Edging closer to smooth

Now that 622 has a motor, I hooked up some alligator clips to see how it behaves. There is a hesitancy at the quarters, which makes me believe there is a slight quartering issue, despite my efforts to limit it. It’s unsurprising because I could feel some stiffness when rolling the chassis by hand.

Really the only adjustment available at this point is to ease out the holes in the coupling rods. How much to ease them is one of those things that can only be learned by experience, unfortunately, and compared to steam modellers who start with RTR models or even kits, I have relatively little. Knowing that it will be difficult to add material back, I made the holes in the coupling rods only big enough to allow their crankpins to turn.

Before reaming any holes, though, I checked how the special hat-shaped washers compared to their related crankpins. I found two that were indeed as much as .07 mm (.003”) wider than their related crankpins, meaning they will have a tighter fit on the coupling rod, and be prone to unscrewing the crankpin bolt (I had indeed encountered this with one of them already).

I made two new washers to better match those rear crank pins. The process was to centre-drill some 1/8″ rod, turn the hat profile, and part it off with a razor saw. This left the washers a little too thick, so I dropped them into a hole in a piece of scrap so I could hold onto them whilst filing them to the correct thickness.

With the new washers, the chassis rolls with less hesitancy. We’re almost there.

2 thoughts on “Edging closer to smooth

  1. I am not sure whether you have now got past this stage, but there is a useful tip if you are having to put on and take off the coupling rods. When you put the nut on the end of the crankpin before doing a test run, add a touch of nail varnish. This will be enough to stop it undoing while you run the motor, but it is easy to remove if you need to do further work.
    Hint: best to get an old nail varnish that has fallen out of favour and it is OK if it has gone a bit gloopy. The colour does not matter!
    Best wishes
    Eric

    1. Thanks Eric; I’m still very much at that stage, so your advice is timely. In 622’s case, they’re actually bolts, but the same tip would work on the back of the wheel. How does the varnish compare to Loc Tite green or blue? These M0.6 bolts are very delicate, and I worry that they mightn’t survive the stress of breaking a bond.

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