I thought I would have a play with the Jacob's chucks today on my Vicmarc VL150 lathe. Drilling Sierra pen blanks that have been turned to 19 mm dia between centers then held in a Vermec ER32 Collet Chuck on the head stock, and conventional setup with MT2 Jacobs chuck with drill bit in the tail stock, using the drilling sequence I posted above.
Jacobs chuck #1 - best I could achieve was a 0.5 to 0.6 mm off set hole on the hole exit end of the pen blank. i.e. the resulting tube wall thickness difference from thinnest to thickest ranged between 1 & 1.2 mm. I was becoming concerned as I had achieved far better results in the past then the penny dropped - different Jacobs chuck!
Jacobs chuck #2 - far superior result with far less than 0.1 mm off set hole on the hole exit end of the pen blank. i.e. the resulting tube wall thickness difference from thinnest to thickest was less than 0.2 mm.
Repeated this with dozens of blanks and with both 27/64" & a 10.5 mm quality reduced shank (10 mm) drill bit in both chucks. I can't explain the difference other than Jacob's chuck #1 is a cheapie and #2 a much higher quality chuck. I cleaned the MT, even tried multiple positions of chuck #1 in the MT2 in the tailstock, reset the drill bit in the jaws of the chuck etc. etc. - same result, very occasionally the drill actually wandered onto the correct axis.
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Originally Posted by John Keeton
