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Making Patterns for a Rider Engine

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Pekka just asked me if I'd explain how I make patterns. I'm making some for a Rider type Stirling engine I'd like to build from iron castings, so I'll try to explain as best I can. This isn't going to be a how-to from the perspective of correct traditional practice, but just the way I do them and think about them.

First step is to draw out what you want as a finished machined product -- at least in its outer form. In this case it's two different cylinders. Both are water jacketed, so there are two round outer rims on each, which a tube jacket will fit to in order to make the water space.

At one end also is a square flange to fit the head of the individual cylinder. There will be ports drilled into those flanges, so they are rather thick. The drawing is really an idealized yet incomplete rendering. There are no fillets, chamfering, shrink allowances, machining allowances, or draft here. It's just something to work with as a start towards what I want to do. I design patterns in my head for simpler stuff, on paper with pencil for more complicated stuff, or as a last resort where I'm having a hard time visualizing probably unwise complications I want to inflict on a casting, in the old free Google SketchUp 7. As I have here.

Note that this is not at all what a pattern will look like, and something this shape would likely be a failure to cast if you tried to. But it is a good picture of the intended shape and dimensions of what I want to end up with.

I decided to simplify the pattern so that one pattern can be used to make both cylinders, even though they vary in both height and the location of the water chamber rings. The casting can be shortened, and the rings turned in where needed. The new shape is on the right, compared to the smaller cylinder of the pair.

I have also increased slightly the dimensions all around to allow for shrinkage, and a machining allowance. I made the central bore smaller for the same reasons, but also, my cores (which will be placed in the mold) are already made up at 2" diameter, so I drew that size. Actually the central finished bore will be 2.4 inches in diameter, but I didn't have a pipe "corebox" closer to that dimension. So I will need to bore a fair amount on the finished cylinders.

I have not yet added any fillets/ chamfers, etc yet.

I now need to add core prints. These are the extensions needed to make a space in the sand mold for the cores, which will in turn create the hole in the casting which is the bore. The core prints are the darker sections below. These will be part of the pattern.

This shape is not an ideal one. Right angles in fairly thick castings have a tendency to create shrink cavities right at the inside corners. It helps to fillet those areas. I will, but I'm not going to draw that in here, because it's a pain in SketchUp, plus I already know that I will add those later after turning the cylinder part of the wood pattern, rather than try to turn in a radius.

Also in general one should determine a parting line and add draft to vertically drawn surfaces, like the flange. But I hope that here I may be able to rap the pattern to give me enough clearance to draw it as a straight section.

Maybe, maybe not. I'll probably decide once I start making the pattern. It's easier for me to get the feel of something like this when I'm holding it in my hand, rather than trying to draw it.

I forgot to mention that the sharp corners on the squared portion could also benefit from being slightly radiused. Sharp corers are harder to mold, requiring more care in drawing the pattern out, though doable if absolutely necessary.

In this case again, I'd do that on the pattern rather than draw it out. I know what I need to do. Well that wouldn't work if you were sending your pattern out to be made, and cast from -- playing strictly the engineer.

The next decision is whether to make this a split pattern or just a single pattern. Or a half pattern.  If a half pattern it will likely be mounted on a match plate. If a split pattern, each half could be mounted on one side of a match plate, and then two castings could be molded at one time (that only works on a symmetrical shape like this one). Or a split pattern can be doweled together with a slip fit to be molded as a loose pattern.

These considerations also may relate to how you're going to form the pattern. If you have a wood lathe, or wood turning capability, you will likely make either a full loose pattern, or split it. But if you are going to form the shape without a lathe, you may decide to go with a half pattern on a match plate, as easier and quicker to make.

For me the best option would be a split pattern on a match plate -- I could mold both cylinders at once. But since I'm "rusty" at iron molding after several years, and with a new furnace and unfamiliar pattern, it would seem wiser to start with trying to pour just one casting. Pouring and gating for two is less certain of a good result, and also why waste metal for two if you need to adjust something?

If I did do a lathe turned pattern, I'd probably do it as a split pattern, which would be molded loose. At least for the present. Later, if desired it could be mounted to a match plate

Now, while I do have a wood lathe, it would have to be used outdoors, and this winter makes that uncertain. I could use my metal lathe as a wood lathe, but that makes a real mess in my very small metal working space, and wood sawdust and fibers get everywhere on the lathe.

The last option is to make a half pattern, and that I could do indoors fairly easily. It's harder to shape and sand than making a whole or split pattern on a lathe, but it's just a little more time and work.

Hmm.... which to do?


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