Lathe faceplate index pin question

[LittleWatchShop]

I posted my question here, but it may not have gotten bumped, so I will post again here.

I have a couple of faceplates for use on lathes.  Neither have the index pin.  I recently bought 4.5mm rod stock to make an index pin.  They fit perfectly except for the fact that both my faceplates have a key in the collet, so any pin must have a notch cut to fit into the key.

Why?  Seems to me the index pin does not need a fixed orientation.  But both of my faceplates require a keyway.  Just like the spindles in WW lathes, the key appears to be a soft material and can be easily knocked out.

Thoughts?

[nickelsilver]

The index pin rides in a bore, and that bore may not be perfectly true to the rotation of the lathe spindle. It's quite hard to make a small bore like that in (what should be) hard steel perfectly. But if the pin is rectified in the bore, in the lathe, it's now true- as long as it doesn't turn in the bore. Thus the indexing pin and slot.

 

When we used faceplates at school, we got the part close with the pin, then bang-on with a wobble stick. Nowadays, I use optics to center in the faceplate.