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31
The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 31, 2025, 10:46:51 PM »
There is a difference though.

Aluminum expansion with temperature is several times as much as cast iron's. When an aluminum piston works in a cast iron cylinder the piston clearance reduces with temperature. When a cast iron piston works in an aluminum cylinder, the piston clearance increases with temperature.

I would expect that in any case, the aluminum alloy used for a piston has a relatively low thermal expansion coefficient -- for aluminum, so it would still be a good idea to try to use that material as a cylinder in this experiment.

Also, the use I have in mind probably means the piston is operating at higher temperature than the cylinder, since the cylinder is a cooling heat exchanger for a hot air engine. So that might offset to some extent their different material rates of expansion.


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The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 31, 2025, 10:59:17 AM »
Thanks Bill, good suggestion.  :beer: I have automotive pistons I can use as the melting stock.
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New from Old / Re: Eurospark H425 Die Sinker EDM reborn
« Last post by BillTodd on May 31, 2025, 05:09:15 AM »
Just resurrecting this to say that hthey are scrapping a couple of Ingersoll centre 400s at my old works and wondering of there are any parts that might be useful for  other sparkers ?


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The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by BillTodd on May 30, 2025, 12:44:10 PM »
I can't see why it wouldn't  work . There are plenty of aluminium pistons running in cast iron barrels.

The piston thinks the barrel  is moving most of the time .

Your barrel would probably best be made of a suitable piston alloy .
35
The Design Shop / Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 30, 2025, 10:37:08 AM »
Has anyone used or heard of a cast iron piston running in an aluminum cylinder? No, not Nikasil or chrome plated.

This would be for a model sized hot air engine (say 1", 25mm  bore), run dry, not oiled. Depending only on the natural aluminum oxide formation to harden the cylinder interior, and the graphite of the cast iron for lubrication. High degree of polish for both to start with. No rings, of course.

Natural inclination would be to guess issues might be galling, and differential expansion due to heat opening up too much clearance.

But maybe not.......?

36
Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by vtsteam on May 30, 2025, 10:11:08 AM »
I'm not convinced that it doesn't work as drawn. But if it doesn't, it would be instructive to make the changes to get it running. Part of the fun actually, and a place to learn things.

I will give one interesting side note (clue). Robinson engines used a perforated displacer with regenerative material inside. The Westbury design does not, and E.T.W. seems not to have realized this, and had drawn a sealed hollow conventional displacer.

Still, I believe it should work that way, as well.
37
Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by nickle on May 30, 2025, 04:24:39 AM »
Isn't that design in the book? that would be a shame as i was eying that build off for a some day project.
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Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by vtsteam on May 29, 2025, 09:41:25 PM »
Hi Nickle!  :beer: I do have Westbury's treatise, also a lot of old Model Engineers, which I've collected over the years. Coincidentally I have just been reading yesterday about Westbury's last engine, a Robinson type, published a couple months before he passed away. And a then an editor's note, about a year later:, 

  [ You are not allowed to view attachments ]  

I find this not only a little sad, but also somewhat mysterious, and it makes me want to build that engine just to see for myself whether this is true, or not.
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Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by nickle on May 29, 2025, 07:07:19 PM »
They are fascinating things... I still have not built one despite them being the main reason for buying my first lathe 20 years ago! I have the beginnings of a fizgig on the shelf in my shed. The surface area for heat exchange thing is an interesting aspect of Stirling Engine design... I wonder if some sort of ribbed displacer cylinder would help... but then you get more drag from a larger surface area and the thermal mass of the cylinder can become an issue. As you say, there are so many variables. Aside from the ability to use low energy density fuel, I think one of the most interesting aspects of these engines is the ability to use waste heat. there are endless industrial and domestic processes that have heat disposal as a problem... including a Stirling cycle in that process allows some energy extraction to happen. It wont always be optimal temperature differentials between the hot and cold sides, but its waste heat so efficiency doesn't have to be the main driver. Have you got a copy of Edgar Westbury's "A Practical Treatise on Hot Air Engines"?
40
Software Tools / Re: Playing with Blender For Artists (Bforartists) - Easier way to learn Blender?
« Last post by sorveltaja on May 28, 2025, 11:08:24 PM »
After taking closer look at the bones system, objects are linked to bones, so they just follow their parents. It appears that there doesn't need to be surface reaction related factors involved. Also it seems to provide more accurate movements, when compared to rigid bodies, where there may be some lag between objects.

Although it's very 'clean' way to make objects move, I find rigid body simulation more tempting at this point.

An example, where rotating purple frame is the animated object:



3D files (STL) and instructions for the mechanism that I used, are available at https://www.instructables.com/Antikythera-Orrery-3d-Printed/
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