On Sun, 14 Mar 2004 12:51:52 -0500, "Paul Farber" wrote:
I'm a hobbiest just starting out, so I really don't want to throw a lot of
money into a mill for a few years.

I've been looking at low end mills (some import, some domestic) and one
thing that I've read about is freeplay on the shafts. Most that i've
fiddled with seem to have almost 1/4 turn of the shaft before the gears will
start moving the table. Is that normal?

Shouldn't be any gears in the table feeds. The handwheels should be
connected to threaded shafts called leadscrews with a nut fixed to the
movable portion of the table. Turning the handwheels causes the nut
to advance on the shaft, pulling (or pushing) the table along with it.

Anyway, the leadscrews have to turn in bearings in the fixed portions
of the machine. If there is no end clearance here, the shafts will bind.
This clearance is likely most of the free play you're feeling in a new
machine. This is adjustable with a preload nut, usually on the same
end of the shaft as the handwheel.

If the end clearance preload is adjusted tight, there's very little free
play. But it becomes difficult to turn the handwheel (binding). A half a
turn is too much, but some free play is required, maybe about 1/20th
of a turn. Most of the inexpensive imports are *not* adjusted properly
as shipped. You have to make this adjustment yourself.

The other source of sloppiness is the clearance needed between
the nut and the leadscrew. Again, zero clearance causes the
motion to bind. Some high end machines have an adjustment for
this too, called an anti-backlash adjustment, but most cheaper
machines don't. You have to live with whatever the factory provided.

Typically, this nut clearance will allow something on the order of
2 to 5 thousandths of an inch table motion in a new machine.
Less is better up to the point where the nut binds on the screw.

The problem is, just turning the handwheels in the store won't tell
you which source of sloppinesss it is that you are feeling. That's
because the display units typically don't have the end clearance
properly adjusted (because the store clerks either don't know how,
or don't care).

You can shove in fairly hard on the handwheel, then turn it back
and forth. Pressing in removes most of the leadscrew end play,
so the play you feel then is only the slop of the nut on the leadscrew.
That's what you'll have to live with if you buy the machine.

Gary