Time to build something.

I started with Elmers #25 a simple wobbler type steam engine. Free plans available here.

Next up was drilling the holes in the frame, it was relatively straight forwards, I actually spiral milled the larger hole and the relief on the hole for the cylinder pivot.

The plans call for a jig to position the steam inlet and exhaust holes, since I’m in CNC land I just computed the positions, we’ll see if I regret that decision later.

I made machining the cylinder block blank harder than it needed to be. I started with a pice of 1x1 bar stock, trying not to cut it too small I cut it way to big and ended up having to remove almost 75% of the material.

At 0.01” of material per pass it took a while, and it made me think about better approaches on a CNC mill. Usually I just type the code in by hand one pass at a time, but I think I’ll build a library of these common operations with parameters going forwards.

I marked up the cylinder block, I actually misread the plans, and marked the steam inlet/exhaust hole incorrectly. However I caught it when I overlaid the frame and verified the drilling points were aligned with the frame holes.

I don’t intend to use the markup to line up the drill, I’ll do that with the CNC mill from the edges, but they provide a great gut check before drilling starts.

There were a couple of possible ways for me to bore the cylinder, it could be done either on the lathe using drill + boring bar or using a drill + boring head on the mill. As it happens I owned neither a 4 jaw chuck for my lathe nor a drill chuck large enough to drill the initial hole on my mill.

I ended up buying both.

It’s important to understand the critical dimensions when machining a part, the exact diameter of the cylinder is unimportant as long as the piston fits it snuggly, and the finish is acceptable. Probably the right/easy way to do this is with a machine reamer, I chose to do it with a boring bar, so we’ll see if the finish is adequate.

The cylinder center and the 2 holes need to be in line, and the distance between the holes needs to be relatively accurate so that the inlet/exhaust port lines up with the holes in the frame during the cycle. If I’d built the drill jig, it would take care of any slight alignment offset here.

Having not built the drill rig, my first thought is to bore the cylinder on the mill, it’s relatively easy to exactly align the holes that way.

However it’s probably easier to bore the cylinder on the lathe, and looking at the design, specifically the size of the inlet/exhaust ports, I think it would likely tolerate 10 or 20 thousandths of an inch error in the center of the cylinder bore.


As far as I can see reading the original plans, the pivot pin hole is supposed to go through into the cylinder wall, and be silver soldered there, using a plug to prevent it interfering with the piston. It seems a lot easier to me to just not cut through the cylinder wall, so I’ll try that.

In the end I had no choice but to bore the cylinder on the lathe, there simple wasn’t enough vertical clearance on the mill for the widest drill I’d need.

Instead I drilled the pilot hole for the cylinder on the mill, at the same time I drilled the other holes, to ensure at least the measurements were as accurate as possible.


The cylinder pivot hole is 1/8th deep, which should leave 1/16th between it and the cylinder wall. Having tested it with the back end of the drill it should provide adequate support, even after I machine the relief in the block wall.


I verified the hole alignments with the frame using the 1/16 and 1/8 drills, both fitted snugly through the frame and into the cylinder.

This is the result of boring the cylinder on the lathe.

Trying to get any sort of  accurate alignment out of the 4 jaw chuck was an exercise in frustration, it’s as close as I could get it, by my measurements it’s off by about 0.01” hopefully that’s close enough.

The finish on the cylinder wall is a testament to my skills on the lathe, i.e. not very good.

Next time I probably need to increase the spindle speed and slow way down on rate at which I feed the boring bar.

I’ve seen a couple of solutions to smooth this out.

But it’s slightly under the target 3/8, so I will probably just run a reamer though it.

I ran a reamer through the cylinder and it produced a much better finish.

I then machined the piston, pictured here with the #38 drill I used for the hole that will be tapped for the rod.

Again the plans here seem to show the rod going through the piston, again I stopped short of that, which will make measuring the final rod a little harder.

The critical dimension here is the distance from the rod’s pivot point to the top of the cylinder,

but in practice I don’t think it’s all that critical, all it does is affect the compression ratio, as long as it’s close and the piston doesn’t hit the top of the cylinder I can’t see it being all that important.

The fit of the Piston in the Cylinder seemed pretty good, the engineers blue put on to mark the cutoff point was enough to prevent it fitting. With it removed I can feel some compression in the cylinder when moving the piston in and out, hopefully it’s sufficient.

I got a fair amount done over the weekend.

There were very few problems with most of it.

I used the mill to cut the Crank.

The rest was done on the lathe. Nothing really tricky, the only real issue was the small diameter parts deflecting away from the cutter. It took me longer than it probably should have to drill a small hole in the end and move up the tail stock.

The hole in the piston rod that connects to the crank pin is a little looser that I would prefer,

and I might have to make up another piston rod, but I think it’s going to be OK.

I used Red Loctite to secure the crank shaft and the pin in the crank, I’ll leave the silver solder learning experience for another time.

The whole thing moves very freely, and rotating the flywheel by hand and gently applying pressure to the cylinder to keep it on the frame, I can actually feel compression in the exhaust port. That seems like a very positive sign.

Still a number of things left to do, I have to make up a base, cut some of the parts to length, machine the relief on the inside of the cylinder wall, make up an adapter to get air to it and generally clean up the parts. At the moment I don’ have any small hardware to do the final assembly, so it’ll probably be next week before I get to it.

And here is the finished article.

A lot more time in polishing it than I had imagined.

I did finally get to connect it to a compressor over the weekend and it ran!

I spent way more time marveling at it going round and round than was really justified, I’ll upload a video when I get a chance.


My final thoughts on this.

It was a really good learning experience, and quite rewarding to finally see it run, this particular model is extremely forgiving.

It was also a good forcing function in making me use the lathe, and I feel a lot more confident in my ability to do so having completed this project.


I was happy with the tolerances I achieved on most of the parts, but there are few dimensions that really unforgiving on the model. So it will be interesting to try something a little less forgiving.


I’m going to look for something a bit more challenging for the next engine, but I have another project to complete in the interim.