The Shop > Metal Stuff

Making Patterns for a Rider Engine

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vtsteam:
Time for sanding sealer. I make my own. Lacquer, talcum powder, thinned with a small amount of lacquer thinner.

I need the real talcum powder with talc in it. Not cornstarch. Important to check the label on that. For amounts, I just mix it in a small jar -- I put maybe a heaping tablespoon in about 8 oz of lacquer -- not critical, because the talcum powder will settle out so you have to shake it once in awhile to mix it in again. Whatever amount it wants to suspend is fine.

I add a small amount of thinner sometimes so it brushes easily, not too thick or drippy, and dries quickly. We're talking like 15 minutes. BTW I do this outdoors in summer or in an unoccupied shop or garage with plenty of proper ventilation. and no open flames or hot elements. The vapors are both harmful and very flammable.:zap:

vtsteam:
I've brushed two coats of sanding sealer on, and sanded lightly with 320 grit paper, then sprayed on a light coat of Krylon (tm) yellow over. This gives me a good chance to look over the pattern for flaws and make any modifications before final painting.

After I brush on each sanding sealer coat, I wrap my cheap disposable bristle brush in aluminum foil -- that way it can be used again without having to clean it out in lacquer thinner -- avoiding both fumes and expense. Generally you can paint, then sand, and then re-coat all within an hour with lacquer.

I use Krylon for my color coat (not strictly necessary) because the standard Krylon is lacquer based and compatible with the lacquer sanding sealer. It also dries in about 15 minutes. If I tried to paint with an oil based enamel like Rustoleum (tm) I'd be waiting a long time, and may have compatibility problems if the sealer is not absolutely hard and dry. Nothing like ruining a finish with orange peel after you've worked hard to make the pattern and undercoat as smooth as you can.

Now, looking at the pattern halves in real life I'm feeling that I should add some draft to the flange edges. There's plenty of extra material for a machining allowance there. I could also taper the flange thickness, but not sure I want t do that because then it makes it difficult to mount on a faceplate. It would have to be faced first with a 3 jaw on the cylinder and a steady rest, etc.

I dunno, we'll see. Anyway, tapering the edges for now.

vtsteam:
Okay, 1/32" of draft taper (1 degree) was added to the edges of the flange. This was done easily by setting the table of my little disk sander to that angle and just running the edges of the flange against it until it started to take paint off the bottom of the bevel.

A belt sander (linisher) or disk sander is indispensable for adding draft to patterns, and for forming shapes like disks after rough cutting them with a saw. You just mark out the shape on your rough cut blank and sand to your outline. If you have set the sanding table to the draft angle, then draft is added automatically as you shape the pattern, even on curves.

vtsteam:
The finished pattern. I had a little trouble with the yellow Krylon I'd sprayed it with before. It wouldn't dry fully over the sanding sealer. Not sure why, it was an old can and I dunno, they seem to keep changing their formulations. That one was labeled "Indoor/Outdoor".

I had a can of Krylon "Fusion" in Navy blue, so I tried that and it dried well, so that's the color we've ended up with. In my earlier casting days, I sometimes would paint patterns for aluminum castings yellow, and those for iron black. But I never was very consistent about it. I had lots of John Deere green patterns, too. Besides simple patterns get re--used, often with a variety of metals so coloring for type doesn't make sense.

I did once look up what "real" foundries used, and interestingly, though there was a US joint standard published in 1932 where all unfinished surfaces were to be painted black, machined surfaces red, core prints and seats for loose core prints yellow. Loose pieces and their seats were to be painted yellow with red stripes. And stop-offs were yellow with black stripes.

On the other hand I've seen commercial patterns using those same colors, but switched around, so that unfinished surfaces were red, core prints black, etc.

Blue apparently is way out in left field, but there you are.......

Now, is this a good pattern?
Well, in order not to mislead, um, not totally....: it doesn't have draft on all vertical surfaces, so I'll be depending on it separating properly and then on my rapping and flask lifting skills (not unquestionable!) Also being of a light hardwood, it's kind of heavy compared to a pine pattern, so the cope half of the pattern might drop out.

Finally, the design, with a very heavy flange section at right angles to the main cylinder looks like it's asking for trouble with a shrink cavity, or tear on the inside corner. In aluminum, it would be pretty likely unless you find some way to feed it as it cools. I'm not sure about iron, since it shrinks about half as much as aluminum. It might be okay. Zinc shrinks even more than Al. This one is intended for iron, of course. Anyway we'll find out.

A smarter or at least easier design would just be to cast a plain cylinder and machine a fitted flange after. But there's always the temptation in casting to include shaped features and reduce machining time and numbers of parts in an assembly. It will be interesting to see how this casts in iron.

awemawson:
Looking smart in blue Steve :thumbup:

I'd be very tempted to feed that flange, once you deduct the core volume it's a significant proportion of the whole casting.

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