"Harold and Susan Vordos" wrote in
:


"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.

They are resin bonded. The one we are using is designed for low pressure
double disk grinding. The main advantage is you can go a long time
between dressing. So they pay for themselves in high volume applications.
We were able to go about an hour between dressing with the CBN. We were
doing 140 pcs. per load. The cycle time starts out under 30 seconds. In
an hours time the cycle time gets to be about a minute and we dress the
wheel which takes about five minutes. If the blanks were off of a cnc
rather than a multi spindle we could do better. The amount being ground
was excessive IMO.



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.

I've got a bit of an update. A customer has comitted to the machine
today, and for his application I'll be using conventional wheels and
double disk grinding. Funny you should mention turbos...



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.

A-286. Ick. Ive cut a bit of that. I worked on an interesting turnkey
once on a Y-axis sub spindle twin turret lathe. We rough and finish
milled then rough and finish ground an A-286 part. We were running parts
and I heard something heavy fall into the chip pan. I looked into the
machine and couldn't see anything wrong so I went around and looked in
the chip pan. There was half of a 3/4" end mill in there. Never stopped
the machine and didn't damage the part. The end mill just got dull and
broke. Tough stuff that A-286.
I'm amazed at how guys like you made parts back in the day. If you were
to watch us grind on this machine you wouldn't believe how easy it is.
That is once you get the fixture right, get a good wheel, and dial in the
program. On manual grinders you pretty much have to be on your game all
day. Now-a-days all the development is up front (my favorite part) then
once it's dialed in and proven capable there is not a lot of skill
required in running the machine.



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.

Yeah it would be too many ops. That customer is surface grinding now.
Productivity and quality is not nearly as good as what we are doing.


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.

They were passing the buck without a doubt. That's what prompted me to
ask you about double disk grinding. I was hoping to find another supplier
and maybe get some advice. Anyway I don't have a lot of faith in these
guys anymore. I'm going to give Norton a call for this new deal.



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'm telling you that you would find no joy in operating this machine.
Setting it up and wringing it out is where the fun is. The way the gage
works is that you set it with gage blocks and zero it out. During the
cycle the gage advances and measures the part or dummy part in process.
As the machine gets close to size it reduces the pressure (and therefor
the feed rate), then when the gage measures zero it pulls the wheel up,
then stops.


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.

Thats our current theory. If that customer wants to carry on, I'm
thinking we either change the wheel or use a fixture where the part stays
under the wheel and we use a dummy for gaging.


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

Sort of except the wheel is not segmented. The whole face of the wheel is
used.

snip


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.

That's why talking things out is always good. It gets your mind out of
the rut it's in and suddenly you see the problem. Unfortunately I wasn't
involved in this project until recently, so I'm trying to get up to speed
fast. I'll let you know how the new deal goes, and if we ever get back to
the other jobs I'll let you know what we figure out.

--

Dan