Piston Design and Machining

The piston design is fairly simple and straight forward. We have already determined that it needs to be 1" diameter. The piston itself needs to be very smooth and have a high tolerance to maintain a good seal. We used our judgement and chose a 1" height for the piston to help keep it from racking (wobbling) inside of the cylinder, causing heightened wear or binding. Also, the more surface area that the piston has in contact with the cylinder wall, the less air will blow by the piston into the crank case.

The pin hole for the connecting rod was chosen to be 1/8" thick and located halfway up the piston. The reason it's located halfway up is once again to help prevent racking. The 1/8" pin was chosen for two reasons. First, it's a standard size, making it cheaper and easier to acquire. Second, it's more than strong enough as a quick calculation for a pin in double shear shows :

Shear Stress = Force / (2 * Area) = 117.8 lbs / (2 * π * (.5 * (1/8)) ²) = 4800 psi

(As a reference, mild steel has a yield strength of about 40,000 psi at it's fatigue limit)

The next decision made was to make the piston pocket 1/4" wide by 3/4" deep by 3/4" long. This allows us to use a 1/4" long flute mill bit to machine the pocket (pocket depth = 3x the diameter). A different diameter pocket could have been chosen, but our supply of long flute mill bits was limited. Also, we just so happened to have some .220" thick aluminum plate handy from which we could make our connecting rods (no facing 1/4" plate to fit inside the slot). The reason for the length and depth of the pocket is so that the crank rod doesn't rub on the pocket's walls. If you would like to download an engineering drawing with all of the dimensions, click below:

Piston_Dimensions.pdf

Having the design laid out, a material needed to be chosen. As mentioned above, it needs to be as smooth as possible and have a high tolerance. There just so happened to be some 1" chromoly bar from a bent axle sitting in the scrap pile of our shop. This is an ideal material for use as a piston. The exact material is 4140 steel, with a cold rolled finish and Rockwell hardness of ~40. This means severals things. The first two numbers in type of steel tell us that it's an alloy designated 41, in this case that it has a high Chromium and Molybdenum content in the mixture. The last two digits, 40, tells us that the steel has a .4% Carbon content (Note: the higher the carbon content, the stronger but more brittle the material). The fact that the steel was cold rolled tells us that it was plastically deformed without being annealed afterwards. This process leaves residual stresses in the material which can cause it to warp if heated. However, the residual stresses also make the material stronger. The cold rolling process itself produces a very smooth and high tolerance surface finish.

So what makes this such a good material to use? The cold rolling gives us a wonderfully smooth surface finish. The hardness of the material means that any wear which is going to occur will most likely occur on the cylinder walls, which in our design are very cheap and easy to replace. Quick measurements give the diameter of the 4140 to be .999-1.001". No need to do extensive machining on the lathe to maintain tight tolerances!

Finally, on to the machining:

1. Use a drop bandsaw to cut the 4140 stock to within ~.040" of the finished length (1.000"). You don't want to cut it too close or else you won't be able to face the ends to make them smooth.
	2. Face the pieces on the lathe in order to get close to 1.000" lengths. As you face each end, add a small chamfer to remove any burrs and make the pistons easier to slide into and out of the cylinders.
3. Grab a pair of v-blocks, clamp the piston in the mill as shown on the right. The v-blocks will provide a very good grip on the stock without leaving marks like the vice would. Now Cut a 1/4" wide slot 1/4" deep (see two pictures below). Remember, 4140 is a very hard steel. We took .025" passes to help reduce cutter wobble/chatter that would prevent straight cuts. This gets extremely important the deeper you get into the pocket, where any wobble can easily break the cutter or cause the work piece to slip.
	4. Pre drill holes at each end of the slot for the 1/4" end mill. Most extra long end mills are not centercutting. Ours was no exception. If you aren't paying attention and try to plunge cut, the cutter will snap in half before you crank the table more than a couple thousandths. We used a 1/4" drill bit, but anything between 1/8" and 1/4" should be enough to remove the material from the dead spot below the cutter.
5. Once your holes are pre-drilled, mill out the pockets to the full depth (another 1/2" from the bottom of the slot). This is probably the hardest step on making the pistons. Edge finding on a round object and moving precise amounts, it's easy to be off by a few thousandths. We even screwed up ourselves several times. Luckily, the pocket in our piston design gives room for a lot of error without causing further troubles.
	6. Put the piston on it's side in the v-blocks and orient the slot to be parallel with the table. Edge find on the end of the piston, then move in 1/2" to put the pin hole in. If you haven't moved the table at all in the y-direction, you should have been able to flip the piston onto its side and still be located dead center.
7. Deburr the pin hole and slot using a file, sandpaper, deburring tool, center drill, or similar items. Just don't get overzealous and put large gouges in the side of the piston.
	8. You're finished the piston(s). This was by far the most tedius and boring work on the entire engine. Grab a beer and take the rest of your day/night off. The next step is to make the connecting rods.

Next: Design and Machining of the connecting rod...