I recently acquired a somewhat rusty used Rockwell 6" bench grinder
(model No 23-612 F573 G6-202-17 1/3 HM 115vac single phase, 3250 rpm).
It is in the process of getting new bearings (the old ones tick when
running, probably due to bits of metal in the grease).
This unit is old-time US, made of cast iron, and weighs a ton. And runs
smoothly, except for the ticks.
The mystery is the wheel flanges. The arbor is 0.500" diameter, and the
wheel flanges (that hold the grinding wheel) are die-cast aluminum alloy
and are about 0.425" thick along the arbor shaft, and are equipped with
two keyslots spaced 180 degrees apart. In the arbor shaft there was a
0.095" diameter by 5/16" deep radial hole with a spiral pin pressed in,
with the tip sticking out, and engaging one of the two keyway slots
(which are 0.117" wide) of the inner flanges. The spiral pins had
largely sheared off, and had chewed a groove and raised a divot on the
inside of the flange, where the radial pin rested. Both sides were
I was able to drill the pin stubs out without damaging the arbor shaft,
using a 0.088" diameter HSS drill in a hand drill, using black sulfur
My first thought was that this should have been a woodruff key, but the
keyslot is halfway between the standard sizes. But I could mill a
woodruff key slot in the shaft and widen the keyslot in the flange
My second though was that the original design probably had a single
close-fitting but loose pin that went all the way through the arbor, and
engaged both keyslots.
My third thought was to wonder why the scroll pins both sheared off.
They don't lead that hard a life, so what happened? Maybe I don't want
to make this too strong.
My fourth thought was to observe that few modern bench grinders have
such pins or keys. What problem did they solve?
The only 6" grinders I've worked with haven't been that elaborate.
Usually had pressed steel washers on each side just large enough to
stay on the cardboard washers on the wheel, the inner sides abutted a
shoulder on the arbor, friction was the only thing that kept things
from spinning on the arbor. Never had a problem with that. So I
think your arbor is overengineered unless it was intended for driving
some other gizzy as well as doing grinding duty. Might be the pin
engaged with a flex shaft attachment or something similar. You could
probably put the thing back together sans pins and it'd work fine as a
Stanley Schaefer fired this volley in news:9f7cc9e0-
I'll agree, and go further with that thought.
Spiral pins are easy to remove (as opposed to roll pins).
There is no apparent reason for that pin to be easily removable, unless
it's intended to be removed.
Yes. These were roll pins (look like a fat letter C, not a two-turn
spiral). There wasn't enough left to grab and pull the pin stub out.
I agree with both of you, but still Rockwell went to some trouble and
expense to put those pins and key slots in, so they must have thought it
necessary or at least quite desirable. The question is why. What
problems were they solving?
Joseph Gwinn fired this volley in news:joegwinn-
Pretty simple. The fact that they were roll pins somewhat suggests they
were NOT designed to be removed.
So, they were simply enforcing that the face of the friction washer would
be the ultimate yield point if something slipped, rather than having the
washer slip on the shoulder of the shaft.
Although my grinders have a mere shoulder, _I_ would rather have them
slip on the paper washer than to slip on the shaft -- if they ever
slipped; which they have not.
How do you know that they haven't slipped?
In fact, Joe posted that his flange is scored from it slipping on the
shaft and the pin's stub cutting it.
BTW - the talk has been "flanges" and I say "flange", cause only the
inner flange is/was pinned, right? Varying thickness of wheels would
prohibit pinning the outer flange. Oh, wait ... "flanges" could mean
the 2 inner ones (both sides).
The shaft has a substantial shoulder, upon which the big flange washer
rests. The pin is in a machined keyslot, and does not prevent motion
along the shaft. Only rotation is prevented, and only for the two inner
flange washers (nearest to the motor).
I suppose they could have cast the slot in, but they didn't, and they
use only one of the two keyslots, so my guess is that they simplified
the attachment but left the washers alone because they already had the
molds and tooling.
Certainly. But my theory was that the original design had a pin through
the shaft so the pin could engage both slots in the washer. Although my
guess is that the pin was loose, trapped by the washer, it could have
been a roll pin pushed all the way through as well.
That makes sense. Prevents damage to the shoulder and washer.
Given that both pins were partly sheared, it was certainly trying to
Well then I'm stumped. If the pins only prevent rotation then maybe
some engineer though it was a good idea. I have never seen a grinder
like this and have never needed a pin to prevent wheel rotation
There doesn't seem to be a good reason for the pins. Maybe the only way to
find out what problem (real or imagined) Rockwell designers thought they
were addressing, would be to find some advertising literature from the time
when this type of "feature" was introduced.
Putting a thru hole that size in a 1/2" shaft doesn't seem reasonable to me,
but I assume someone thought it would be a good idea.
If there were wrench flats (or pin spanner holes) on the inner wheel flanges
to fit a wrench when removing and replacing the shaft nuts, I could see some
perceived practical use for the pins, but then only really useful if the
nuts were nylock types (any type of locking nut is not actually required for
mounting grinding wheels), because grinding wheel nuts don't require
significant torque to make the mounting secure.
I've seen used bench grinder arbor threads that have been chewed up by
someone using a plier on them, apparently by someone not smart enough to
figure out the right direction to turn the nuts, or not confident in having
the nuts just snug enough to firmly secure the grinding wheel.
I've also seen wheels with significant gouges in them, so if an accidental
jam on one side caused the motor's rotor to stop, it's possible the other
wheel could continue to rotate, but that's still not likely to unscrew the
nut from the arbor threads.
On that sort of grinder, you don't really need to hold the stone at
all when screwing on the nut. It ends up wherever it ends up and you
true it after everything is tightened up.
I still say that cross pin is for driving something other than a
Stanley Schaefer wrote:
Hmm. Maybe. The motor unit et al are the same. The buffer's roll pins
are 1" long and go all the way through, with 1/4" sticking out on either
side. By contrast, on the grinder, the hole is blind, and the pin
sticks out a bit less than 1/8". But maybe.
Yes, but it seems like a lot of work to find that one ad. Maybe someone
will know where to look.
Why is a 0.096" diameter hole in a 0.500" shaft a problem? This does
not seem large compared to the shaft.
There are no such holes. Just RH and LH 1/2-20 hex nuts.
Yes, but no such damage seen.
This could be - Bubba at work. But I don't see any other evidence. The
wheels are quite worn, one 6" stone being worn to maybe 4", but no big
gouges in stone or metal.
I agree.. searching for the actual purpose/intent of a short-lived decades
old design feature could result in a huge waste of time.
We'll await the day when someone poses the question: Who was asking about
pins in the bench grinder shaft arbors, because..
All we might do at this point is speculate.. and someone probably nailed it,
as there are always good ideas found here, IMO.
Actually, speaking of speculation, it occurs to me that the worn-down
stones may be the clue: Maybe a user was leaning into the wheels to
grind faster, and what was carrying the resulting torque was the inner
flanges and roll pins. As I recall, the arbor nuts were snug but not
overly tight. If the wheel was able to rotate on the shaft under such
heavy use, the nuts would have become very tight, perhaps too tight.
A+, good conclusion Joe.. why all the mystery? heheh
I think you meant to state 1/3 HP rating in the OP.. so it's likely to stall
before crushing the grinding wheels, me thinks.
(however, 1/3 HP is much more powerful than today's 3/4+ HP "ratings")