waviness in cylindrical grinding
A PM member recently raised concerns about waviness in final passes of od grinding. This article addresses this concern from a different view point. It is directly copied from Cutting Tool Engineering online magazine. I have had previous phone and email contact with editor and post this with his prior general approval in the interest of education.
Dear Doc: I cylindrical grind nitrided shafts with 20 spark-out revolutions. Sometimes I can sparkout forever and still never remove the waviness, and other times it takes only a
few spark-out passes. I spark-out at 200 rpm with a constant wheel speed of 40
m/sec. (8,000 sfm) and a wheel diameter from 500mm to 350mm (19.7" to 13.8").
The level of waviness seems to depend on wheel diameter. Why is that?
The Doc Replies: No grinding wheel is perfectly true, and that imperfect
roundness on the wheel inevitably gets put into the workpiece in the form of waves.
The key is not to “catch the waves” when the workpiece makes successive spark-out
revolutions. Take the wheel rpm and divide it by the workpiece rpm. That’s the rpm ratio. If the rpm ratio is an integer value, such as eight, then the workpiece catches the wheel in
the exact same wave and exacerbates the waviness (see Figure). If the rpm ratio is a
fractional value, such as ¼, 1⁄3, ½, 2⁄3 or ¾, the situation is a little better as the previous
wave is obliterated by the next wave, providing a series of scallops. The ideal situation is a long-decimal value, such as 7.84923785. Here, you never “catch a wave,” but just keep
obliterating previous waves. I’ve seen unbalanced, out-of-true wheels generate little waviness because they found such a value. In your situation, the wheel rpm ranges from 1,528 to 2,182 rpm. The rpm ratio is going from 7.63 to 10.91, passing through three integer values (eight, nine and 10) and numerous fractional values. How do you make this happen on the machine? You can constantly monitor the rpm ratio to avoid integers
and fractions, which is a hassle on an automated machine. You can run at a constant wheel
rpm, which means the wheel surface speed will change throughout the life of the wheel.
Or, the CNC programmer can choose a combination wheel and workpiece rpm that
always gives a long-decimal value. That’s the best method. Why don’t more CNC programmers program in long-decimal rpm ratios? It would take 10 minutes to do, but they
typically don’t know about it. Too many of them have an attitude of “leave the grinding
to the grinder.” That may change, but, in the meantime, you’ll have to sort it out yourself."
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I run a large surface grinder, 36" x 168". Dealing with the waviness is what seperates the men from the boys. It all has to do with what sfpm you are running and even down to how your diamond is in you holder. Sometimes just turning your diamond solves the problem.
It is all part of the game.
A guy can tell you what his computer program tells him what to do but unless you are actualy truning the handles on the machine you do not know what it is all about.
IMHO grinding is 90% feel.
Originally Posted by cash
Here in S.E. WI I have worked with more card-carying life-long
machinists and tool-makers who seem to have managed to learn
nothing from the feed-back that the machine (grinder) gives you......
Than milling and turning and benchwork all totalled.
I kept my grinding abillities hid in the bottom drawer of my toolbox
for ten years at the place where CNC milling expertice was top-shelf.
Anytime I was loaned to the mold-makers,
I waited till first shift all went home and then
allmost ready to throw-up from my instructions,
I let loose and enjoyed myself. Often finishing in a fraction
of the time given, and always with geometric integritty
that they were taken a'back by the next day.
I always enjoy your posts Cash.
They left me with over 20 .500 ejector pins that were all .152 too long
due to a design change. Not enough lenght left to recut them,
they hoped I would be done in one shift. They did the pins for the twin mold.
All the pins were burned on the end, he said he spent half the day redressing.
The way they held the pins didn't allow for all the heat and a few were
too short once cooled.
I had 20 pins perfect by lunch, two wheel dressings, and his 80 wheel back
on the hub so-as to avoid interigation. Mine were all ground dry,
lenght +/- .0005 and never heated to any color.
Was met with cold indifference the next day.
Wouldn't ever ask how to do it a better way.
Did you use a standard cut off wheel to get them close, and then finish grind to length. This is the process we use for grinding our ejector pins and core pins for molds
They eventually bought a EJ pin machine, it does both.
Originally Posted by M Owens
But when I was there we had an old grinder that always had a
fixture for chopping pins in the carbon room. 3/32 C-off wheels.
Always all used up.
I didn't like this fixture or their vertical V-block based fixture
that was used for finishing the pins.
Over the years I have put hours in at three different mold-shops.
To me a feeble half-assed V-block and a wing-nut are not up to the task.
I had rectangular bars with reamed holes in each end.
The holes were split and had a cross-screw for pinching shut.
Each bar did two EJ pin sizes. Once a pin was slid up into the bar
I settled it on a 1-2-3 block. clamped shut then remove the block.
If any heat did run down the pin, it didn't affect it's vertical position.
To trim 20 1/2" pins .152 with a .093 wheel half burned up
I would still need two set-ups. So I used my bar for 1/2" pins.
About 1/4" sticking out the top. White Norton 46 sft wheel.
Head set to peel off .149 in one (1) pass.
Rather fast 'X' travercing. cutting from behind the pin first,
bringing the wheel toward me in steps course enough to
sacrifice a little wheel with each touch. (The wheel perishes)
just enough to stay fresh the entire batch.
Once the pin end is ground to center...... I stop and cut from
the front to the center same technique.
The two cuts meet in the middle. Pin never gets hot.
Then kiss the last three thou off with the same wheel.
Very slow 'X' feed, normal use of S.-grinder.
Bingo 20 pins , Perfect lenght, no edge burr.
Only reason to dress is the wheel gets quite round after a while.
So take it back to flat.
I just like to tell it as it is.
I started running the grinders when I was 18 and 17 years later still love doing it. I always try to learn somthing new every day on the machines.
In this time I have gotten pretty good at it as well.
That's my motto too.
Originally Posted by cash
My wife has orders to shoot me if 3 or more coworkers
tell her I have become closed-minded and set in my ways.