According to :

[ ... ]

just got a nichols (semi-automatic converted back to run via hand
feed) set-up in the shop, and it works beautifully.

Congratulations!

i'm turning it into a dedicated tube mitering (aka fishmouthing /
coping) machine, and as such i'd like to remove the arbor from the
spindle in order to run holesaws.

[ ... ]

how the heck do i get the arbor out?

i took the whole overarm assembly off, knocked out the dowel pin that
runs through the arbor, and removed the two hex cap screws that are on
the piece immediately next to the arbor (the arbor runs through this
piece into the head). i thought maybe the arbor would just slide out,
but it definitely did not.

You're working on the wrong end of the spindle. The arbor is
held in by a drawbar from the other end of the spindle. Look in the
center of the pulley. It may be a cylinder with a pair of flats milled
to fit a wrench, or it may be a hex nut. In any case, put a wrench on
that, loosen it by about 1/2 to one full turn, and then tap the end of
the drawbar with a dead-blow hammer once or twice, which should free the
arbor. Then grip the arbor and loosen the drawbar until the arbor comes
free.

The two cap screws in the end of the spindle are to simply keep
the arbor from spinning before you get the drawbar tight enough. The
cross-bar (dowel pin) which you removed engages the heads of the hex
screws, and should stay in the arbor all the time. I presume that you
found the setscrew which held it centered?

Note that many 40 taper tool holders will have a flange with a
pair of notches which straddle the hex head screws. The dowel pin is
unique to the genuine Nichols arbors.

You'll have to find one of these appropriate to holding your
fishmouthing saws.

You do need to at least loosen the overarm and slide and rotate
the arm with the bearing enough to allow the arbor to pull out.

can anybody offer any explanations? as detailed as possible, as again
i'm a total greenhorn.

i've searched the archives, but couldn't find anything that directly
helped me out.

The drawbar is common to almost all milling spindles, except
those with built-in quick-change systems.

Do you have a manual for the Nichols mill? If not, drop me an
e-mail.

Enjoy,
DoN.

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