Building Log Bunks for Logging Flat Cars inOn30 for NMRA AP Master Builder Cars

 

I'm building four logging flat cars (log bunk flat cars) for my Hebard Cypress Company in On30. The master and mold shown above are the basis for the cars. 

The log bunks are the brackets on the flat car that hold the logs as shown on the photo above. Essentially it is a timber across the flat car with metal cleats on each end, and a chain for securing the log. My plan is to build two log bunks for each car, or eight all together. I may actually want to build more, so I decided to build a master and mold, then cast all that I need.

This is a wood prototype but I'm using styrene for the master as it slips out of the mold more easily. This mold design will trap air so I'm building it to be pressure cast. The depth of the timber is going to be slightly higher than the prototype so that I can sand it down to the precise size. Pressure casting leaves a "fillet" or raised edge around the casting that needs to be removed.

I cut two identical timbers which are just a tad longer than the width of the flat car frame. The steel cleats on the ends will be made with thin strips of styrene.

The cleats are made each with three pieces of thin styrene. I cut and glue each piece then cut off the extra and file it clean and square.

Using a #72 bit I drill 7 holes, two on each cleat and three across the timber for bolts and washer castings.

Her is a completed piece, on my hand so you can guage its size. 

The thre bolt washer castings hold the piece to the frame, and the cleat castings hold the cleat assembly to the timber.

I made two of them so each casting would give me enough for a single car, although on some cars I may use three instead of two for some variety. Logging operations often build their own cars from kits they get from the steel casting manufacturer so there is usually some variance between groups of cars.

The assemblies are glued with white glue to a plast base. The glue is not permanent but hold the timbers in place so the don't move or float in the mold making compound.  You can remove them from the base later.

The mold box is made from thin strips of styrene that I buy in large sheets. I cement them with Plastruct styrene cement. This system works great because the styrene is melted and forms a leak proof bond to the other styrene. The star pattern allows the strips to be pealed off quickly. The mold material was mixed as per the directions, poured in and allowed to cure for 24 hours. This is Smooth On Mold Max 40, which is a bit too hard for this kind of mold, but it did work well.

After curing I gently removed the mold dam from the mold rubber and removed the block.

This is why I use the Elmer's Glue. The masters break clean from the base and can be used again.

Carefully I removed the masters from the mold, and trimmed the flashing from the mold. Looks good!

To cast this mold I will heat it in the microwave oven for two minutes to get it warm to the touch but not so hot I can't handle it. This allows the resin to flow in the mold better, and to help release bubbles.

This casting was made without a pressure vessel (pressure pot) that I would normall use. It is still in the mail, but I wanted to get a test shot immediately.

The impression is good and the mold will work. This casting has some critical bubbles in it so that it was rejected, but when I pressure cast it there won't be any bubbles as the 50 PSI pressure squishes the bubbles so small you can't see them.

On to sub assembly!