Chapter 17.12 – Return CranksThis week I worked on the Return Cranks. These are the little arms that clamp onto the crankpin on the rear wheels and provide an out-of-phase motion that is contrary to the motion of the crosshead bracket. These two motions are combined through the combination lever to produce the final motion for the steam valves.
The return cranks were made together, end to end, out of a length of 1/4" x 1/2" 1018 bar. After thinning the bar to 0.200” I drilled and reamed four 1/8” holes, two in each part. The outside holes are used to hold the part on the jig for many of the machining steps but will later be enlarged for the crank pin. The inside holes are the ones that will connect to the radius rod via a 1/8” pin.
Using the outer holes, I mounted it on a jig and used a 1/8” end mill to define the edges of the 1/2" diameter round section that is centered around the eventual crankpin connection.
Then I milled the outside ends flat to the correct width. This will be where the screws go that will clamp the part onto the crankpins.
Next, I moved the part into a second hole in the jig that provided a ~5
o angle so I could cut the angled sides of the crank. This shows just after the first edge has been angled. Note that the angle only goes to the center of the piece because there are TWO parts here.
Flipping it around and turning the piece over I cut the opposite side angle. These two angles meet in the middle.
Then I completed the same two operations on the other side. Here are all four angled sides completed. You can begin to see the shape of the return cranks.
Moving the piece back to the horizontal position on the jig, I mounted the jig vertically and used a 1/8” end mill to cut a recess on one side. This covers both parts of course.
And while the part is upright and clocked in, I drilled #49 holes through both ends. This is not the right size for the #3-48 screws, but it is the right size to hold my filing buttons, and it is small enough that it doesn’t mess up the #3-48 holes. We’ll widen these holes later.
Next, I used 1/2" filing buttons to round the area around the crankpin holes.
Then I filed the ends round, again using filing buttons. These filing buttons are held with a #1 screw, which is why I used the #49 here earlier.
Now that I’ve rounded the ends I can go ahead and drill the end holes to their final size. They were first drilled through for #3-48 tapping, then I drilled halfway through with a 3-48 clearance hole. After this, I tapped the other half of the hole.
And then I widened the crankpin holes and reamed them to 7/32” to match the crankpins.
The last major operation here is to cut a 1/32” slit on the end of the crank to allow the 3-48 screw to actually clamp the part onto the pin
With all major operations complete it’s now time to cut the piece in half on the bandsaw and use 9/32” filing buttons to round the ends of the crankpins.
And here are the two completed Return Cranks!
That was a lot more work than it had any right to be, but I’m pleased with how they look. And yes, they do in fact, tighten nicely onto the crankpins, I checked! But before I can really assemble all of these pieces I’ve got a bunch of little pins to make. So that’s what I’ll be doing next.
Thanks for looking in!
Kim
