the spindle requires to be accurately pointed.
The steam ports are next drilled in the top of the boiler. Three of them
should be equally spaced (120 degrees apart) on a circle of 1-inch radius
drawn about the spindle poppet as centre. The holes must be as small as
possible--1/40 to 1/50 inch--and inclined at an angle of not more than
45 degrees to the top plate. The best drills for the purpose are tiny Morse
twists, sold at from 2d. to 3d. each, held in a pin vice rotated by the
fingers. The points for drilling should be marked with a punch, to give the
drills a hold. Commence drilling almost vertically, and as the drill enters
tilt it gradually over till the correct angle is attained.
If a little extra trouble is not objected to, a better job will be made of
this operation if three little bits of brass, filed to a triangular section
(Fig. 76 a), are soldered to the top plate at the proper places, so that
the drilling can be done squarely to one face and a perfectly clear hole
obtained.
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