Work has been slowly progressing on the Class 56. I've been focused on building the tender to get it out of the way. I've also started machining the driver centers and fitting the tires.
For riveting on this project I'm using a technique/tooling that I haven't seen before. I turned a rivet profile tool out of tool steel on the lathe and chucked it in the cnc mill. I mount the part to be cut out to an MDF carrier and create a toolpath in my cam software to punch the rivets, drill holes and finally cut out the part. I'm doing the tender wrapper in .005" thick brass sheet and the punch code in CAM does a -.0025" travel in the z axis to punch the rivets half way. This process works extremely well with accurate rivet spacing and much better alignment than I could achieve by hand.
Here's a shot of the cnc mill punching the rivets for the tender top. Notice how closely spaced the rivets are! This would be near impossible without a special jig and a TON of time to do this by hand/manually.
Once the "punch" operation is complete, I next fit the appropriate drill in the mill to drill the holes for the mounting brackets, hand grabs, tender fill etc. Finally, a 1mm diameter end mill is chucked and the mill cuts out the part needed. I like the results better than photo etching the parts. For one, you get a proper profile to the rivet rather than an etched cylinder look from etching. Second, you don't get the undercut/sharp edges from the photo etching process. Third, you have a clean brass surface rather than the matte etched surface. Fourth, you don't have to wait weeks to get the parts!
Here's a shot of all the major tender wrapper components after being processed on the mill.
The tender base (second from the left) was cut out of .010" thick brass to provide more structural support to the base and provide a good sturdy substrate to solder the sides to.
One of the most challenging pieces of the tender is the flare at the top. To model this, I made a 3D model of the tender and modeled the flare at the correct angle and profile. I exported this 3d surface to a program used to make paper models from 3D models called Pepakura. The software flattens a 3d model and creates a pattern to cut out. Here's a shot of the paper pattern before being cut from brass.
The software adds tabs and a fold/bend line where the tabs can be folded. Figuring this pattern out by hand or even in cad alone would be a very difficult task! By modeling it in 3D and using the paper model software, I was able to create this pattern in about 20 minutes!
The tender frame was fabricated using brass "C" channel with the end beams cut from .060" thick brass on the cnc mill.
The center beam is a brass "I" beam or H profile beam. The plates on the ends were cut from .005" thick brass sheet with tiny stainless steel rivets welded in. The side "C" channels had the open side facing the center of the frame and I cut .005" thick strips to enclose the open side of the channel. This makes the sides look like they're solid when in fact they're hollow. Precision Scale NBW's were added along the side frame pieces in cnc drilled holes.
On the under-side of the frame, I turned truck bolster bearing plates on the lathe and soldered them on the bolster straps.
In the above shot, you can see the floor boards for the tender. I cut each one from brass strips which are extremely close to the thickness they should be on the prototype. They were grained with the edge of a razor saw to give the effect of wood grain.
The following shots show the progress of soldering up the tender wrapper components.
The bends on the engine side were carefully rolled to match the curve of the base plate and top.
The above shot shows the major pieces fo the tender in place and soldered up.
Above is the flare in place. Note that beading was added around the top of the flare! This is thin gauge brass wire used in jewelry making that I picked up at the local craft store. Soldering this thin wire around the compound curves of the flare was a challenge to say the least. This process would not be possible (without a lot of swearing) without a resistance soldering unit with nice tweezer style tips!
Above is a close up shot of the compound curves in the flare. You can see that it flares out in both directions while wrapping around the curve!
The tender floor boards were soldered on after being grained on both sides. I spaced them as I've seen in some photos of these engines. The spacing didn't appear to be consistent on the prototype and so I replicated that on the model.
Shot from below:
In the bottom left you can see 2 round discs with NBW's on them. These are the ends of the tender air tank. Rather than buy a casting, I decided to make these per the prototype photos I have.
Here are a few tips;
- You'll note that I have the tender sitting on a piece of shiny metal. This is a slab of 1/4" thick T6 aluminum. When soldering on a major assembly like this, I like to keep the model on a slab of aluminum. The aluminum wicks the heat away from the model quickly. I also use this slab of aluminum when soldering in general and especially on tiny pieces. I'll solder on a piece of wood and then quickly move the part to the aluminum to cool it down.
Here are a few shots with some tips on building an air tank.
Soldering up a wrapper in a cylinder form can be a challenge. I calculated the wrapper dimensions in cad by checking the circumference of the tank diameter. I punched the rivets using the same cnc technique as above on the major tender sides. I turned a solid brass cylinder the inside diameter of the tank. Next I wrapped the rivetted wrapper around the cylinder and used steel floral wire to tightly secure the wrapper to the cylinder.
This is an extremely useful trick! Twist the steel wire down to tighten the wrapper to the inner cylinder. This same technique is useful for boiler wrappers as well. Rather than use a solid brass cylinder inside, you can use rings turned on the lathe or heavier gauge brass tube or use a ferrous metal like steel or non-ferrous aluminum (basically anything you can't solder to) as an inner form while soldering the wrapper.
Once the part is soldered, you can remove the wire and clean up the part.
Finally, a quick shot of one of the driver centers chucked in the lathe for processing. I machined a fixture to hold the center while machining. These centers were bought from Bill Meredith from the Leadsville Shops. Beautiful castings!
The fixture is basically a brass rod turned down slightly smaller than the inside diameter of the tire with 2 holes drilled and tapped for .5mm bolts which correspond with the exact location needed to hold the center tight on the fixture through the gaps in the spokes. There are actually 2 of these fixtures. One has the center drilled out so that the axle hole can be drilled/reamed to the proper diameter and the other has a piece of slightly under axle stock friction fit in it so that the axle hole can slide over it perfectly without forcing the hole open any more. This 2nd fixture (shown) is for turning the outside diameter down to the proper diameter for the tire. The fixture can also be flipped for facing off the back of the center so that it can fit flush and true on the fixture for turning the outside diameter. It's quite a lot of work to process these precisely but well worth it to prevent any kind of wobble in the mechanism.
I've still got quite a bit to do on the tender. Mostly piping the air lines, installing the brake hangers and hardware, the fill hatch, rear coal bulkhead, front coal bulkhead, fabricate the toolboxes (will have working hinges and have tools inside), add the valves and drain cocks, coupler, engine link and the small strap that covers the edge of the floor boards.
I intend to finish the tender completely before moving back on to the locomotive. While the list seems long, the tender shouldn't take a whole lot more time. I'm anxious to get the locomotive frame finished and a rolling chassis built. The engine will take quite a bit more time. I'm focusing on fabricating as much as possible for the engine so that it has a consistent, hand-built look.
Thanks for looking!