The connecting rods connect the three axles, so the steam engine's power is evenly distributed on the track. They are made from cold-drawn mild steel and are fully machined. They were usually forged for full-scale locos, before being milled to shape and having a ground finish.
There were two versions of these rods. With and without flutes (the recess in the middle, to reduce weight). The locomotive I'm building, no 5361, will have the non-fluted (plain) connecting rods.
With the band saw the bar is cut to length.
Because the steel was cold-drawn, they were annealed. This relieves the tension of the steel, so it stays reasonably straight after machining.
The middle part of the rods is cut away with the shaping machine.
The pre-machining is done.
The bearing holes are drilled and reamed in the milling machine.
Then I wanted to use the CNC-milling machine for the outer contour of the rods, but the machine showed a fault. The Y-axis was not moving constantly and didn't get to the home-position.
It turned out that one of the flexible couplings between the motor and ball screw axle was broken.
No new one could be bought easily on a Sunday, so a new one had to be made.
With some quick measurements, a sketch was made and a new part could be turned and milled.
And ready to continue the work on the connection rods.
The outer contour was programmed directly from the SolidWorks drawing. The CAM software calculates the toolpath using the DXF file.
It is a small milling machine, so light cuts have to be taken. Although it takes some time to mill the rods, the result is good and much faster than sawing, chain-drilling, and filing by hand, as I used to do when making these kinds of rods for the 3½" 'Mona' locomotive in the '80s.
The recess for the coupling pin between the rear and front rods is milled in the conventional milling machine.
For this machine, I have many more tool holders and different cutters. The slitting cutter was used for the 4 mm groove.
The sides were cut with a 16 mm endmill.
With files and sandpaper the rods were worked, so that no machine marks were visible.
This was done with almost rods on British locomotives. Appearance was even in the days of steam important.
A quick test fitting and check.
Bearing bushes turned from bronze.
The oil holes were centred on the milling machine.
They are press-fit size (+0.02mm)....
....and pressed home in the vice.
Small top nuts are turned and the hexagonal shape is cut with a dividing head.
A small bearing bush is also fitted between the two rods.
This hinge is needed, so the sprung axles can move up and down in their hornblocks, without binding the motion.
The connecting pin has a simple screw to hold in place.
The retaining or locking pin is only a dummy one. This would be impracticable for a working live steam model. Disassembling or assembling would become a tiresome job. Especially when this is needed to be done at a steam event at a model engineering society. 😐
