What I am I missing in the second sentence?
From Dumore instructions:
External grinding is done with the work rotating in the same direction as the grinding wheel, while internal grinding is done with the work rotating in the opposite direction. In both cases, the result is the contact faces of the work and wheel moving in opposite directions, thus maximizing cutting action.
pghlost
What you are missing is probably the import of the FIRST sentence.
Try this:
- lathe is rotating the work counterclockwise. Surface of the OD that the wheel will contact is moving
downward, taking the toolpost as fixed reference point.
- grinder is rotating its wheel counterclockwise as well. Surface of
the side of the wheel that will contact the work is moving
upward, taking that same toolpost as fixed reference point.
Extrapolate respective surfaces and which side of the wheel is in-contact to an ID bore.
CAVEAT: Grinders can be back-mounted. ID's can be ground on the back up-curve. Both players are reversible.
That's why Dumore stated the GOAL of maintaining opposite directions, regardless.
There is a bit more to it, given that the grinder's wheel is nearly always far faster moving than the work.
Rotating such that gravity's UPSIDE surfaces are converging rather than departing would increase the possibility of debris being pinched and wedged before gravity and centrifugal force could fling it out of harm's way.
If grokking that is still "not convenient", to use the Chinese excuse?
Peddle the Dumore!!!