Could I get tips on dumpster diving for materials for your build?

[Dick_Morris]

Side rods from aluminum? Sure. but it's a trade off. Easier to machine and less reciprocating weight, but more likely to be scratched or bent in the inevitable major derailment and is doesn't retain paint as well. I used a piece of aluminum bar for my American and it works fine (except for the funny color). Allen uses (or did use) cast aluminum. LE did or does use bronze for their rods. With the right grade you don't even have to fit bushings until it's worn to the point that it needs a shopping.

An example of too soft is aluminum for car or tender wheels. It doesn't give a reliable press fit on the axle (Locktite is more reliable) and the flange and tread wear quickly. Can you use it? Sure. The Alaska Live Steamers had a bunch of cast aluminum wheels many years ago on our passenger cars. The truck side frames were also cast aluminum without bushings. In fact, I still have a couple of wheels in my scrap pile to be melted down and used for something else. In light use they allowed us to get started when resources were thin, but their cast iron or steel replacements were much more suitable and the side frames lasted much longer when bronze bushes were fitted for the axle bearings.

A friend initially built a heavy Mikado with wheels cast from plumbing fittings. Too soft and too slippery but they did get him on the track for a couple of years before he had to replace them.

[John Hasler]

I think that if you must interference fit aluminum onto steel you need to shrink it on, not press it.

[NP317]

Aluminum side rods are OK IF you use the correct alloy! Usually a silicon aluminum alloy will provide the machinability and strength required.
A 7000 series aluminum hardened to T6 or T651 can also work. (Did I get those numbers correct?)
6000 series soft aluminum, not so much.
For example, look at internal combustion engine pistons: Silicon Aluminum. Softer alloys would simply melt.

My Allen Ten Wheeler has silicon aluminum (?) main rods (from Allen Models) and mild steel connecting rods.
My Mikado has stainless steel rods. I do not remember the specific alloy but is it was carefully selected for machinability, strength and corrosion resistance.
I asked professional Machinist friends for their advice. Priceless.

I hope this illustrates the complexity of alloy choices, and specifications. Certainly worth doing carefully.
RussN

[Harold_V]

I think that if you must interference fit aluminum onto steel you need to shrink it on, not press it.

Depends. If one has used aged or heat treated aluminum, there's risk of annealing.
Nothing wrong with press fitting aluminum. Just make sure that you provide the proper dimensions for the desired fit, and that the surface finishes are acceptable.

Mind you, I'm not promoting the use of aluminum, but 7075-T6 or T651 rival mild steel in tensile strength.

H

[Steggy]

I'm confused about this one. Already learned first hand that if you try to machine something too hard, you'll put a lot of wear on the cutters and it will take a long time.

Hardness and strength are not the same thing. There are steels, for example, which have elevated strength compared to a basic low carbon steel such as C1010, but are still comfortably machined with high speed steel tooling. On the other hand, there are steels such as 304SS which are no stronger than a medium carbon steel, such as C1045, but are tough on cutters.

The mechanical properties of steel are a function of carbon content, presence of alloying agents (especially chromium), the process used to finish the stock to size (cold working vs. hot rolling), level of impurities, etc. As a machinist, you need to have some understanding of metallurgy in order to choose the right tooling and procedure to make the finished part. If the drawing doesn't specify the exact material then you need to know enough about metallurgy to select an appropriate metal.

But you say I could pick something too soft and it will bend? What are you talking about? If I made side rods out of aluminum?

I'm talking about using steel with unknown mechanical properties in an application that demands a combination of strength and fatigue resistance. Rods are an example of such an application, as they see cyclic compression and tension forces, and some incidental bending. A rod is a compromise between strength, fatigue and mass—however, hardness is rarely a requirement. If you want to minimize a rod's mass so it doesn't heavily stress mating parts when your loco is highballing down the track you can't make the rod out of unknown steel retrieved from someone's scrap barrel.