Lightweight compressed air engine

[b.lindsey]

Dave,
When I was in high  school, a small group of us really got into .049 control line airplanes too. Back then we would get out of school and head to the playground of the local elementary school. I can only imagine that today they would arrest us for doing that...kind of sad in a way. Any progress on your half scale radial?

Bill

[sshire]

.049. I had forgotten about mine. Perpetually bruised finger from prop hits

[moerman]

I've done some measurements. The cylinder I have is 7.23 mm internal diameter, the piston is 7.15 external diameter. So that's is a 0.08 mm clearance, or 0.003". Not to bad for of the shelf tubing, but a bit much for a good functioning engine of this size, I think. I'll try to turn a piston cap with a better fit so the top of the piston is slightly wider than the rest of the piston.

[Stuart]

Think of the fits as a percentage of size

Eg

25 mm dia with 0.05 mm slop would not be to bad

But a 5 mm dia 0.05 would be massive

For a air engine of that size you will need a lapped fit of bore to piston but with dissimilar metals

Eg brass with bronze or drawn bronze and cast bronze would work as as would a CI bore with a Ali piston

Hope that helps it's not a exact rule but the percentage idead will help

Stuart

[moerman]

I've done it! I machined a bronze top for the piston which fits snugly in the cylinder. and has a step to fit in the piston. No measurements done as yet, I did the last part of the turning until I had a good fit in the cylinder. It took me about a cm of brass to make this 3 mm thick part. First tries were undersized.

The piston gets an aluminium center which takes the conrod and pin. I already machined a rod of alu in the right diameter. As the aluminium will be glued in the piston I think I will glue the bronze piston top as well. I am thinking about using cyano-glue for this.

[moerman]

I just received a 2nd hand Moore&Wright 1965M micrometer, which makes measuring a lot easier. I see that the brass piston tube is 7.16 mm in diameter, while the bronze top is 7.22 mm diameter. I started worrying if the top was sufficiently larger in diameter because I noticed some grooves in the surface which will be polished away. But now I'm confident that also after polishing the diameter is larger than the brass tube piston.

[moerman]

Yesterday I finished the parts for the piston and a connecting rod. The piston is build up from brass tube, a bronze top and an aluminium core. The core is slotted and drilled to take the conrod and pin. I decided to drill through the bronze top to get some weight reduction. The aluminium core is stepped to fit the hole in the bronze. It all fits beautifully! I'm really learning a lot by making these parts on my new lathe.

I made the conrod out of 1 mm dural instead of 2 mm hobby shop aluminium. I think it will be strong enough.

Pictures will follow later.

Wout

[moerman]

Here is a picture of the progress I've made. Not much .... But I learned a lot!

The piston and connecting rod are now together and the piston fits the cylinder nicely. The black line is oil which turned black during lapping. That is, if you can call moving the piston through the cylinder up and down by hand lapping. The brass bearing is still twice the length it should be. I have drilled it to 4 mm ID now and it seems the original ID was not constant. The drill went in easily but found some resistance at some parts of the length. Making a fitting shaft is a lot easier now. The shaft is still a bit undersized at 3.94 mm after polishing.
It rocks slightly in the bearing. I hope the next one will be perfect!

Wout

[moerman]

YES, the next one was perfect. WAS .... The fit is very good but when I wanted to part the extra length of the axle it went wrong. It bent and broke just outside the lathe jaws. I have parted the useable remaining part of the axle, but it is a few mm short. Now I have two options: make a new axle or use the shorter axle. I first will try the second option. On the plus side: it saves weight! I can make the engine just a few mm shorter.

Next question is: how shall I connect the iron cranck to the iron axle? Is silver soldering required? Haven't done that before, but that has never stopped me. I heard there is only one rule: cleanliness, is that right? But another method of joining is welcome, too. Any suggestions?

Wout

[Ian S C]

Another rule of silver soldering, get the joint hot enough to melt the solder, don't melt the solder with the torch. Another way is to arrange the crankshaft so you can assemble it with Loctite.
                                           Ian S C

[Stuart]

for your light engine I would go with loctite to

you can undo it with a small amount of heat so so easy with SS

I use 638 or 603 your choice  don't buy a big bottle its to expensive , it does not go off but cost is cost

Stuart

[moerman]

I have not done any work on the CA engine because I first had to tackle some issues with my lathe. But I'm back on the job now. First I will make another cranckshaft and front bearing. For the bearing I have bought a set of 4 mm reamers and a 3.9 mm drill, so the diameter of the shaft will be 4.00 mm. I also bought a length of 2.5 mm brass tubing so now I have everything at hand to build the engine. now just hoping I will find the time this weekend....

[ChipMaker]

I love this simple Compressed Air Engine!  I am going to add it to my "to do list"!  I make simple engines as well as some a bit more complex such as Elmer's Geared Steam Engine #5, which is now in my lathe!  I cheated a bit on this one though; I spent the "big buck" and bought my gears!  Regards, CM