< Expansion Links>
The expansion links are considered one of
the most difficult parts of the valve gear.
Traditional method to cut the arc shape is
mounting a blank on the lathe's pace plate
against a parting tool and turning by hand.
But I employeed milling machine and rotary
table to complete these troublesome items
more easily and quickly.
The die block is almost a square block but
has two curved surfaces along the expansion
link groove. A phosphoric bronze round bar
was turned and parted off to a disc which
was then temporary soft soldered to a brass
sheet, milled, drilled and reamed on the
rotary table.
A bright mild steel blank for the expansion
link is bolted to a jig on the rotary table
in a correct distance from the center, then
drilled a series of holes for rough shaping.
Incidentally I used a steel pin threaded
at both end for clamping the blank so as
to ensure accurate positioning.
Mount the blank again and the inner and outer
arcs are milled. Then the holes for the trunnion
pin and the eccentric rod connection are
drilled and reamed. Finally at the stage,
recess of side window is slot drilled.
With a rotary table we can cut straight after
curved line or curved after straight line
in a time. Also with Y-axis offset, the two
lines can be connected with desired angle.
The drawing shows how to cut such a shape
with Y-axis offset. The light blue circles
show locus of the end mill.
Reversing the job and milling the groove.
The width of the groove is determined using
the die block as a gauge.
Centering the hole for the eccentric connection,
the round shape in the lower part is milled.
After all of the cutting work, I case hardened
the groove with "Kasenit".
The trunnion pin is made of silver steel
and press fitted with a bench drill. To maintain
toughness, it is better silver solder here
but it should be careful not to disturb the
groove surface. After heating for soldering,
quench the job into cold water for hardening.
Cleaning the job with oil stone for flat
surfaces and rolled emery cloth for curved
surfaces.
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