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Model brake component size comparison to prototype


Paul Woods <paul@...>
 




An example of the casting work I have done - O-scale freight truck with functioning equalisation - and genuine American black walnut bolster.  I had expected to find some shrinkage in the axle-boxes because they are a big lump compared to the arch-bars but they were fine, all portions of the sideframes turned out at intended size.

Regards
Paul


Dennis Storzek
 

On Sun, Apr 5, 2020 at 09:04 PM, Tony Thompson wrote:
  Richard Hendrickson did the same to get some durable bracket-mount grabs. I still have a couple dozen of his parts, very nice brass. Don't know what reject rate occurred.
Those may well be my parts, I intended to sell part of the run to recoup some of my cost, but when Richard heard about it, he bought all I wanted to sell.

Dennis Storzek


Dennis Storzek
 

A couple of things are not being accounted for in this discussion. Yes, metal shrinks as it solidifies, as does most things, water excepted. But in the investment casting process, the molds are at somewhere around 1000 deg. F when the metal is poured, so thermal expansion has made the molds bigger.

Shrinkage does not have to be equal in all directions; if the part is constrained as it solidifies, most of the shrinkage will be in the thickness of the part. An example would be painting your house. Paint shrinks as it cures, but that doesn't mean that you house gets smaller as it dries. Since the house is sturdier than the shrinkage forces, 100% of the shrinkage is in the thickness of the paint film.

The big problem calculating size with the traditional lost wax process is accounting for the size change in all the steps; the master is used to make a rubber mold, and traditionally it wasn't RTV, but some heat cure rubber, which shrinks. Wax is poured into the mold, and it shrinks. The wax part is then used to make a plaster mold, which is then heated to burn out the wax and drive any moisture out of the plaster, so it doesn't flash to steam when the metal is poured, causing voids, or worst case, causing the mold to burst open. Finally, the metal shrinks as it cools. The shrink rate of each of these steps is only an estimation unless the job is such a long run that actual data was tracked for the purpose of adjusting the final part size. That just doesn't happen in model railroading. 

Another story, told to my by a professional modelmaker who was a friend of Bill Clouser:

Clouser modeled in 1/4" scale using the prototype track gauge, what we now call P:48. One of the nicest freightcar trucks available in those days was made by Carl Auel (sp?), but it was five foot gauge. Bill took one of the bolsters and used ti as a pattern, had enough cast in brass to provide multiple patterns to fill the typical spin casting mold, and then used that mold for waxes to have parts cast for himself and some friends who also modeled to exact scale. The resulting parts were 5% shorter than the original, which corrected the width of the truck.

Dennis Storzek


Tim O'Connor
 


I have some too - I'd forgotten where they come from. I only have enough for
one or two cars, but now there are the Kadees, so...


On 4/6/2020 12:04 AM, Tony Thompson wrote:
Dennis Storzek wrote:

I've only ever done one investment cast project; I wanted some bracket grabs that were more robust than the typical styrene parts, so back when Intermountain was still molding parts here in the US, I bought a hundred detail sprues from their PS-1, clipped the runner with the grabs out of them, and sent them out as investment masters. I was warned about the potential for ash to plug the long narrow cavity, and indeed, the reject rate was someplace around 20%, but the properly filled parts were nice.

    Richard Hendrickson did the same to get some durable bracket-mount grabs. I still have a couple dozen of his parts, very nice brass. Don't know what reject rate occurred.

Tony Thompson


--
Tim O'Connor
Sterling, Massachusetts


spsalso
 

Paul,

And what did you think of MY tone?

Are you saying metal, in its solid form, does not shrink if it is cooled from 900 C down to 25 C?  And would that shrinking, if it does indeed happen, not be in line with what we call the "coefficient of thermal expansion"?

Brass solidifies at around 920 C.  So when the part gets to (around) 920 C, it will become a solid.  IF there is a huge liquid reservoir STILL pressing down on the part, it will indeed compress the part.  Unfortunately for your argument, the amount of compression is literally microscopic--as in, close to immeasurable.  And please note that, when the part is no longer being compressed, it will spring back to its former size.  Also, please note that all this springing back and forth STILL occurs at high temperature--the part has a long way to go to cool down to room temperature, and the shrinkage described in the previous paragraph will STILL happen.

Put more directly, I am saying you are wrong.  Again.  And I look forward to your attempt at disproving the concept of coefficient of thermal expansion.


Ed

Edward Sutorik


On Mon, Apr 6, 2020 at 04:22 AM, Paul Woods wrote:
Tony

I am greatly insulted by your tone and it appears that you have not put any thought into your comment.  Seriously?  You are trying to tell us that ALL the metal in a casting cools to freezing point at exactly the same moment?  You might know a lot about metallurgy but I know a bit about building things from metal.  I hadn't bothered mentioning that the brass caster's tale matched my own experience gained building steel ships because it didn't seem important, but fine: the metal might eventually shrink equally, but not all at the same time because cooling happens from the outside in, and this can be used to our advantage.  I have dealt with plenty of  castings, such as A-frames for supporting ships' propeller shafts, with significant variations in section size.  The orientation of the casting as it is poured is quite important, with the thicker part intentionally placed above the thinner part so that it acts as a reservoir keeping the thin section filled as it cools....as a metallurgist you should be aware of the high density of metals such as steel and brass, and thus the pressure that will be exerted that can keep pushing semi-molten metal down towards the bottom of a mould when it is cooled slowly enough - that is the key, achieving a slow enough rate of cooling.  You can't tell me it doesn't work because I've already done it; yeah sure, the metal shrinks but if it's done right it is possible to control WHERE the shrinkage occurs.  The column that lost-wax brass parts are stuck to when they are cast serves as one heck of a big reservoir of molten metal as the 'branches' are freezing so I don't have any reason to doubt what the caster said.

Paul Woods
retired mechanical engineer


Tim O'Connor
 

Please take this dispute off line.

Thank you.

On 4/6/2020 11:43 AM, spsalso via groups.io wrote:
Paul,

And what did you think of MY tone?
--
*Tim O'Connor*
*Sterling, Massachusetts*


Tony Thompson
 

Paul Woods wrote:

I am greatly insulted by your tone and it appears that you have not put any thought into your comment.  Seriously? 

       Yes, I was blunt. Not any thought? How's your tone yourself?

You are trying to tell us that ALL the metal in a casting cools to freezing point at exactly the same moment?  

      No, I didn't, nor does it matter.

the metal might eventually shrink equally, but not all at the same time because cooling happens from the outside in, and this can be used to our advantage. 

      Ah, you do know that it shrinks equally. This discussion began with the question of how much lost-wax patterns are made oversize to account for this shrinkage. It most certainly IS the entire shrinkage that is relevant to this topic.

....as a metallurgist you should be aware of the high density of metals such as steel and brass, and thus the pressure that will be exerted that can keep pushing semi-molten metal down towards the bottom of a mould when it is cooled slowly enough

     Mold filling is NOT the issue: important, of course, but not the issue here. So on this point, what you say is entirely true, but irrelevant.

Tony Thompson




Tony Thompson
 

I wrote:
  Richard Hendrickson did the same to get some durable bracket-mount grabs. I still have a couple dozen of his parts, very nice brass. Don't know what reject rate occurred.
Dennis Storzek replied: 
Those may well be my parts, I intended to sell part of the run to recoup some of my cost, but when Richard heard about it, he bought all I wanted to sell.

          Likely true. He described the process by which they were made, but didn't ever say, that I recall, that HE had gotten it done.
          The parts are lovely. I am slowly using them.

Tony Thompson




Paul Woods <paul@...>
 

Dennis: Thank you for that very succinct explanation.  I believe it supports if not outright proves that what I said was not nonsense. 

Ed:  There is a vast difference between theoretical principles and real-world applications.  I think I was being fair in objecting to Tony's manner of writing, which was unnecessarily harsh, especially in light of what Dennis has said.  What about your tone indeed!  There are nicer ways of saying that you think someone is wrong, and 'tis easier to remove one's foot from one's mouth when it has not already been shoved all the way in ;-).

Moderator:  Don't bother excommunicating me from this group; I am already leaving voluntarily as I don't need the unpleasantness of being attacked like this.  When I read something that I think is wrong, I first ask, "Excuse me but I thought it was actually ____.  How have you arrived at your facts?"  To get a whole lot of "You're wrong!  That's nonsense!" fresh out of the blocks is not the behaviour of gentlemen.

Regards
Paul