I'm trying to design and simulate a simple knurling device. enter image description here

The shaft ( blue part ) has a flat surface at the other end, where the small grey pin connect the green body to the shaft. I guess you can see it clearly in this view:

enter image description here

My first question is, why we do even need to give the surface such a flat shape?

I think maybe it's easier to bore a hole, but I'm far from being sure.

And is there any other flat surface on the other side ( opposite side ) or there is only one flat side ?

  • $\begingroup$ Have you used a knurling tool on a lathe? The top thumbscrew needs to be tightened over the course of the operation, as the depth of knurl on the part gets deeper. $\endgroup$
    – Criggie
    Commented Mar 21, 2021 at 9:36

1 Answer 1


Yes, it would be easier to drill the hole but there are other reasons.

  • It's makes the shaft the same width as the red bearing also clamped by the green sides.
  • The shaft will experience torque when the knob at the top is tightened. A hole though a round shaft would have high pressure on the curved edge at a point on each side some little distance away from the green faces. Holding the flattened shaft between flat plates gives a spanner-like grip on the shaft.
  • $\begingroup$ After reading your answer then I conclude that, there should be two flat surfaces, each in contact with green parts. Am I right ? $\endgroup$
    – Sam B
    Commented Mar 20, 2021 at 20:47
  • 1
    $\begingroup$ Correct. It is more than likely that the whole assembly would be symmetrical. $\endgroup$
    – Transistor
    Commented Mar 20, 2021 at 20:53

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