Stacking Parallels Is a Balancing Act
Parallels lift a part up off the vise base so the cutter has clearance to machine all the way through or near the bottom without touching the vise itself, but stack too high and you risk running out of spindle clearance or having the part flex if it's only gripped by a thin sliver above the jaws. Getting the parallel height right means looking at both ends of the stack at once.
Why Stock-Above-Jaw Clearance Matters
If the top of the stock barely pokes above the vise jaws, an end mill or face mill working near the top surface can clip the jaw before it ever touches the part, especially with larger-diameter tools or holders that stick out wider than the cutting edge. A small margin above the jaw top, not just barely above zero, keeps the tool path clear of the fixture.
Checking the Other End: Spindle Clearance
The same stack-up that solves your jaw clearance problem can create a new one at the top, where a taller-than-expected stack eats into the available distance between the spindle nose or tool holder and the table. Checking both ends before you actually set up the job avoids finding out the hard way mid-cycle.