"Catherine", a USA "Sweet Pea" engine build

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ironhorseriley
Posts: 192
Joined: Fri Apr 05, 2013 12:02 pm
Location: Rogers, AR

Re: "Catherine", a USA "Sweet Pea" engine build

Post by ironhorseriley » Mon Apr 03, 2017 7:34 pm

Great to see you back at it & that you survived the move! I am encouraged by your ways of getting the "bugs" out.
Jim, Former railroader, fascinated by wood working & “all things engineered”.

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Benjamin Maggi
Posts: 1065
Joined: Wed Apr 25, 2007 10:38 pm
Location: Albany, NY

Re: "Catherine", a USA "Sweet Pea" engine build

Post by Benjamin Maggi » Mon Oct 23, 2017 12:52 pm

Because the wheel/axle box assemblies are loose in the horns (the suspension springs are not installed yet), the wheels slid around and trying to measure the proper length the rods should be was tough. So, I kept the frame vertical on the floor which at least forced the axle boxes into the full “rear” position due to gravity. I used a long yardstick (which isn’t nearly accurate enough for measuring the rod dimensions, but since I hadn’t blocked the wheels and this was going to be a rough process anyway I just proceeded) and measured the axle-to-axle center spacing as the exactly 13+5/16” as called for in the plans.

I took a piece of the aluminum stock I bought and laid out holes exactly 13+5/16” apart. Being frugal, I oriented them on the aluminum bar so that if I could fit another pair of holes on the same stock if I needed to. Then, I drilled the holes in steps up to the 3/4" which is the diameter of the crankpins. The two bars were slipped on either side of the engine and I tried to rotate the wheels but they bound up. Clearly, they were either a little too long or a little too short. So, I cut each rod in half and then drilled/tapped the ends with a 5/8-16 tap. Some steel plates were then milled 5/16” to allow for slots and tightening bolts. I took one of the rods to Home Depot to size up some hardware and unfortunately must have left it there, because I couldn’t find it anywhere. I called and they didn’t have it, so I had to remake it.
Making adjustable rods.jpg
Making adjustable rods.jpg (41.7 KiB) Viewed 347 times
Then, the rods were loosely bolted together and installed on the engine’s crank pins (which were for right now just essentially 3/4" diameter extensions from the wheels. Little by little, the wheels were rotated and the bolts adjusted and tightened until the wheels ran free with the rods in place. When we switched them around, they still rolled well. So, I tightened the bolts really well, tested the rods one more time to make sure I didn’t screw them up, and set them aside. It was, frankly, amazing that we hit the perfect length without so much as a measurement.
Adjustable rods.jpg
Adjustable rods.jpg (38.16 KiB) Viewed 347 times
"One cannot learn to swim without getting his feet wet." - Benjamin Maggi
- Building: 7.25" gauge "Sweet Pea" named "Catherine"

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Benjamin Maggi
Posts: 1065
Joined: Wed Apr 25, 2007 10:38 pm
Location: Albany, NY

Re: "Catherine", a USA "Sweet Pea" engine build

Post by Benjamin Maggi » Mon Oct 23, 2017 1:05 pm

I have been pondering the crank pins for several years. The Sweet Pea designed called for marine style drive and connecting rods with the ends separate pieces that would bolt around the pins, which were shaped sort of like mushrooms. Thus, any incorrect measure of the rod length could be corrected by filing or shimming the rods. I didn't particularly care for the appearance of the round cross-section marine rods and wanted to build conventional, one-piece rods. These would slip over the crank pins and somehow be retained. This meant that I had to redesign the crank pins. The community here provided me a wealth of information (http://www.chaski.org/homemachinist/vie ... =8&t=95312). After looking at crank pins used for Allen Models' mogul, Little Engines' 0-6-0 switcher, and Kozo's A3, I decided to go with the LE design.

However, the passage of time has led me to instead go back to the original design of a three-piece rod and a mushroom end on the crank pin. Not only will be rods be more prototypical, but they should be easier to adjust for wear down the road.

There have been quite a few threads on the topic of what metal makes the best crank pins, and after doing some research online here, I ordered two feet of 4140 hardened steel rod from McMaster-Carr. http://www.chaski.org/homemachinist/vie ... hp?t=96360 This is more than I will need, but I have a track record of making imperfect parts.

I adjusted the specified measurements of the crank pins by allowing for extra thickness on the first flange of the pin where it fits into the wheel. Since the valve gear was shimmed out from the frame ¼,” I added ¼” to the crank pin dimensions. Once I have a more concrete numbers for the valve gear clearances, I can always turn away the extra part of the flange (on the inside portion) which will move the areas of the crank pins where the rods will ride in. Of course, that will also require shortening the crank pin or else it will stick out the back of the wheel. Anyway, I made that portion longer than necessary at this point too because it is easier to remove metal than add it back.

One of the things I decided upon early on was that the crankpins were going to be secured with Loctite instead of press fitting. If properly made to the correct tolerances, they should hold up well and not work loose. Key to that is accurately measuring the crank pin holes in the drivers.
Measuring Crank Pin holes.jpg
Measuring Crank Pin holes.jpg (37.49 KiB) Viewed 344 times
One tool that was especially useful was a round cutting tool. This allowed me to carve out the portions of the wheels where the rod bearings will ride and also smooth over the edges nicely. I guess it could be done with a round file, but that would take a bit more work. I also used various grits of emery tape (80, 140, 220) to get a nice smoother surface on everything.
Turning crank pins.jpg
Turning crank pins.jpg (33.45 KiB) Viewed 344 times
The ends of the leading crank pins were rounded over with a file to give them a nice smooth appearance. The ends of the driving crank pins have a shoulder turned to accept the linkage of the valve gear, which clamps on it. This not only functions as part of the valve gear but it also keeps the rod from falling off the crank pin. I don’t know yet if I will need to drill through the crank pin, but I likely will have to.

With a bit of luck and carefully thinking through everything before doing it, I managed to make all four crank pins on my first attempt. (Well, my "first" attempt at making crank pins to the original plans). The four in the front are the "keepers", but the ones in the back are my previous attempts.
Lots of crank pins.jpg
Lots of crank pins.jpg (30.68 KiB) Viewed 344 times
I won't secure the crank pins to the wheels until the rods are done, in case something else must be modified.
"One cannot learn to swim without getting his feet wet." - Benjamin Maggi
- Building: 7.25" gauge "Sweet Pea" named "Catherine"

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