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1,517 Posts
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

1,517 Posts
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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1,517 Posts
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

1,517 Posts
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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1,517 Posts
Discussion Starter · #8 ·
I quarter the wheels on my home made jig
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The screws for the crank pins need to be home made also, I use 1/4" 303 stainless Hex and thread it to 4-40
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Here is the crank pin with the screw in it,
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And the frames with the wheels and side rods installed.
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dennis cut the crosshead brackets on the CNC mill and I bead blasted them. They still have some brackets to be silver soldered and some to be bolted on.
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The cylinder head covers are turned from aluminum stock. They are 1/16" larger than the head
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you can see where the front cylinder cover covers the head but the rear cylinder just covers the head bolts
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I use hardwood strips which are shaped on the belt sander to get the outside shape I want to get the cast piston valve look
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The faux piston valve covers are installed and the cylinder is painted
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1,517 Posts
Discussion Starter · #9 ·
It has been two months since my last post. A lot has been going on at home so I haven't been building at my regular speed but here is my progress to date. It will probably take more than one post because of the ten photo rule.

This photo shows how I mount my cylinders. I turn the spacers from stainless rod and mount them with 4-40 stainless screws. The Stainless slows the heat loss from the cylinders to the frame.
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And here they are mounted.
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1/8" square strips are drilled out and silver soldered to the crosshead bracket for mounting on the frame. A block of foundry brick is cut for this purpose.
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This makes the strips square and perpendicular to the bracket but to get them 100% correct, they are finish milled on the mill.
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1/16 x 1/8" tool steel strips are silver soldered into the slots for the crosshead guide mounts.
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The crossheads are milled from 1/4" brass stock and later will be tin plated. The front and rear engines have different size crossheads.

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The valve stem and combination lever take some extra care. A 1/8" square piece is soldered on to the 3/32" stainless rod. It is then slit with a 1/16" slitting saw on the mill to form a fork.
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Because of the close proximity of the valve stem and radius rod holes on the combination lever, the fork needs to be trimmed on the 1" belt sander.
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Here is the expansion link with its double ball bearing pivot shaft and the radius rod connected. You can see where the radius rod is trimmed down matching the valve stem fork.

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This shows the valve gear with all of the pieces in place.
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1,517 Posts
Discussion Starter · #11 ·
Thanks Russell
This is kind of a 2 0f 2 so the prior post should lead up to this one.
Here is how I trim the valve rods
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The lifting lever for reverse needs to go around the expansion link. To do this. I solver solder a pin onto the reverse arm.
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It then works like this
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Here is a shot of the underside of the axle pump. It is also used as the mount for the rear truck. The two banjo fittings which are not fitted with the piping yet keep the truck from rotating too far, left or right. There is a spring washer under the truck tongue which keeps the wheels on the track.
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here is the top view of the pump with the ball bearing eccentric and scotch yoke follower
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Here are the automatic cylinder drains I use
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A top view of the piping. On the left side is the lubricator line and adjusting valve, The compound starting valve and adjuster are on the right side.
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1,517 Posts
Discussion Starter · #14 ·
It is a socket head SS 2-56 x 1/8" screw The head diameter is .136" and I design the slot for this size. I screw it into the radius rod and then sand off the back where it protrudes till it is flush before assembly to get the right length.

1,517 Posts
Discussion Starter · #16 ·
the boiler mounts are one at the rear under the firebox and one above the front driver of the rear engine. These provide a fixed attachment of the boiler to the rear chassis. The front chassis needs to sway left to right as it goes around curves. To do this, we need a slotted mounting plate with a curved slot. The radius of the slot is the measurement from the front driver of the rear chassis and the plate. This slot can be milled with a jig made for that purpose but a simpler way is to draw out the curve and on CAD measure the deflection every .050". Plunge cuts are then done with a 1/4" end mill and the ridges left are easily removed with a needle file.
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Mounts are made the same height about 1/2" lower that the boiler mount will be and and spacers will be use to get the proper front/rear balance.
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The outer boiler shell is cut to length and the firebox sides are cut out. the bronze boiler bushings are turned out and the corresponding holes are drilled in the boiler shell.
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This shows the filler plug, the upper sight glass fitting and the auxiliary steam fitting. Below that is the steam dome fitting.
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On the bottom side are the two boiler mounts and the lower sight glass fitting.
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Here is a photo of the underneath of the two chassis,
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And a closeup of the coupler between the two chassis. Not shown id the silicone rubber spacer cut from tubing which gives it a little give for minute changes when going around curves and through switches.
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Here is a look at the axle springs and journals. The ball bearing does not show as it is on the wheel side of the journal
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1,517 Posts
Discussion Starter · #17 ·
I am waiting for the flue tubes to arrive, Here I am turning the front tube sheet on the lathe. I center pop a hole and then use a compass to draw a circle 1/16" larger than the plate will be. I cut the shape on the band saw and drill out the center hole to 5/32" for my arbor. The copper tends to flatten out where it is turned but it is cleaned up with a file before removing from the lathe
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From my CAD drawing I center pop the flue holes and then center drill them. i hold the piece with a vice grip to prevent it frpm being spun by the drill.
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I then drill out the holes with a step drill and clean them up with a reamer
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Here is the throat sheet and front tube sheet ready for soldering. you can see where the center hole has been closed by silver soldering a copper peg in place.
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1,517 Posts
Discussion Starter · #18 ·
The tubes are silver soldered to the throat sheet first. The tubes are held in place by setting them in the front tube sheet and the throat sheet. Ithen wrap 1/32" solder around a 5/16" rod to get 3/8" rings which I place around each tube in the inside and then heat the outside till the solder comes through.
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This assembly is then soldered into the boiler shell
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And then pickled in citric acid.
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The front tube sheet is then soldered in place
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The crown sheet is cut to size and the fit is checked.
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The baxkhead is cut and the holes are drilled. The three at the top are for the throttle. The one on the right is for th clack valve and the one on the left is the superheater one. The square one is for the mica window.
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