SCD tools not just for specialized machines

Single-crystal-diamond tools are routinely used to cut optical-quality components on special turning machines. But they also can run on conventional machine tools—often to great advantage.

The main reasons are that many of today’s CNC machining centers and lathes incorporate ultraprecision ball bearing spindles, which couple with high-speed servodrives. And, control of axes motion has improved, thanks to linear motors and submicron-positioning feedback systems. Augmenting these improvements are ongoing CAD/CAM software developments that help the end user take full advantage of a machine’s enhanced capabilities.

Prior to milling this copper, high-pressure manifold with a diamond-tipped carbide insert, shop personnel hand-lapped the sealing surfaces with a series of fine abrasives. One SCD tool can cut hundreds of parts, saving many hours of labor-intensive processing. All images courtesy Chardon Tool.

Cutting tools have evolved, too. There are literally thousands of ways geometries, wear-resistant coatings, substrates and other tool elements can be combined to optimize performance. Most of these tools effectively remove material, have acceptable wear life and meet the user’s surface-finish requirements. But there are applications where no other cutting tool material can duplicate the results achievable with single-crystal diamond.

Like the name suggests, SCD is a solid crystal made from tightly packed carbon atoms. Using special equipment and techniques, SCDs are shaped into tools whose cutting edges have no apparent chips at magnification up to 1,200× and exhibit extremely good wear resistance. Additionally, an SCD can be vacuum-brazed to almost any carbide insert.

Single-crystal diamond vs. other cutting tool materials

1 Single-crystal diamond. 2 Polycrystalline diamond. 3 Tungsten carbide. 4 Polycrystalline cubic boron nitride.

To use or not

Choosing whether or not to apply SCD tools depends on a number of factors. The following questions and answers are intended to help you decide if an SCD tool is right for your application.

When should I use SCD tools? When superior surface finish, part precision and long tool life are required. Depending on the machine tool, it’s possible to achieve a surface finish of Ra 0.5µin. or better with an SCD. The structure of diamond is such that it can be sharpened to an almost perfect cutting edge. This unique property lets it produce mirror-like finishes. Diamond is also the hardest and most abrasion-resistant substance known, allowing it to outlast carbide tools by 1,000:1 in certain applications.

The shop that made this aluminum airframe casting was hand lapping the part’s difficult-to-reach locating pads. Each surface had to be inspected. The company installed a diamond-tipped insert into its existing tooling and facemilled the part. This eliminated all hand work, and the process is now so repeatable that quality-control requirements are met with batch checks.

How do I know if my machine tool is rigid enough for SCD tools? Spindles should run quietly and be free from vibration. Slideways and lead screws must have minimal slop and backlash. Most modern CNC machines meet these requirements, making them suitable to run SCD tools. Styles are available for turning, facing, grooving, milling, spot facing, flycutting, boring and trepanning.

What kind of materials can I machine? Just about any nonferrous metal—including aluminum, copper and brass—as well as plastics and many kinds of crystals. SCD is not suitable for metals that contain iron.

What kind of feeds and speeds should I use? In general, SCD tools are for fine finishing, so light cuts are the rule. Depths of cut range from 0.0001" to 0.001". Feed rates depend on the tool tip radius and desired surface finish. A good starting point for the feed is 0.400 ipm. Due to the cutting edge’s sharpness, cutting speed is not overly important. However, 1,000 sfm is a good initial speed.

What about coolant? Users can opt for spray mist or microdrop lubrication. If flood coolant is used, it should be fine-filtered to remove swarf and fines that can become trapped between the cutting edge and workpiece surface. These particles can chip the cutting edge or degrade the surface finish.

Is an SCD tool expensive? The initial cost is significantly higher than for a normal cutting tool. A typical diamond-tipped insert sells for approximately $300. However, utilizing SCD tools can often eliminate costly, time-consuming lapping, polishing, honing and buffing of parts that have stringent requirements for geometric-form accuracy and surface finish. Moreover, SCD tools can be resharpened many times, and at a relatively low cost.

What else do I need to know before cutting with SCD? Because of its extreme hardness, SCD is brittle. Care must be taken when handling tools. Don’t touch the cutting edge with fingers, dirty chip brushes or rags. When not in use, the tools should be kept in their protective packaging.

An end user was machining this housing for an underwater camera on a conventional CNC lathe. The domes were hand-polished to make them optically clear. The end user installed a diamond tool to make several fine finishing cuts, a move that eliminated the labor-intensive polishing process.

Workholding and toolholding devices should be rigid and tight. Vibration is one of the main causes of excessive SCD wear and poor cutting performance.

When possible, use a noncontact tool-presetting system. If that’s not possible, place a plastic shim between the diamond and the surface of the preset device to avoid hard contact.

SCD isn’t for everyone or every job, of course. The machine tool must be in good condition, the workpiece material must be of good quality and the operator needs to be trained on tool usage. But when mirror-like finishes and submicron accuracies are required, an SCD might be the tool to turn to. µ

About the author: Hugh McAllister is sales engineer at Chardon (Ohio) Tool, a producer of diamond tools for ultraprecision machining applications. Telephone: (440) 286-6440. E-mail: hmcallister@chardontool.com. Web site: www.chardontool.com.