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Face Grinding Graphite

dweekley

Plastic
Joined
Nov 26, 2023
Hello, New to the forum.

We have a part made from graphite. It has a .0002" OAL tolerance with a 32 RA Surface finish requirement on both faces. Surfaces also have a .0002" flatness and parallelism callout. Attempting to face grind it on our jig grinder with some success. The surface finish is the current issue.
Looking to see if anyone has any experience with graphite that could shed some light.


Thanks
 
Graphite grinds so well dry that you should be able to use a 60 or finer grit wheel for a decent surface finish..
Dress the wheel face and the OD if using the wheel face.
Hand bump/derss a small bevel on the wheel corner. 64th to 32 OK.
Use a sharp facet single point diamond .

I would mount a spotter on the table with a slip of masking tape, and then a grease pencil on the spotter to get within .0002 with no measuring.
Have a little air to blow chips off/ about as much pressuer as you blow with you breath blowing
 
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Most our graphite parts are VERY small, but we usually need the flatness and dimensional numbers your talking about.

We've had great success with lapping these parts in a matrix made of 4140 tooling plate ground to the dimension required.
Using 1500 grit wet dry and moving to 2000 grit and a finish pass with a clean sheet of printer paper for the final lap.

Your mileage may vary.
 
Hi dweekley:
If you are hoping to face grind something like this on a jig grinder, I expect you will always have difficulties holding your finish and I say so because of the mechanics of face grinding.
The culprit as I see it, is that the contact time between any single abrasive grain in the wheel and your graphite part is very long.
Contrast that with a surface or cylindrical grinder where the contact time of one abrasive grain in the wheel is very short.
This matters because each grain is a like a small boulder, scouring across the workpiece and ploughing up a furrow of debris.
The longer the contact time, the more debris packs up ahead of the boulder and the more chance to plug up the wheel, making it skate over the surface and trashing everything around it...it's one of the reasons coolant on a grinding wheel is so helpful to getting a good finish.

So you may have no choice but to surface grind for finish and tolerance and jig grind only the features you cannot surface grind.
It's a PITA for sure, but one you may have to accept.
An alternative you may consider, is to index the part so you can run the wheel past the surface as if it were a miniature surface grinder...the hassle of doing so is one you can evaluate better than we can.

I may be wrong on this and you may find a wheel, speed and debris removal combination that lets you get there while face grinding but I'm not hopeful.
One thing for sure...if you can keep the debris under control I expect you'd have a better chance.
An air blast may help you although the crap it throws into the air may well make this a non-starter.
Maybe a coolant blast instead.

Also, I've had better success using silicon carbide wheels rather than aluminum oxide for surface grinding graphite, and I don't know why.
I also don't know whether that experience translates to jig grinding.

One caveat with silicon carbide wheels...they tend to leave abrasive grains behind on the wheel when you dress them and those can plough furrows into your surface in a heartbeat, so condition your wheels with a bit of wood to knock off those grains before you grind.
Some grinder hands just use their finger...watching it always made me squeamish so I always used a bit of maple, but I haven't ground graphite in decades.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com
 
One caveat with silicon carbide wheels...they tend to leave abrasive grains behind on the wheel when you dress them and those can plough furrows into your surface in a heartbeat, so condition your wheels with a bit of wood to knock off those grains before you grind.
+10 on this^^^^^^^^^^
If you're going to go this route and want to keep your fingerprints in tact, you can simply find a piece of scrap, hardened A2 or the like and take a .0003 pass on your grinder to "dull" the surface. But I don't feel confident in this method for surface finish reasons.
 
Dressing the wheel sides, it would seem that three part sides could be ground in one holding, and a sides and face dressed wheel runs smother.

For smaller parts one can parallel clamp two bars to a Jo Block or Jo block stack to quickly make go and no-go gauges. A simple surface grinder is quick to make gauges , and good back-up for making graphite parts.
A wood sticking the wheel to knock off loose grits can help surface finish, along with experimenting with grit size.
A set-height diamond can be an asset, so the target height may be .001 down feed from the dress height dial zero. With that your down number stays the same .001 (or what) from the dress number.

Usually the dressed wheel OD gives the best surface finish.
 
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