In a bridgeport type mill why use an er25 collet chuck and collet? I assume it is easier to eject. But what advantage does it offer over r8 or mt3? I have never heard of a native mill spindle in ER size. Are the collets more accurate and grip better with more fingers?
Bill D
ER are dirt-common on Asian "mill drills" and "native" as well on loose building-block spindles some folks use to
make a vertical head add-on. Also CNC tool-changers and their magazines, residing on taper-tail (NMTB/ANS/ISO, CAT, BT, and newer-yet).
TG might be more beloved for "live" tooling, CNC world, but ER serve that market, too.
Most things grip better than Are Ate. As with 5C, it has a narrow "collapse range".
ER, TG, SK have very low runout even in "standard" grades. There are extra-precision ones as well.
The "handy" advantage to ER, TG, SK, on an all-manual machine is that once in place, there is not as much need to mess with a drawbar - tool changes are done right in front of you with a spanner wrench - be it hex, castellated, or "hook", all of which nut types fit the same collets and body.
The ER family claim to globally wide acceptance (which it has earned a great deal of) comes off the back of very good grip over a
very wide "collapse range".
A full millimeter per collet, though half-mm sets are better, and "zero collapse" or "on-size" are better-yet for taps and such.
Already half-mm "TG" has even Tighter Grip at the downside of needing twice as many collets for the same coverage.
Both (and SK) are "meant to be" precisely tightened with a torque wrench, but as they work well-enough with "feels right", it doesn't happen often.
FWIW-nothing-really, the "number" in ER is the length of the collet, not the max diameter. One has to look that up in a table. I have ER 20 and ER 40. ER 50 exists. ER 60 is 'mentioned", very rarely. In theory, the design scales, could go larger, yet. But I have never seen even an ER 50 "in the metal", nor an ER 60 even for sale.
NB:
ER 32 is perhaps a better size than ER 25 as to torque to support BirdPort-class spindle power?
There is a trade-off between clamping nut size and stick-out, vs max diameter & grip, though.