"D Murphy" wrote in message
...
"Harold and Susan Vordos" wrote in
:

The feedrate is determind by pressure, same as flat
lapping.

That sends up a red flag. I'm sure you've not talking about lots of
pressure, but I can't help but wonder if it's more than the matrix of the
wheel can withstand (as you alluded, maybe a bad wheel).. How are the CBN
wheels bonded? I've never seen one.


The one job we had trouble with we were single side grinding. The
material is 52100 low 60's Hrc. I can't remember exactly and don't have
the drawing here at home. CBN is the only way to do this type of
grinding.

I'd still be inclined to go with a 38A Norton wheel, but that may not work
with that type of machinery. It sure would be my choice for the level of
technology with which I'm familiar. Years ago I made three sets of
supercharger planetary components for Auburn and Cord, each of which used
the same blower guts, but a different housing (Auburn is straight 8, Cord
V8). I recall it ground beautifully, as do the vast majority of hard
materials.


To tell you the truth it's surprisingly easy to achieve these tolerances
on this machine. (When the wheel doesn't fall apart) One of the other
parts we did double disk grinding, we ran for 24 hours and had a total
thickness deviation of 1.6 microns and flatness and parallelism never
exceeded 1 micron. I'm willing to bet that if we re-cleaned and rechecked
the parts that were at the extremes we would find that the deviation was
smaller.

To be perfectly honest, you're way out of my league. While we worked to
close tolerances, I don't recall anything tighter than .0001" (for
flatness). We used to grind a thin cover for the bearing housings of the
guidance system of the missile. They were made of A286 stainless, drilled
and countersunk, then hardened. We'd rough them on a surface grinder, but
to get them flat, we'd spin them in a fixture that, for all practical
purposes, emulated the machine you are using, but very crude and old. It
was an old B&S grinder with drip oilers, if that helps put things in
perspective. We relieved the side of the wheel, using only the outside
1/4" or so, then, with the work head and grinding heat perfectly parallel,
we'd float the part in a pocket fixture, driven by a loose fitting pin in
one of the holes. We could get the parts flat (less than .0001") by
grinding both sides. It's an excellent way to grind, because there's
nothing distorting the part, which was actually held in place by the wheel.


About the chipping. I can't help but wonder if it's not being caused
by your parts chattering slightly.

That's our current thought. The parts that the customer has supplied have
way too much variation, so the tall ones are under pressure and the short
ones might be jumping around as they pass beyond the edge of the wheel.
Either that or we have a bad wheel.

While it may not be what you're hoping for, it might be a good idea to hit
them on a surface grinder to make them uniform in size before using the
double disk machine. Sort of a rough grinding operation, leaving only
enough stock to bring them flat and to size. Dunno. Might be too much
handling.

Your operation being a total
stranger to me, could it be that your coolant plays into the problem?

You might not be familiar with what we are doing but you're thinking is
dead on. The wheel manufacurer was certain it was the coolant. We were
using an equivelant to the one they sell. We then switched to their own
brand and the wheel still chips. They were claiming that the damage was
already done by the coolant we were using.

Then I'd suggest to them that they change the matrix in their wheels!
That's absurd.

Then I pointed out that the
lower wheel didn't chip and its flooded in the coolant. Hmmmm. Well then
you must have crashed the wheel. I'm telling you, that never happened. If
it did I would just get another wheel and get on with my life.

Sounds like the old "pass the buck" thing to me. These wheels must be
fairly expensive, or they'd pony up another, at least to see if it, too,
chipped. I have a philosophy when it comes to such matters. If, in the
end, I find they're the problem, I make certain to use their competitors
for all future transactions.


Like I said the feed rate is controlled by pressure. We are starting out
at low pressure until we get to a point where we are certain that the
wheel is contacting all of the work. Then it shifts to a higher pressure
and then back to low as it finishes.

Do you have any idea when the wheel fractures? It might not be when the
piece finally dislodges. I'm still wrestling with the pressure.

The lubricity idea is interesting.
I'm going to look into that some more. When we are single side grinding
we are using hardened fixtures that ride on a hardened plate. The part we
are grinding locates on a shoulder in the fixture. So there is
opportunity for something to stick. The speeds are fairly slow though and
the flood of coolant is immense. I am thinking that we could re make the
fixture so that the workpiece doesn't come out from under the wheel as
the fixture rotates around. The problem then becomes we would have to
desin a dummy part for the in process gage. (this part is very small) The
double disk ground workpieces use a dummy part mounted on the periphery
of each fixture for gaging. These dummy parts are larger in diameter than
the part being ground.

So the gage part acts like a stop, but electronically? You'll have to
forgive me, for any machine I've ever run, everything was determined by my
hand. I've never used so much as a DRO in my entire machining career.
Everything was done the hard way.

I like the idea that the part never leaves the wheel. It is likely where
the chipping occurs. By its nature, there's not really a need for the parts
to go outside the wheel boundary, so it might be an excellent thing to
pursue, especially if coolant can lift a part occasionally.

Curious. How much of the wheel face is being used? Is it like a Blanchard?

Thanks for your candor on the manual machines. I'll certainly keep your
kind offer in mind.

Anyway thanks for bouncing this around with me. I've got a couple more
ideas as a result.

Not sure I did you any good, but I'd sure enjoy hearing what you conclude.
Why don't you keep us all posted? These are the kinds of problems that are
interesting, and often very difficult to solve. In the end, you'll wonder
how you missed it, assuming it's not a bad wheel, anyway.

Harold