Brassy-2

Completed 9/19/2006

All of my steam engines so far have been only partially planned -- that is, I'd do a concept sketch, and design parts as I went along. For that reason, I never knew precisely what the parts would look like, how big they'd be, or what material they would be made of until I had the finished engine. Then I'd draw as-built plans for posting on this website. So Brassy-2 is a departure from my usual modus operande. This time I did the drawings first, and built according to those drawings. And wha-da-yah-no? It worked!

I call this engine Brassy-2 because it's somewhat like one of my early engines, Brassy Babe. Its base and frame are two pieces instead of one, and it has a half-inch bore and stroke -- twice the bore of Brassy Babe. I'll describe these items in the same order as I made them. A link to the drawings is at the end of this file.

Frame

The frame is made from a piece of brass, 1/2" x 1" by 3". Before I was clued in, I made the frame in two operations. First, I would do all the cutting and drilling required, except for the port holes (A and B in the photo). Then I would put the frame aside and make the cylinder, crank/crankshaft and piston/piston rod. Finally, I would assemble the parts, stick a pencil lead in the cylinder's port and turn the crank. The pencil would mark the arc the port made on the frame. Then I would disassemble the engine and drill the port holes in the frame, and reassemble the engine.

There is a better way.

When planning the engine, you need to make three decisions that affect placement of the port holes on the frame:

1. Length of stroke. In the case of this engine, I decided on a one-half inch stroke.

2. Distance between the cylinder's pivot point and the crankshaft. I decided on one inch.

3. Distance on the cylinder between its pivot point and its port. I decided on one-half inch.

After making those three decisions, you can make a drawing such as the one below.

First draw a circle to represent the crank. Its center is point A in the drawing. Mark two points (B) horizontally in line with A, one a quarter-inch left of A and the other a quarter-inch right of A. Total distance between these two marks is one-half inch, the previously decided length of stroke.

Next, mark a point representing the distance from the center of the crankshaft to the pivot point (C), and a third mark (D), which is the distance between the pivot point and the port.

Finally, draw a line from the two points marked B, through C to D. The intersection of these two lines with the horizontal line at D are the required locations of the port holes in the frame.

There is a tiny error in locating the port holes by this method, for the pivoting action of the cylinder makes an arc, the bottom of which is at point D. That means the "perfect" location of these ports is slightly higher than shown in the drawing. But the error is so small it can be discounted. If you draw an arc as shown in the drawing, you can see that the error is not big enough to matter.

Cylinder

I made the cylinder out of aluminum, 1-1/8" x 13/16" x 5/8", with the 1/2" bore 1" deep. The bore is offset from center, to provide enough "meat" to anchor the pivot screw.

The bore is drilled with a 7/16" bit, 1" deep, and reamed to 1/2". The pivot hole (near the open end of the cylinder) is drilled just short of the bore and threaded 4-40. Since the tap is slightly tapered, the threads end a little short of the bottom of the hole. This is good, for the pivot screw will jam in the unthreaded bottom of the hole, so it will not come out during motor operation.

The port is 1/2" below the pivot, and is drilled 3/32" diameter into the bore.

Crankshaft, Crank and Pin

This assembly is of three materials. The crankshaft is 3/16" brass rod. The crank is aluminum turned to 5/8", and the pin is 1/16" diameter steel wire. The three pieces are press fit, and sealed with Loktite.

Piston

I turned the piston and piston rod as one piece, with an oil groove in the piston. The piston is turned to 1/2" for a slip fit into the cylinder. The piston rod is 3/16", with the knob turned to 1/4". The diameters of the rod and the knob are strictly cosmetic; they could be as thick as the piston itself, though that might make it too heavy. Note the 3/32" hole in the knob. That's for a running fit with the crank pin, above.

Flywheel

The flywheel is brass, 3/8" thick and 2" diameter. The center hole is 3/16" diameter for a slip fit on the crankshaft, secured with a 10-32 set screw. The other three holes are cosmetic. I used the jaws of the 3-jaw chuck in which the piece was mounted, to mark their location.

Spring/Bolt

The bolt is 4-40 x 3/4", and the spring is hand-wound. Years ago I purchased a couple of 3-foot pieces of piano wire and this spring is wound from that. To wind it, I stuck a length of 1/8" rod in the lathe's 3-jaw chuck, anchored one end of the wire to the chuck and hand turned the lathe while feeding the wire onto the rod. In this engine, the spring from a ball-point pen would have worked just fine.

Base

The base plate is simply a 2-7/8" x 4" x 1/4" piece of brass from my junk box. It could as easily be any material you may have -- wood, plastic, aluminum, etc., of any convenient size. The holes are 1/8" diameter, countersunk 1/4" to accomodate the screw heads and spaced 1/2" apart to fit the 4-40 threaded mounting holes in the frame.

Plans for this engine can be found at b2-plans, a pdf document. Click here to see it run.