Machining Graphite ?

My son's senior class project (Aerospace Engineering)is for his group to construct a model rocket with hybrid motor.

I've been asked to machine to motor's nozzle(diverging/converging)using graphite rod that they're sending me. No knowledge/experience with this material- and no idea of "grade" or other info as to exactly what they're sending me. Small stuff- only a couple of inches long and IIRC about 1-1/2" diameter.

I use a manual lathe, typically run carbide tooling but have HSS I can grind up if that's preferable.

Suggestions for machining this stuff (tooling, speeds/feeds)? Cover the ways? Vacuum for dust?

Thanks in advance for any tips.

Take a look here: will graphite powder damage v-ways

tobnpr said:

My son's senior class project (Aerospace Engineering)is for his group to construct a model rocket with hybrid motor.

I've been asked to machine to motor's nozzle(diverging/converging)using graphite rod that they're sending me. No knowledge/experience with this material- and no idea of "grade" or other info as to exactly what they're sending me. Small stuff- only a couple of inches long and IIRC about 1-1/2" diameter.

I use a manual lathe, typically run carbide tooling but have HSS I can grind up if that's preferable.

Suggestions for machining this stuff (tooling, speeds/feeds)? Cover the ways? Vacuum for dust?

Thanks in advance for any tips.

Click to expand...

I mill and lathe graphite regularly.(almost every day)The grade is very important. The finer the grain size, the better.

Depending on the shape, size and cross section, you must do things differently when machining carbon.

If you have very thin areas, you need to be careful not to use too much air blasts. Also a simple dropped toolbit or file can destroy a work piece.

There are tips and tricks to accompanist things that you normally wouldn't think of. For instance:

Lets say your project requires a large diameter at one end and a very small diameter nipple at the other. with a smaller hole all the way thru.

If you drill the hole first, and then turn the od, it will simply shatter as you get close to the finish size.

The only way you'll get this done is to turn the small od, in one pass. No finish cuts. they will only break the nipple off.

Then spot and drill the small hole in the center, clearing the swarf constantly. Lots of light pecks with your insanely sharp drill bit.

Also, you can do everything right, and still break a part. Move on and deal with it. You should treat your first attempt as practice from the start. If you nail it on the first try, great, but you need to allow enough material for a second attempt.

Very sharp tools are a must. H.S.S. is just fine, but pay close attention to the cutting edges. Super sharp, honed edges are a must. If you can, try to stone a .015 radius on your point.

Don't spin your lathe too fast. I would suggest using the speeds for free machining brass as a reference. Very light feeds.

Hope this helps.

SEMCO has a great blog about graphite. Check it out here.

The OP may not intend to do this but graphite is great to use on the surface grinder for checking/mapping a dressed radius/form without causing wheel wear. Often grinds in one pass (careful of heat and grinding force) and throws a reflection free profile in the optical comparator. Thought somebody might find this useful.

Absolutely use a good vacuum for dust control.

In thin cross section it's fragile as an eggshell.

Converging/diverging in a small (1/2" and under) can probably be bought made from carbide
as a sandblasting nozzle....Albeit the carbide isn't as heat resistant.

Thanks for the info, much appreciated.

This is the general idea. I'm not clear on the sketch as to whether the 1.52" dimension is the OD of the nozzle or the exit diameter of the converging nozzle, but whichever it is, I need the other...



Commercial nozzle:




Getting a 12" length (will find out more specifics on the material), cut to length off the lathe (how)? or part off on the machine?

Will be using a 6-jaw Buck (no collets).

Thinking...

Turn OD. Then drill as close to .295 throat as I can, then bore? Or should I order a reamer if time permits? I'd rather ream as boring skinny holes has it's own challenges but don't know how this material will work with a HSS reamer?

After OD, throat is cut, then set compound to desired angle and cut nozzle until exit diameter is achieved.
Flip over (and probably dial-in the throat, not sure if half-thou of runout in my chuck is a big deal), then machine the other nozzle.

I "think" there will need to be O-ring grooves, but no info on that yet.

Sound like the right plan, or changes needed?