So my shaft problem woke me up early this morning without realizing it. Immediately it was on my mind and I wanted to get to it. I thought the alarm had gone off but half way through the drive in I looked at the clock on the dash and I was a solid fifteen minutes ahead of schedule. Now I know why traffic was so light. After the first cup of coffee I was ready to do some forensic investigating with my test indicator. First place I went was the lathe which was still set up from the day before.
This is when I started to realize that I had probably out tricked myself about the cause of the bad end face parallelism. When It showed some runout (couple of grand) out at the end and near zero runout near the collet I started to come around to the idea that the shaft was just be bent. Duhh. This would pretty much explain everything I had seen and measured the day before. Nothing like a good nights sleep to clear out the cobwebs.
What Bob measured was a series of slices of the round cross section of the shaft. From this he then created a three dimensional model of the shaft and found the best fit of all the axes of these cross sections. From this he can determine the smallest circular tolerance zone that these can fit on. In the case of the bad shaft it showed that the zone was .002 in diameter. Incidentally this is very close to the bends in the shaft I measured with the test indicator on the surface plate.
This was a real fun mental exercise for me, and Bob actually had some fun also. I guess he never had to display the twist like that before so he got a little learning out of this seemingly simple little shaft. So in closing nothing is ever as simple as it seems and the devil is always in the details.
To err is human
To forgive is divine
To check is Engineering.
Thanks for looking.