Compensating for something

I started my build for an O scale 4-4-0 locomotive. My plan to build engine Leach got derailed when I accidentally ordered only two drivers from Slaters instead of the four that I needed. So I had to drop another order for a second set of drivers and some other parts. They will not arrive for about a week or so. 

In the meantime, I started a test build with the wheels I have on hand. They match up pretty well with the engine Talisman. John White has a good set of plans for this locomotive in his book, "American Locomotives." The locomotive is similar to the USMRR Deveraux engine, so maybe that is what this will become.  The next step is to adapt those plans to an equalized (compensated) suspension model locomotive.

A scaled drawing sized to fit the wheels I have on hand. The gold and brown object at the bottom is the template I made to hold the hornblocks in position for soldering. 

I plan to use a pivot beam design. You can find out a little about it at this link, but if you are like me, you will find the explanations very hard to follow. Luckily, Eric Gates, has built locos like this and is helping coach me.

I built my pivot beam sub-chassis with sections of 0.090 inch square brass bar and brass hornblocks bushings.  The first thing I did was to solder two sections of the brass tube together, trimmed them to the same length and drilled a 0.040 inch hole through both with my drill press. That way they were identical. They will be the pivot beams. Then  I made a laser cut template with 0.025 inch thick resin-impregnated laser board (see the drawing above.) The templates held the hornblocks in position while I soldered the bar to the top of the blocks.  That worked pretty well. I used the axles in the hornblock bushings to square up the hornblock to the pivot beam. Note the hornblock bushings do not slide up and down in the hornblock guides in this design. They are fixed to the pivot beam. So there is no need for springs etc. This is one of the key advantages of this design. 

Pivot beam sub-chassis

The other advantage is that the compensating sub-chassis is "hidden" behind the drivers and main frame. There are no out-of-scale pivot bars connecting the pilot truck to the suspension. This is important for US style 4-4-0s where there is a lot of open space under the boiler. 

Laser cut MDF farm for testing fit and function

Next I laser cut a test frame from 1/16th inch MDF. The purpose of this was to check for fit and function. Everything seems to be working well. The Scale Four diagram says this frame is "cosmetic", but it does have some structural purposes. It holds the pivots beam,  locates the pilot truck and carries the boiler and cab. 

So I will use the MDF frame as a template to build up the frame from brass. I plan to make the frame with 0.090 inch brass bar stock. That should be fun. 

The last remaining critical task is to make the side rods. They need to be precise to avoid binding. I plan to use a laser cut  template to make them. I hope that works.  I have two drill presses, including a miniature one for model building, but I don't have a vertical milling machine.  So I have to use templates and other techniques to make identical flat parts.