Wobbler Steam Engine – Piston and Cylinder

Posted: 6th December 2017 by Quinn Dunki in Hacks

Reciprocation and Salivation

 

Last time, we made the crankshaft and main bearing. These are the elements that convert linear motion into rotational motion, so they are obviously pretty important in any reciprocating engine. Now we’re getting to the meat of it though- the cylinder and piston! These are the parts, when very precisely fitted, that perform the dance that built the world.

That “precisely fitted” clause is the critical part. The piston and cylinder are a mechanical system with a difficult job to do. The piston must be able to slide with as little friction as possible, yet be a gas-tight seal through a large temperature range. It must travel in a very straight line, and do so for potentially millions of cycles in its lifetime. That’s a tall order, but the tool that gets this done is precision. Precision is so cheap and plentiful in the world today, that it’s easy to forget how difficult it used to be. Pull a one-dollar package of drinking straws off the shelf, and they’ll all be equal in length, diameter, and wall thickness to a very impressive degree. This is because the machines that make them are extremely precisely built, which allows the straws to be built quickly and with minimal wasted material.

Machine tools create precision, and precision makes possible things like a functional piston-cylinder pair that can drive a textile mill, a racecar, or a toy on your desk. Now, in most real-world applications, pistons have rings on them. Piston rings are a whole other sophisticated mechanical system designed to cope with additional challenges, such as containing pressurized oil, maintaining extremely long life, containing high gas pressures in the combustion chamber, and being manufacturable inexpensively. Luckily, small steam engines are rather less demanding than internal combustion, and desk-toy steam engines even more so. The basic precision of a machine tool will get us to where we need to go.

Since we don’t have piston rings, we are relying on the body of the piston to be a very good fit to the cylinder. The easiest way to do that is to make the cylinder with good machining practices, then make the piston custom-fitted to it. This would not be efficient for manufacturing purposes, but luckily we’re not trying kickstart a second industrial revolution here. We just wanna see a thing we made do a neat thing. We don’t have any sort of steam seal on the cylinder (nor, as mentioned, do we have rings) so this is certainly a lossy design. The best-made piston will still have appreciable blow-by when made this way. Again, though, we just wanna see a thing yada yada.

The piston and cylinder will both be made from brass. They will move smoothly together, and respond roughly the same to temperature changes. A very nice surface finish is fairly easy to achieve with 360-grade free-machining brass, which helps our cause quite a bit. Good surface finish isn’t just for looks- it’s also a way to reduce friction and increase gas-tightness (especially when we don’t have the luxury of gaskets to hide our mistakes).

As before refer to the drawings for all dimensions in the parts you’re about to see being made. Here’s a look at the cylinder, to start with:

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These drawings were all generated by Fusion 360, mostly automatically from the 3D model of the engine. Print the PDF (linked above) to get a complete set of 1:1 scale drawings.

 

The cylinder starts out very straightforward- simply turn a piece of brass stock to dimension, face the end, center-drill, drill, and ream to the correct bore (as shown in the drawings).

 

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Reamers make a very precise hole dimension, which is good because a very precise hole is what we need. It’s also critical that this hole be very concentric with the outside. We’ll see why in a moment. Luckily we can do this in a single setup in the three-jaw chuck to guarantee good concentricity.

 

At this point you can flip the cylinder around in the chuck and face the other end. A secondary setup is fine for the facing operation, since we have achieved our goal of concentricity already. Here’s where things get interesting however. A cylinder needs to be mounted to something, right? Furthermore, once mounted, the piston needs to move inside it such that the end of the piston rod follows a precise linear motion relative to whatever the cylinder is mounted to. That’s why we needed concentricity in the cylinder- because the piston will be moving relative to the mounting point of the cylinder, and if that system isn’t entirely parallel, something will bind (or leak).

To make matters even more interesting, the cylinder mounting area in a wobbler engine is also the valve system. That means we need a very flat surface for sealing, and that surface will be moving. The plane of that motion needs to be parallel to the piston’s travel, which means our flat surface needs to be parallel to the cylinder bore. This is probably the most critical operation in the whole engine, so it’s worth taking the time to get this right. The question is, how to we create a flat surface on the side of a cylinder with just a lathe? Easy! The four-jaw chuck. Recall from last time, we talked about how three-jaw chucks are precise but not repeatable. Well, four-jaw chucks are precise and repeatable! This is because the jaws are independent, and thus you can “cancel out” the runout in the rotational system formed by the spindle and chuck of the lathe.

Before we get into how we’re going to set up this cylinder-flattening stunt, let’s cover the basics of four-jaw chuck use. I would practice this before trying to do the funky setup on the cylinder. Here’s a crash course in the independent four-jaw.

The first step is to get your part centered in the chuck to within 100 thou. That’s one sweep of the indicator, and being within that makes this process way less mind-bending. With experience you can easily eyeball it to get it within 100 thou, but here’s another method. Align all four chuck jaws to one of the rings on the chuck- that’s what they’re for. Then, hold your piece in the opening, and tighten two opposite jaws the same number of turns of the wrench until they touch the piece, holding it lightly. Then do the same with the other two- always counting the number of wrench turns to keep them roughly equal. When all four jaws are touching the piece, we can move on to the indicator.

 

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Here’s the basic indicator setup. I like to put the mag-base on the cross-slide, because it makes it easy to adjust the position of the indicator as needed. You want the indicator horizontal, on center (by eye is fine), and perpendicular to the material (again, by eye is fine). Ride the indicator on a machined surface, if possible. That will give better results. Make sure the indicator is compressed by roughly half its travel. If it runs out of travel in either direction, the whole process is invalid. Note that in this position, the indicator is aligned with a chuck jaw when that jaw is horizontal. This is important, as we’ll see.

 

Now we’re going to spin the chuck by hand, always in the same direction. The mag-base arm will have a tiny bit of slack, and the indicator stem will have some drag on the surface. That means the system effectively has backlash in it, like the lathe cranks. Always turning in the same direction corrects for this. As you turn a full revolution, the indicator will tell you your highest spot (closest to the dial, furthest from you), and your lowest spot (furthest from the dial, closest to you). The distance between the two is how much we have to move the jaws.

“But wait,” you may be thinking, “how do I know which jaw to move, when the high spot could be anywhere?”. Aligning the indicator with one jaw allows us to pretend that the high spot is always aligned with a jaw, so we always know exactly which one to move. Let’s take a look at how this works. Below is a diagram of how the four jaws might be gripping the material, exaggerated for effect.

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Here you can see that the high spot is 45° off from either jaw. However, if we only look at the indicator when it is aligned with a jaw, we’ll move the left jaw inwards, and watch what happens.

 

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We’ve reduced the overall runout, and now the remaining runout really is aligned with a jaw (the bottom one, in this case).

 

As you can see, only looking at the indicator when it’s aligned with a jaw will get you where you want to go, without having to do brainastics trying to move two jaws at once.

 

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Here’s my high spot (as read when a jaw is aligned with the indicator). For big moves (more than 5 thou), put the high spot at the indicator, and loosen the jaw closest to you a little bit. When you loosen, the indicator won’t move much. You’re just making room. Then tighten the opposite side as described below.

 

Make small turns on the wrench (10-15 degrees) until you get used to how much wrench turning maps to how many thou on the indicator. This will depend on your particular chuck and your particular indicator.

 

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Here’s my low spot. Since I already loosened the opposite side, now I can tighten this side, watching the needle. You want to tighten to the midpoint of the extremes. My high spot was 79, my low spot is 55, so I tighten until the dial reads 67.

 

One or two iterations of those big moves will get you within 5 thou. Now it’s time to “tighten in”. Continuing to rotate in the same direction as before, turn until the indicator shows a low spot, and tighten the jaw facing you. Again, because of how the indicator is set up, the high and low spots will always align with a jaw. Each tighten should be to the midpoint between your high and low point, which by now should mean you need to move at most 2-3 thou.

If you do something and the run-out gets worse, that means you moved a jaw in the wrong direction. This is easy to do by mistake. Just stop, clear your head, and restart the process. The first time I did this process, it probably took 20 minutes. You get better quickly with practice, and I can now do it within a couple of minutes. Real Machinists™ can do it in 30 seconds or less.

After two or three iterations of the tightening-in, our runout is reduced to ¼ to ½ a thousandth. That’s plenty precise for anything getting built in Blondihacks Labs.

 

It’s also a good idea to go around one more time and make sure all four jaws are tight at the end. Do this carefully to avoid messing up your hard-earned setup. If the tightening-in was done well, none of the jaws will be loose anyway.

What’s cool about this process is that the material we’re going to machine now has effectively no run-out in it, but the jaws will not necessarily be centered on the spindle. The process above has “cancelled out” any runout in the chuck, the jaws, or the material. The material could be a crazy shape, and we can still dial in the exposed part to have zero runout. This is the magic of the four-jaw, and the reason your lathe cannot be without one.

Okay, with four-jaw basics covered, let’s get to a more creative use of this awesome chuck.

Because the outside of the cylinder is machined, it’s a valid reference surface. When placed against the face of the chuck, we know we’ll get cuts that are parallel to the bore inside, since we know we maintained concentricity between the outside and the bore. Tricky, but these are the mental gymnastics of machining.

 

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Here’s the setup. The cylinder is gripped with shop-made soft jaws to protect the finish. A dial indicator is used to make sure both ends of the side of the cylinder are the exact same distance from the cutting tool (we expect this, because the cylinder is seated against the chuck face. Still good to check). This guarantees a cut parallel to the bore. A caliper is used to get the cylinder roughly centered on the axis of the chuck. We then use the dial indicator on the side (as shown earlier) to get the cylinder centered.

 

The four-jaw technique for centering the cylinder is the same for a normal part, by we need to do it twice- once for each axis. Start by eliminating the runout in the long end. When you rotate the chuck from one end to the other, pull the plunger out by hand, then let it touch the other end when you’ve spun the chuck 180°. Compare the two ends and make adjustments until they read the same. Then do the same for the short sides, to get the center of the side of the cylinder right on the center axis of the lathe.

With the cylinder in this position, we do a normal facing cut to the depth shown in the drawings. It will be as though we’re facing off a large round bar with a diameter the same as the length of the cylinder. This is an interrupted cut, so don’t get aggressive. Take your time and make as many shallow passes as needed to face down our flat side. Ten thou at a time for a small bench lathe like this is a good starting point.

 

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And here we are! We’ve made a lovely flat spot on the side of our cylinder. Note that I used a dial indicator on the carriage to get the depth just right. This is an important dimension, because it sets the relationship between the piston rod and the crankshaft. If this isn’t clear right now, don’t sweat it- follow the drawings and the engine will run.

 

At this point you may note that the cylinder has smooth sides (as called for in the drawings). This is the easiest way to make the cylinder, and is how beginners should do it. However you may notice in later videos that my cylinder has decorative ribs on it. This is because I made the cylinder twice. In my case, I botched the dimensions in a pretty unrecoverable way, and had to make the part again. Don’t be afraid to do this- it’s going to happen to you! Think of it as an opportunity to practice your skills. Making the part the second time always goes much quicker anyway, so the time lost is not as bad as it seems. You’ll learn something every time you make a part, so appreciate these growth moments for what they are.

With the side flattened, the next step is to drill and tap the pivot hole. This is what makes a wobbler wobble, after all! Since we took the time to align the cylinder, we can now simply chuck up our center drill, knowing it will land right in the center of the flat spot- no layout required. Resist the urge to drill this hole on the drill press. We want to use the lathe for this to ensure the axis of rotation of the cylinder is precisely parallel with the crankshaft. The lathe will do a much better job of this than a drill press.

 

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Here we are, all set up for drilling. The depth of this hole is really critical, because it needs to be as deep as possible (for sufficient threads to hold the pivot rod) but not go through the side-wall of the cylinder. To that end, I’m using a dial indicator on the back of the tail stock chuck to get a very precise depth measurement. Note that when you have the indicator at an angle like that, you introduce a little bit of so-called “cosine error”. This is because you’re actually measuring the hypotenuse of a triangle formed by the motion of the chuck, not the chuck’s motion itself. This is usually not enough error to worry about though.

 

With that hole drilled, we can tap the threads for the cylinder pivot, as shown in the drawings. You will need a bottoming-tap for this. This is a blind hole that is much too shallow for normal taper taps. Tap on the lathe or drill press to get a nice straight thread.

 

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Here’s an example of using the drill press (with a center punch) to ensure straight threads. In this case I’m tapping the cylinder head holes, but the same technique can be used for the pivot hole. The tool in question (lathe or drill press) is not running. It’s just there to keep things straight, as they are very good at doing. If you’re smart, you’ll tap the hole before removing it from the chuck in the previous step, since we already aligned it for the drilling operation. I was not smart.

 

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With the pivot hole drilled and tapped, we can now drill the steam port. Triple-check your layout here- the position needs to be precise, since it’s lining up with stationary holes in the frame. Center-punch and center-drill your mark to be really sure the drill press puts that hole where you want it.

 

The last operation we’ll do is to hone the cylinder. The purpose of honing is to get as smooth a finish as possible in the bore, thus ensuring a good seal and friction-free operation. In some cases, such as internal combustion engines, the hone also helps retain oil on the surface (via a cross-hatch pattern achieved with a special machine).

A serviceable cylinder hone can be made with a scrap of round bar. Cut a slot in the end, slide in a piece of folded emery paper, and chuck the whole thing in a hand drill. This creates a high-speed abrasive that you can run up and down the cylinder. Start with 400 grit, then go to 800, then 2000. If you’re feeling ambitious, you can finish by stuffing a piece of rag in the honing tool, saturated with brass polish.

 

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The jury-rigged cylinder hone in action. A Z-folded piece of emery paper will expand under the centripetal force of the spinning, creating a pretty nice hone.

 

Next up, the piston! Making the piston itself is a pretty straightforward turning operation. There are a few details that are interesting to cover though.

 

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Here’s our piston. Making everything round means we can do it all in one setup on the lathe, thus maintaining that all-important concentricity. Note that we need a cross-hole drilled for the crank pin. It’s almost always easiest to drill cross-holes in things before turning them. I forgot to do so, but was able to save the part with a complicated cross-drilling setup, as we’ll see.

 

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Turn the stock down to the largest diameter. This is the most critical one, so get it on the money! A micrometer is mandatory here. I recommend aiming for 0.5 thousandths over-sized, then fitting to the cylinder (as we’ll see later).

 

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This is a long, thin piece, so tail-stock support is required. Simply mark and turn down each section to the diameters shown on the drawing.

 

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To give a nice aesthetic transition between the diameters, I ground this 90° forming tool from a piece of high speed steel. Square shoulders would be just fine also!

 

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Next, we need to mark and drill the “big end” of the piston rod for the crank pin. I used the surface plate and a height gage to lay it out. A caliper would work fine as well.

 

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The cross-hole was made on the drill press with an overly elaborate setup. It would have been easier to drill and ream this cross-hole first, before turning the piston. Live and learn! Note the aluminum beverage can shims. Aluminum cans are remarkably consistent in thickness (there’s that cheap modern precision again), so keep scraps around for shimming.

 

The final step is fitting the piston to the cylinder. Back into the chuck it goes, but we’re not going to be using any cutting tools.

 

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Use a strip of 400-grit emery paper wrapped partway around the piston to remove very small amounts of material at a time. The piston is gripped by the tiny nubbin at the end, but we aren’t putting any appreciable force on it, so this is fine. Also note the towel to protect the ways. Sanding is an abrasive process, and you don’t want grit raining down on your lathe’s precision surfaces. Abrasive grit is very bad for machine tools and you want to keep them away from each other as much as possible.

 

Wrap the emery paper about 90° around the back of the part, holding it lightly with your finger tips. In the unlikely event that the lathe grabs it, you want the paper to be pulled out of your hands, rather than your hands getting pulled into the chuck. Sand the piston for several seconds at a time, with the lathe at a moderate RPM (perhaps 400). Test fit against the cylinder with each pass until you get the right fit. Then polish with 800 and 2000 to remove the scratches from the 400 grit paper.

The “right” fit in this case is that the piston slides easily, but when you remove it quickly, you hear an audible “pop”. To test this, cover the steam port and the top of the bore with your fingers. Slide the cylinder over the piston, then slide it off quickly. You should hear a pop like a cartoon bubble popping. Also remember that oil occupies a little bit of space, so test fit with the piston lightly oiled with a 3-in-1 machine oil. When you’re done, the piston will be loose enough that it can fall out of the cylinder from its own weight, but still audibly pops when pulled out quickly. If you’re unsure, err on the side of the piston moving more freely. If it leaks a bit, that’s okay- you can add more steam pressure to compensate. The engine will be less efficient, but will still run. On the other hand, if the piston is sticky or binding at any point in its travel, the engine won’t run.

The last part we’ll throw into this post is the cylinder head. This is a very straightforward turning operation, so I won’t show that here.

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The cylinder head is very easy to turn, but it is good practice for laying out bolt circles. See the previous post in this series for information on that process.

 

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Once turned to dimension, lay out the bolt circle on the top, and then drill.

 

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Next, use the cylinder head as a guide and a transfer punch to make the matching marks on the cylinder. This is much easier than trying to make two bolt circles that match perfectly.

 

Make sure the cylinder head bolts don’t align with the steam port! You don’t want them interfering.

 

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The holes are drilled and tapped, as with anything else. Note the aluminum can material being used to protect the finish of the cylinder from the vise jaws. Handy stuff to have in the shop!

 

Okay, that’s all for now. The hard pieces are made, and we’re in the home stretch. Soon we’ll be able to start assembling parts into something that resembles an engine! This is the point at which we begin to salivate at the thought of an engine running under its own power. Grab your bibs and stay tuned!