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Discussion Starter · #1 ·
The 108 was a saddle tank Mallett loco originally built for Potlatch Lumber Co. and later sold to Weyerhaeuser
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It went through several changes while owned by Weyerhaeuser
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I don't think I will be modeling from the last photo
While I was making the cylinders, Dennis was making the frame rails and axle boxes on the CNC mill
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The cylinders are made in similar fashion to my other builds. The high pressure cylinders have a .625" bore and the Low pressure are .813 the stroke is .900". I am using slide valves with cross port plates to give it a piston valve look.

I make crossmembers for the frames, assemble and bead blast them and then paint them black
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I will post more photos as I go along. If anyone wants any photos of things before I assemble things, let me know
 

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That's a very unique looking loco! High and low pressure cylinders - how do you determine cylinder size? Looks Fantastic so far!!
 

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Discussion Starter · #3 ·
Thanks Pete
I usually scale down the bore of the prototype and make mine as close as practical. I tend to go with larger bores than normally used by the manufactures which I feel are mostly undersized. A misconception is that a cylinder with twice the cross section will use twice the steam. I have found this not to be true at all, as the larger cylinder will do the same work with half the pressure. My ceramic burner boilers produce a lot of steam so consumption isn't really a problem.

On this one, the high pressure cylinder scales down to almost exactly 5/8" so that is what I am using. The low pressure cylinder can't be scaled down as the original compound engines ran on 200 psi to 300 psi and at our low pressures, the ratios of 3 to 1 cross section won't work. I was able to get a copy of an article by JVR who helped design the Aster compounds. Through his experimenting, he determined that the ratio between the cross sections of the HP & LP cylinders should be 1:1.7 so that is what I have been successfully using.

Taking a closer look at some parts, you can see the steam ports on the LP cylinder. They are oval shaped to get a free breathing cylinder.
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The cross port plate transfers the front port to the rear and vise versa
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I get the rough wheel castings from Walsall in GB
I use several lathe settings to get the wheels turned to just over spec. Then on Dennis' 36" lathe we do the plunge cut to get the proper tread profile. The next thing is to drill the holes for the crank pins. These have to be all perfectly matched so I use this jig I made up which just has a piece of the axle material set in a block of brass. It is then mounted in the mill vice. the mill is centered on the axle shaft and the mill is moved over 1/2 the cylinder stroke.
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Then each wheel is set in place and drilled and tapped
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Counterweights are milled out of steel flat stock and attached with JB Weld
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And they are primed ready for painting
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The side rods consist of two sections which are joined together with a tongue and groove pivot joint. The groove is cut out on the mill with a 1/16" slitting saw. The stainless needs to be cut at only 350 rpm to keep the blade from overheating.
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The tongue part is done with an end mill and a stainless pin is tuened from 3/16" stock to 1/8" with a .025" wide x .016" deep groove to accommodate an e-clip
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Thanks for the sizing ratio information, I wasn't aware it existed. That cylinder arrangement looks similar to that used on the G1MRA "Project" I built. Interesting that you are using stainless, what grade? How does it machine?
 

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Discussion Starter · #5 ·
I only use 303 stainless
It machines very freely
I have ruined too many end mills on the other grades of stainless
It isn't available in all shapes but enough for most applications

On my crosshead guides which are 1/8 square stock, I use tool steal as 303 isn't available in that size. It polishes up nicely and is fairly rust resistant
 

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Fortunately the real 108 is looking much happier these days than she did in that last picture. She has been restored to steam on the Black Hills Central Railroad in South Dakota, along with sister engine 110.
 

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Discussion Starter · #7 ·
I made the slide valves this morning and lapped them and the cross plate valve port side.
I start the plate on the bench belt sander to get it flat and then lay sand paper on a glass pane and sand un one direction with various grits starting with 220 and then 320, 400, 600, 800, 1500, and 2000

Here is the 220 pass. I go at different directions each time so i know that I have removed all of the scratches from the prior pass
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Then 320
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Here is the 1500 pass

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And the 2000 ones with the slide valves. The final pass is done so the scratches are parallel to the slots
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