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Projects ... A3 Switcher    Pistons, Crossheads and Guide Yoke Assembly 10/8/2010, 11/24/2010


This page covers machining the Pistons, Crossheads and Guide Yoke Assembly ... Section 15.0 & 16.0 pages 93-100

For the update on the Guide see below ....


The next two pieces really needed to be done carefully ... a lot of critical dimensions. I had to scrap a few pistons and crossheads, but I kept at it till finished. For the Piston the gland diameter for the O-ring was the hard part ... had to use two known pin sizes to get the inside diameter, used a micrometer rather than the calipers. See picture below. The Crossheads ... a lot of cutting and drilling where any mistake could (and did) result in a scrap. And of course then there is the tapping 0-80 holes, I would recommend using 1-72s just a bit larger (plenty of room though) but much less prone to snapping off.


Each picture below is a thumbnail so it can be enlarged to get a better view ... back arrow to return.

 
Here's the raw material ... stainless steal and 936 Bronze. Got these from McMaster Carr. Some time later ... the parts.
  The blank has been machine to the given dimensions for two crossheads. Here I'm measuring to be sure that the gap for the Main Rod will be .130". Since the Main Rod has a width of .125" ... there's not much room for error.

Ready for the plugs to be silver solder in place. There is a .003" gap for the SS to flow into ... should be enough.
  Notice that the plugs are little higher than the top surface. This allowed me plenty of room to get the .360" dimension right on. Next operation is cleaning up and milling to dimensions. Then cut in half.
 
Here is the jig I used to cut the 8 "wings tips" and the 1/4" radius. To get the 8 I used a 3 and a 5 angle blocks. Press them against the vice edge to get a snug fit. Used a cad program to locate the xy coordinates ... drilled then just moved the x axis. Done! Well had to do that 8 times...

  Now for that 45 cut ... which is actually a compound cut ... the 45 and the 8. Again used the same jig ... set the 8, tighten the nut then set the 45 and made a very light cut for the groove.
 
Getting close to finishing ... the slipper grooves were next ...set the endmill in the center took light cuts ... flipped it over and repeated until the dimension was .703". Notice the crosshead in the background ... need to keep the vice jaws straight.
  I wanted to use Bronze for the slipper, so I made this jig so that the thickness would be .040" and flat. To do that I faced a piece of round stock, cut it off for a thickness of about .060", milled the edges down for a tight groove fit . Then milled the top surface as shown. Finally sanded the edges for the final .235" dimension.

 
Final product ... need to pin the piston and drill and tap the holes for the Link Bracket. Kozo's New Shay he uses Loctite on the Pin and Rod ... interesting will have to look into that. I wonder if that helps keep the steam from leaking thru the gaps.
  Another look ... the piston is a little tight so I may take a tad off the piston diameter and a little off the cylinder walls when honing. If I drop the piston in the cylinder there is no binding. Only when the rod is attached ... so I guess I have a concentric issue that needs attention.
  Looking ahead I see that I will soon
 be running on air!
Measuring the gland diameter to get the O-ring squeeze between 5% and 10%. My final measurement is .8926 - .2050 = .6876 which gives about a 6.6% squeeze. Of course that assume that the O-ring meets its specs!

   
                                             Update on the Guide Yoke Assembly (Tie Plate, Guide Yokes and Guides ) 11/24/2010

These parts were actually easy to machine, of course with the help of CNC the Tie Plate and Guide Yokes were a piece of cake. CNC is getting easier with each part. It was the assembling the parts together that was the real challenge. The Crossheads and Guides are suppose to slide smoothly over the full stroke.

Wow .... with only a margin of .0025"? Considering that there are so many parts that are assembled together ... that wasn't going to happen. Not at least without a few minor adjustments.

I ended up with Guide width of about .231" vs .235" and a Crosshead groove width of about .244" vs .240". After these changes the parts would slide somewhat smoothly. I still may need to make additional adjustments when I run on air. One side was much smoother then the other ... so far I haven't found the part(s) that are causing the problem.



 
As you see this is the fixture I used to CNC the Guides Yokes. I put in a command to stop the process right after drilling the holes so that I could insert the screws. I used a 1/8" Carbide endmill ... took 6 trips. Total machine time about 15 minutes. Did the Tie Plate the same ... machine time 3 minutes or so.   Back to manual milling. The silver soldering was simple and straight forward. Cutting the gap was done very slowly with many checks.


 
Guide Yoke Assembly..... If you look closely I added an extra recess area for the Drive Wheels.   The Guide assembly ...
That's it for now...
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