Re: Rigid tapping toolholder
I normally use an ER collet to hold taps for rigid tapping, both on the lathe and mill. But for maximum accuracy, I think a dedicated floating tap holder would be the best choice. The Jacobs chuck could have a bit of runout and if the tap is stiff enough it might be able to cut a bit off center and tap the hole oversize. A floating tap holder would cause the tap to follow the hole.
Also, you might look at the fit class of the tap. And confirm that your hole is the proper size.
Jim Dawson
Sandy, Oregon, USA