patterns and shrinkage

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hwboivin3

patterns and shrinkage

Post by hwboivin3 »

Hello,

I've been looking through my collection of foundry books looking for info on shrinkage.


I'm looking to make some drive wheel patterns.

Which direction does the cast iron shrink at the tread? I know the o.d. shrinks, but does the i.d. grow?

I have them drawn up 3d, but my thinking is that simply scaling up the shrink rate is not the correct way.



Just curious......and if anyone knows a pattern maker, that would be even better!
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Harold_V
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Re: patterns and shrinkage

Post by Harold_V »

hwboivin3 wrote: I know the o.d. shrinks, but does the i.d. grow?
No, it doesn't. Think of shrink fitting a sleeve to a shaft. The shaft can remain room temp, with the sleeve heated. It will expand in all directions, and shrink in all directions upon cooling.

Harold
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RONALD
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Re: patterns and shrinkage

Post by RONALD »

If I read you correctly, you want to allow a hole in the center of your pattern.

I would not bother with that. For our size you would be off better having no hole and then boring in a lathe.

In the photo below, you will notice my patterns have no holes.

These are match plates, and I attached the sprue right in the middle, on the back, and poured straight down into the mold.

If you meant the inside diameter of the tread, at ~ 8"s, I would not worry about shrinkage. It will of course decrease the inside diameter, but no one will notice.
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hwboivin3

Re: patterns and shrinkage

Post by hwboivin3 »

Thanks,

I was thinking that if it shrinks in all directions, that the tread ring would shrink towards the center of the ring.

So, I should be able to scale up my 3d model for 1/8 to the foot and add stock on the machined surfaces.....1/16" ??


Also, I'm not a woodworker.....how's aluminum for a pattern?
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steamin10
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Re: patterns and shrinkage

Post by steamin10 »

In model work, you can get away with any number of materials for patterns. Some soft, like styrofoam, to Lucite, wood, body putty, waxes, or broken original parts.

Broken parts are dipped in shellac, or wax to build some thickness, and while they may give a blurry cast without sharpness, all the areas to be machined will have the stock needed for a good finish. (Like a wheel).

Remember a casting is a short cut to fabrication, and lowers the cost in time and labor. In theory, the larger the casting the more savings. Large locomotives had frames completely cast in one major base piece, for that reason. Many examples exist for cylinder blocks and the like for intricate shapes, that eliminate assembly and machine time.
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RONALD
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Re: patterns and shrinkage

Post by RONALD »

Yes, if you want to be a "Rivet Counter", every dimension should be scaled for shrink, but the only one really important is to allow enough material on those surfaces that will be machined; hard to add metal to a casting after it is cast.

Wood, plastic, metal, etc., have been used to make patterns. Most use wood which is easiest to work with. Be sure to add enough draft so that the pattern can be removed from your mold.

If you want to avoid draft, you can make your patterns out of Styrofoam. Of course you only get one casting out of each pattern because they are burned out in that method.

In the photo below, all castings were made from Styrofoam patterns.
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Lost Foam047_2.jpg
jkimberln
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Re: patterns and shrinkage

Post by jkimberln »

hwboivin3 wrote: Just curious......and if anyone knows a pattern maker, that would be even better!
I'd vote for Gary Martin at http://www.martinmodel.com/ I've purchased a lot of castings from Gary and both the rendition and the iron are both just fine.

If you do make patterns, consider just making wheel centers. The resulting cast iron with spokes, etc., can be turned then treads can be made from DOM steel tubing which will wear a lot better in use.

Further, you can now have molds made via 3d printing of resin bonded sand, precluding patterns altogether. It actually works fine and I've seen the results. A friend of mine at GGLS did drivers this way and they turned out good. The spokes are true shape, etc. etc. With printed patterns you don't need to put in draft, just shrinkage.

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Hopefuldave
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Re: patterns and shrinkage

Post by Hopefuldave »

Easiest way is to imagine it scales down by the shrinkage,so if it shrinks 1/8" per foot, that applies to the whole casting - so your pattern needs ALL its dimensions scaled up (including the inside of holes) by the same factor, 97/96ths... This is surprisingly easy to do in most CAD software! Then add your machining allowance, 1/16" minimum on alll machined surfaces (so if you need to machine the inside of a rim, it needs 1/16 SMALLER inside diameter).
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hwboivin3

Re: patterns and shrinkage

Post by hwboivin3 »

Thanks for the help.

I will contact Martin Patterns.

I don't want to do the 3d printed mold because I prefer pattern boards. I am machining my wheels on my cnc mill but want to look into patterns to make more of them if anyone were to want to build the engine and 3d molds don't seem to be a good production method.


Thanks again
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steamin10
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Re: patterns and shrinkage

Post by steamin10 »

If you are capable of machining a locomotive wheel, then machining pvc, or abs plastic masters would be an easy project.

If it is a commonly built engine, then there are probably wheels already in stock. It makes no sense to re-re-create what has already been done and just buy them.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
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hwboivin3

Re: patterns and shrinkage

Post by hwboivin3 »

Dave,

To my knowledge, these aren't available. It's not a commonly built engine.

I thought about plastic, but I have a lot of aluminum laying around the shop.
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steamin10
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Re: patterns and shrinkage

Post by steamin10 »

What size and what do they look like? I have several types of early wheel patterns on tap here. Mostly civil war era high steppers.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.
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