< 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.