It is the time for the next project. This time I have selected the Potty three cylinder air engine. Plans can be found on this site.
I will be making some relatively minor changes to the original design including not using metric screws. I have modeled all of the parts and produced new drawings with inch dimensions. I find it less error prone if I do not need to do the translation from mm to inches on the fly.
I ordered the material I needed and started making chips just over a week ago. I will now try to get this build log up to date.
I started with the largest piece in this engine, the crank case. Starting with a 2.5 inch diameter piece of aluminum I turned a section down to 2.1 inches to reduce the amount of milling. I also bored out the central cavity. I used my 4-jaw since my 3 jaw is on the small size and would not grip as well as the 4-jaw.
Next I move the chuck with the part to the h/v rotary table on the mill. After milling the hex shape I drilled and tapped all of the holes on the hex faces.
Then it was back to the lathe where I parted the crank case from its handle. Back on the rotary table, this time in the horizontal position with the 3-jaw I milled the part to length and did all of the features on the valve face.
After drilling and tapping the holes on the bearing housing face and a little cleanup I had my firs finished part.
Next the bearing housing. This time I started with a 2 inch diameter piece and turned the mounting flange and the boss the fits inside the crank case. I also drilled and bored the bearing recesses.
After drilling the screw clearance holes I made a simple arbor by turning a 1 inch steel rod for a slip fit in the .875" bearing recesses. Drilled a deep tap drill hole with a shallow clearance hole for a 1/4-20 bolt and parted off a thick washer. After tapping the part still in the chuck I could mount the bearing housing and face to length and turn the taper..
Next I worked on the cast iron pistons. This was my first time machining cast iron and it went smoother than I anticipated. First I turned the outer diameter and the groves. Next I used a .375 end mill to "drill" the flat bottomed hole in the bottom of the piston. I finished by flaring the bottom of the piston. I cut the piston from the stock and face it to length. Repeat 3 more times as I wanted an "insurance" piece.
On the mill I drilled and reamed the .125 wrist pin hole in each piston.
To cut the cut the square pocked in the piston I used a .125" rod and a 1-2-3 block to get the wrist pin hole parallel with the mills Y axis. I then used a .375" pin to locate the center of the piston. A few passes around the .375 diameter hole mad it a .375 square with round corners. When I was done I had 4 usable pistons.
I also made the cast iron cylinder liners, No pictures, just simple turning and boring.
I could not resist attaching the bearing housing to the crank case to see how they looked. Here is a photo of the progress to date.
Stew, also thanks for the plans that are the basis of this build.
I got a bit more done on the engine this week. I started on the cylinders. The smallest stock I had that could be used for the cylinders was 2 inch round rod. A bit large but it will work. I started by cutting 3 pieces about 1.25 inches long and then mad lots of chips. I milled the 1 and 1-3/8 dimensions about .025" over sized. This way if I am a little off on the bore it will be OK.
After drilling the center of the bore with the DRO on the mill I went to the lathe and drilled .625" diameter and bored to .750 inches.
Next I modified the arbor that I made for the bearing housing. I turned it to a close fit on the cylinder bores with a length of approximately 1 inch. I made a slight undercut so that the inside radius does not cause any problem. The thick washer was unchanged.
After a detour to make a better parting tool holder I turned down the 1 inch diameter. I will not be making the simulated fins since I do not ave a suitable tool.
I had limited shop time this week but did get to nearly finish the cylinders. Based on Stews comments, Thanks Stew, I made up a tool to cut the fines. I used a .125" diameter center drill that the tips had been broken off to grind a .060" wide tool. I made a simple holder from a 1/2" square piece of steel to told the tool and used it to cut the fins as shown below.The other end of the center drill was ground to use to face the inside of the flanges.
I still need to round the corners of the flanges but that will wait until I have the cylinder heads finished so that they can be done together. Here are the nearly finished cylinders.