Monday, April 30, 2007

Coupling Rods

The spacing of the coupling rod centres is critical to the free running of the loco. I measure each side separately and comparing it with the drawing. Any inaccuracies in making up the axle-boxes or in setting the crank-pins will show up here.

I started by marking out the centres and outline on the rod blank, then drilled the centres, starting with a small centre drill, then drilling out at (say) 2BA clearance at one end, and 2BA tapping at the other.

The rods tapered from 3/8 to 1/4 in. - i.e a taper of 1/16 in. on each side.
So I marked out the centre line on the support bar, and another, offset by 1/16 in. I drilled and tapped a fixing for one end of the bar on the centre line, and attached the rod blank. Then I lined up the other centre on the 1/16 in offset line, clamped and drilled through 2BA tapping. Now I removed the rod blank, and opened the 2BA tapping hole on the rod blank to 2BA clearance. Also I tapped 2BA threads in the support bar and bolted the two together.

I clamped the support bar in the milling vice, parallel with the main axis of the milling table. I then cut away one side of the taper, turned the blank over (i.e. upside down) and cut away the second side.

I then rotated the entire assembly of support bar and blank through 90 degrees in the milling vise, and milled the back to profile. If, as in my case, there was a slight taper on the back of the rod, I set this up by making a spacer washer equal to the taper, and placed it under the appropriate 2BA fixing bolt.

I then opened up the bearing holes to their running diameters - in my case, 1/4 and 3/8 in.

In the case of the trailing coupling rod, I decieded that the most accurate way to measure it was to mark it out directly from the job. I made up a piece of rod as a scribe - the photo says all!
Once all the coupling rods had been made up to this point, I was able to try them on the chassis, and check that they ran freely. The chances are that they will stick at some point and it may be necessary to 'modify some of the hole positions - the drawings show the driving axle bearings as 'reamed', whilst the driven bearings as shown as 'drilled' indicating that some clearance is to be allowed. Once I was satisfied with the hole positions, I enlarged the holes to accept the bushes, which were pressed in, and finally re-reamed. I actually reamed them all, and, had it been necessary, would have opened up the driven bearings with an expanding reamer.

To create the radius around the big-end eye - I cut away as much surplus as possible. With the big-end located on a 3/8 round bar in a machine vice. I clamped this to the table of my bench drill, and presented this to a grinding wheel in the drilling chuck. I held the other end of the coupling rod by hand, and slowly rotated it against the grinding wheel. The radius was soon established.

I've seen a similar arrangement suggested using a milling cutter in a vertical mill. I did try it once, but never again. I thought it dangerous and would strongly advise against it.