My last post was back in September, and about finishing the flywheel for the engine. The next part to do was the piston. The piston and cylinder fit is one of the most critical parts of building these small engines; in my steam engine, I ended up making three pistons out of different materials (two graphite and one cold rolled steel). It's hard to say which method is most common, but it seems to me more people seem to make the cylinder first and fit the piston to it. I had made the cylinder some time ago and that was my intention.
Sometime in October, I put the cylinder back in the lathe's chuck intending to hone it to a constant diameter and smoothness. To my fingertip, the cylinder wall felt like it had ridges on it down toward the left end in this view, so I was sure it needed to be smoothed. To accomplish that, I bought a brake cylinder hone.
I need to talk some numbers here. The cylinder is supposed to be 0.875" diameter - lapped smooth. The piston is supposed to be turned to 0.873 and takes two cast iron piston rings that give that 0.875 but have some compliance. I bought those from a guy who specializes in them.
Measuring the inside diameter of that cylinder isn't as easy as measuring the outside diameter. A common method is to use telescopic gauges like these. Note they all have separate knurled knob at the long end of the “T” shape. The crossbar of the T is spring loaded and that knob locks them in the desired position. They're locked at their minimum span, inserted to depth in the bore, then the knob is released allowing the spring to drive the measuring points to the diameter, and then the knob is tightened again locking the measurement in place. It's tedious to measure diameter and taper this way because it requires multiple measurements and each measurement is subject to errors if the measuring points aren't exactly perpendicular to the walls, and the long part of the handle parallel to the axis of the cylinder.
I honed the cylinder for a while with the three abrasive stones on that brake cylinder hone but it didn't seem to be smoothing the cylinder. The cylinder is curved and the stones are rectangles, so they only rub on their edges. I wrapped the three stones with 150 grit sandpaper and that seemed to smooth the cylinder faster. I spent a minute or two working 150 up and down the bore, then switched to 220 and repeated. Likewise with 320 and 440. The cylinder feels quite a bit smoother but I can still feel the roughness. I tried taking a photo of it; easier said than done.
You're looking into the lathe chuck in this view (from the right looking left in that photo at top). The light colored circle at the top is the bore through the chuck. Just to the left of that circle's bottom center is a large bump - that's one of the chuck's jaws. I stretched the contrast and sharpness a bit, so those light and dark rings aren't that obvious looking in there
The measurements I'm getting make it seem the cylinder is already oversized - around 0.878" - so I'm hesitant to smooth it more. A particle of 400 grit abrasive is 1/400" or .0025 (though that much doesn't stick above the paper). If I took off that much in diameter, it would definitely be too big. The piston is a piece of 0.875 stock which measures around 0.874". I haven't machined the features and details into the piston, yet. Yeah, I could get a 1" aluminum bar and turn it down to something that's a tighter fit in the cylinder, but I have those piston rings to consider. I have enough steel bar to make another cylinder if I had to, but I'd rather only do that if I had to.
As it so often does, it looks like I'm going to have to build it to find out if it works.
You're not honing it dry are you? In my experience, you need to flood it with honing oil as you work the hone back-and-forth through the cylinder. Otherwise the stones/paper will clog up, stop honing, and start putting scratches in the cylinder. And be careful not to "bell mouth" the ends.
ReplyDeleteThat's one of those things that isn't as unanimous as I originally thought. I read some pretty experienced machinists on the two forums I follow and you get both views. Some say they use oil, some work without it.
DeleteWhat I'm most concerned about is the fact that the hone isn't a couple of inches long and working everywhere at once. I should have used a 2" long steel bar and grinding compound - grease with abrasive in it.
There's an intentional bell mouth designed into the part in the first 1/8" or so to ease getting the piston into the cylinder. The stroke is roughly the piston diameter, 7/8", so the part where taper matters is all where the piston sits.
Spent the day making tooling for the mill. Already had some blanks made with centers, taper, threads etc done, was figuring to drop them in the lathe and cut an ER collet recess. I made them a few years ago, was quite happy at the time. Now found the diameter was .0005 small, the taper was a hair off, and the thing was off center, somehow. A good illustration of the learning curve. Took most of the day to get everything back on track.
ReplyDeleteUsed trig for the first time to set a taper, that was fun.
How much of a ridge in cylinder? The fingertip will pick up an amazingly small amount . Ideally, a internal grinder would be the way to go, or maybe a brass lap.
ReplyDeleteSeems like the decisive issue is how much those rings will take up- Making the piston a hair oversize is no big deal- BTW,have you checked the 7/8" piston stock for round?
The cylinder could be bored and sleeved, too, but probably as much work as turning a new one.
How much of a ridge in cylinder?
DeleteI sure wish I could answer that.
I can feel it but that doesn't mean a lot. The cylinder is 2" long so I can't feel that area with the very tip of my finger, which I think is most sensitive, just the fleshy part. I can't run a nail over it, but can brush a nail against it and barely feel it.
The stock slides easily in the cylinder and it was tighter before I started the honing and sandpaper work, so I think I've knocked down maybe a couple of thousandths. A guy who made one of these tells me he thinks it's probably fine but if I get stuck, there might be adventures in making larger diameter piston rings ahead of me.
I'd echo Raven's suggestion of lapping with a brass lap, keeping in mind the possibility for lapping to create bell-mouthing. Another option is honing with CRS bars using light oil to knock down the ridge.
DeleteWhat do your pin gages tell you WRT variations in inside diameter?
What do your pin gages tell you WRT variations in inside diameter?
DeleteThey don't see the ridges. I think the ridges are too small for the domed measuring surfaces to distinguish.
Part of the problem may be that the project is so small. Make it bigger. Bigger. Make it big enough to run a Delco alternator.
ReplyDeleteI have the plans for that one already - a four cylinder engine over 3 cubic inches. I figured I'd make one to get familiar with the parts and processes first.
DeleteAfter the flame eater last year, I wanted to build a 49cc engine like you see on little mopeds and lightweight motor vehicles. Something with a useful amount of horsepower. I found replacement parts for them ready made, just no plans so I could put one together. I settled on the four cylinder because I could get plans.