The manuals for these grinders (all of them) are pretty poor. They only make sense after you already understand the grinders and no longer need the manuals. The newer manuals are just as bad, but only use pictograms for international audiences, so if you are a mime with a chainsaw, you might be in luck.
By 'centering' the vise, we mean that both the Right and Left cutters will be ground to the same length, without adjusting the chain stop. On the 511A (and similar grinders), this is done with the hex bolt and locking nut. The easiest way to do this is to take a new chain and set it in the vise like you were going to grind it. Keep rotating the vise for the R and L cutters, and adjusting the hex nut, until they both are the same distance from the wheel.
Note that this changes slightly as the wheel wears. So you can check the centering periodically, or double check each chain as you grind it (what I do). I grind all the cutters on one side, then just one cutter on the other. I hold this cutter back-to-back with a cutter from the first side, and make any fine adjustments needed. They should be close. If not, center your vise!
The 511AX is supposed to have a self-centering vise.
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More info on this grinder in links referenced in Post #32 above, including :
http://www.arboristsite.com/community/threads/511a-grinder-improvements-tweaks.197073/
Philbert
thanks as always. think i got a better understanding of what its trying to say now.
A