Easy mill vise squaring- Set it once and forget it...

[Toolmaker51]

As expedient vise mounting, this is quite reasonable. However, it doesn't distribute wear over accessible length of lead screw. Many mill owners attach tooling, alternating ends and middle periodically. E23 Fixture keys remedy this, allowing directly squared-up mounting. Commercial versions are often hardened...Unless a vise can be moved without dragging across table, they score the surface. Mild and tool steels work fine. Measure for a close fit to slot in base of vise, machine to width of mill's tee slots, and counterbore keys for cap screw. If slots vary in width this is advantageous. Make them NOT centered so orientation is easily determined visually.
When doweling for removable items: consider tapered pull dowels, or diamond pins. Improved locational positioning, yet easily 'found' by mating part.

Additionally a blatant plug for United Drill Bushing. United Drill Bushing Home Page Cataloging most every fixturing device conceivable. Are not only MADE in USA; but in Downey CA, family owned + operated, they've been there decades, better service than the 'big' names.

Rare for me to offer uncompensated advertisement for ANY business. When I do, rest assured; those are issued sans any qualms what-so-ever.