Disposable inserts with a 1/64- to 1/32-inch nose radius are most effectively employed for turning graphite. A positive rake insert without a chip control groove is preferred. Cutting fluids and/or coolants are not recommended. Adequate and well-maintained fixturing, rigid tool setup and well-adjusted machine tools always provide superior results.
When machining long rods and cylinders, higher speeds and depths of cut can be employed with higher strength graphite materials. Depth of cut should always be maximized when possible without incurring distortion of the part. When distortion or whip are present, feed and depth of cut must be adjusted. Lower feedrates will allow holding deeper cuts. Feedrates of 0.005″ per revolution for roughing and between 0.001″ – 0.003″ for finishing might be necessary. Deeper cuts always generate higher pressures and larger fracturing particles, thereby producing rougher surface finish.
Machine setup and measurements are best done with non-contact of the diamond cutting edge. This includes use of a soft material such as plastic or paper for touching off. Cutting edge contact with any hard material prior to cutting potentially will damage the edge.
Breakout at the end of a pass is always a concern. This can be avoided by having a chamfer cut at the end of the part to ease exit of the tool, or provide stock which can later be cut off. Avoid square-nosed cut-off tools to avoid breaking prior to completion of the cut. A 20-degree angle is recommended. Surface finish improvements can be achieved with tool geometry combined with feedrates. Larger nose radii will produce improved finish but with increased tool pressure. A smaller nose radius will relieve pressure, but feed must be lessened to achieve comparable surface finish. Depth of cut will not affect surface finish unless it causes excess tool pressure – resulting in vibration – or if it is too light (under 0.005″) to create an adequate amount of material removal.
Frankly, it’s not easy to turn the graphite, because the graphite is easy to break if your process is wrong