Last night, I dropped tools when I realized that I didn’t understand why my turntable was not properly aligned with the first rail of the roundhouse tracks in one orientation. Thanks to both Neil and Mark who wrote in with some suggestions.
Neil’s suggestion that my turntable rails were off center was true – by about .010″ or .25mm. That would certainly explain a deviation of double that when the turntable was reversed, and it’s the sort of thing that should be corrected anyway. So, I shifted the rails over so they’re centered, at least relative to the turntable bridge, and we hope the bridge pivots on its centre!
Unfortunately, this actually made matters worse! I think Mark’s conjecture that the turntable is not centered on the centerline of these two tracks may be correct. Mark writes (I hope he will forgive my excerpting his email),
I wonder if this is angular tolerance stack up you are seeing? If you align the *centers* of the approach trackage accurately through the center of the turntable, and then align the approach rails to these centerlines, I wonder if your tolerancing might be more forgiving?
The centerlines of the two tracks on opposite sides of the turntable have been lost, but I did try to centre the lead on the ties, which were centered on the original centerline on the baseboard. That original centerline was a continuous line right through to the third roundhouse stall until I cut the turntable pit out.
Finding that centerline again showed that it is not at all centered on the turntable! Indeed the turntable is a good .020″ / .5mm off-centre. To confirm this, I lined up a straight edge through the re-established centerline and, holding a pencil at the mid-length of the turntable, turned the lazy Susan. This made a circle a good millimetre or more across (visible in the title photo).
Siting down the lead, we can see that when the turntable is facing one way, the alignment at the near end is straight, but when the turntable is reversed, there is a pronounced kink.
So, I think the turntable wandered off-centre relative to those original marks. I actually remember thinking that I should keep the turntable pit permanently fixed to the baseboard once I had found its centre, and now I wish I had. Oh well, I can’t go back and change history now; all I can do is take some time to decide what to do about it.