Cutting the Hard Stuff Right
Precision machining of hard materials requires close attention to processing parameters
By Stanley Hanson
Core Cron Corp.
Compared to standard machining of traditional materials (steel, Al), successful precision machining of hard materials is more sensitive to parameters such as machine-tool accuracy, stiffness, toolholder design, cutting-tool material and geometry, fixturing, coolant presentation, and machining technique.
Hard materials offer designers attractive properties, but machining them is a challenge (Click on image to enlarge).
Hard materials can be classified into two groups--single-component materials and composite materials. Single-component materials include metals such as hardened tool steel, glasses such as Pyrex and borosilicate, and ceramics like SiC and Al2O3. Composite hard materials include metal/ceramics such as metal-matrix composites (MMCs) and tungsten-carbide/cobalt, glass/ceramics such as Zerodur and Cervit; and ceramic/ceramic composites such as silicon-carbide/silicon.
Material hardness is defined by its resistance to permanent deformation. Hardness shouldn't be confused with the modulus or toughness of the material. It's the combination of elastic and plastic properties that determine a material's resistance to permanent deformation (not to be confused with either elastic deformation or exclusively plastic deformation from dislocation, slip, etc).
It's the hardness characteristics of these materials that make them candidates for commercial uses including: semiconductor applications; aerospace instrumentation; and medical and dental implant devices that more closely mimic nature. Machine-tool and metrology-systems manufacturers are moving toward harder materials such as MMCs and straight ceramics to improve the stiffness of machine beds and frames, and to provide enhanced thermal performance, reduced susceptibility to corrosion, and extended service life.
The properties that make these materials attractive for commercial use also make them extremely difficult to machine to the tolerances required by advanced applications.
Precision machining (as used here) is any process using a cutting tool, whether turning, milling, or grinding, which forms a precise dimension, form, and finish of surface. The accuracy held must be 0.0004" (10 µm) or less. Any operation resulting in less accuracy is generally considered conventional machining.
Obtaining tighter tolerances on hard materials is a challenge that must be met if manufacturers are to achieve the improved performance; it's also where the future of manufacturing lies.
A major factor that influences the production of close-tolerance parts from hard materials is the machine tool itself and its parameters, including inherent repeatability, accuracy, stiffness, and the smoothness or uniformity of travel, spindle speed, thermal stability, dampening, machine protection, control capabilities, etc. In more than 20 years of evaluating methods of obtaining tighter tolerances on hard materials, it has been our experience that less than one out of 10 machine tools has the precision, rigidity, and repeatability to produce close-tolerance parts from hard materials efficiently and economically.
Efficiency and economy are the operative terms. Virtually any machine tool can produce some close-tolerance parts if the feed rate is reduced and the cutting tool changed frequently. To successfully produce precision components to meet market demands, however, the machining operation must be cost-effective, as well as accurate and repeatable.
A key design factor in machine tools is the rigidity or stiffness of the cutting tool to the workpiece. Obviously, components and subassemblies must also have high stiffness. Machine stiffness is a major contributing factor to overall machine accuracy and performance. Stiffness is measured by the deflection of an element of the machine when it's subjected to a load; for example, spindle to table. During the evaluation of machine stiffness, we often take up to 100 measurements, using a millionths dial indicator and a force gage, to measure deflection and characterize machine performance. We've found that machines that fall into the 40/10 µin./lbf, and 80/10 µin./lbf range account for a large percentage of machine tools. These machine tools are marginal for precision machining operations. However, there are machine tools that fall into the 5/20 µin./lbf to 20/10 µin./lbf range. These machine tools are excellent candidates for precision machining of hard materials.
Machine accuracy is another critical design parameter. Traditionally, manufacturers have ensured the accuracy of parts by calibrating only the linear motion of machine tools. When making 3-D parts, however, knowing the machine's one-dimensional accuracy ignores squareness, straightness, angular, thermal, and non-rigid body errors that are major contributors to positioning inaccuracies. Calibrating and compensating volumetrically significantly improves the overall accuracy of the machine. Linear positioning accuracies and volumetric positioning accuracies differ substantially. To have the confidence to cut high-precision parts on a production basis, it's necessary that the user know the volumetric-positioning accuracy of the machine tool.
Among the machines qualified for precision hard machining, Core Cron used two HU40A HMCs from Mitsui Seiki (Franklin Lakes, NJ) to gather data. The HU40A Horizontal Machining Center features a very rigid sub-structure. Machine design features such as triangular ribbing and three-point bed support help eliminate dynamic machine distortion that can affect repeatability. The column load is reduced through the use of an air support system with air cavities between the way and the mating surface of the column. Charging these cavities with air reduces the load on the way, and minimizes friction and heat. The way system performs like a high-speed way system without sacrificing the rigidity and stability of conventional box way designs.
The same criteria apply to toolholders. They too must provide precision, rigidity, and repeatability to produce close-tolerance parts, and to do so they must be kinematically correct. Fewer than 5% of the toolholders we've evaluated qualify for precision machining applications in hard materials.
The reasons for this conclusion involve design and manufacturing techniques. For example, if a toolholder is designed to 0.0005" (0.013-mm) tolerance, it may be difficult to hold 0.0002" (5 µm) tolerance on a precision part. Also, toolholders can exhibit very high axial rigidity when drilling or reaming, but exhibit less rigidity when side forces are applied during a lateral cutting operation. It's important to select the appropriate toolholder for the application, with--as always--an eye to cost. Parameters include: accuracy, rigidity, runout, robust design, reasonable cost, long life.
Cutting tools are another element that produce a major effect on the production of precision parts from hard materials. The cutting tools for machining hard materials include PCD, diamond wheels, CBN, diamond on carbide, and coated carbide, and must have the design strength and runout as assembled in the machine tool to provide not only efficient material removal, but also optimum tool life. Parameters to be considered are: material, design, fabrication, tolerance, sharpenability, cost, and availability.
Tool life is an economic issue that must be considered when machining precision parts from hard materials. While it may perform well, a tool that you must change after every 3" (76 mm) of cut length is not an economical solution to machining these materials. Tool life depends upon the material to be machined and the process. In our evaluations, for example, we've found that a proprietary PCD cutting tool has a life of >1000" (25 m) when machining a 30% SiC-Al alloy MMC at a speed of 3000 fpm (76 m/min) with a material removal rate of 4 in3/min (65.6 cm3/min). In this test, tolerance achieved was <0.0002" (5 µm) and precision, or accuracy, was <0.0001" (3 µm), well within the definition of precision machining hard materials. This performance would appear to demonstrate a good combination of tool and speed.
Workholding is another key element. Material considerations are important. Workpiece material may be harder than the material used to make the clamping device. In the case of a vise, for example, soft jaws tend to embed at the first point of contact. This behavior makes it hard to adjust the workpiece--it's difficult to tap it down with a hammer, for instance. The solution is to match the clamping material as closely as possible to the workpiece material. Other solutions include using magnetic fixturing, glue, wax, or vacuum fixturing to hold the workpiece, which must be kinematically correct. Parameters include: design, constant force or displacement or body force, material, tolerance, availability, and user friendliness.
Coolant presentation is an extremely important factor to consider in precision machining of hard materials. In any machining operation, heat must be controlled and removed from the tool and workpiece rapidly and consistently. As much of the heat as possible must be removed with the chips. Cutting with superabrasive materials, like diamond, can create temperatures that go above 3000ºF (1650ºC) within a fraction of a second at the interface of the tool and the workpiece. That is far too much heat to expect the workpiece or the cutting tool to stay intact for long. Consequently, there's a need for the coolant to help dissipate that heat. Parameters involved are: pressure, volume, speed, temperature, cleanliness, and wetting.
Coolant pressure is important, but only for adequate flow. Typically 30 - 100 psi (206 - 689 kPa) is all that's required for machining hard materials, and there doesn't seem to be an advantage to high-pressure systems if the cutting operation is being performed with the proper tool. However, there is an advantage to through-spindle and through-tool coolant presentation. If the coolant is not presented in this manner, it must be very well directed to provide the heat removal required. In some cases, coolant lubricity is a factor, but at the high interface forces necessary in machining hard materials, lubricity is not a major concern.
Optimizing these parameters and combining that optimization with the correct part program is what we refer to as technique. In addition to the parameters already discussed, technique includes selection of the cutting operation. In cutting aluminum with a standard end mill, for example, one of the first things to determine is whether you should climb-cut or cut conventionally. On an older machine that doesn't have backlash control, you have to cut conventionally, but the finish is poor, tool life is shorter, and usually the dimensions can't be held tightly. With hard material, conventional cutting almost certainly chips or blows-out the edge of the part, resulting in poor tolerance control and a bad finish.
Machining technique is critical to success. Even when all other parameters are closely controlled, bad technique can create scrap parts and significantly increase operating costs.
To successfully machine hard materials to precise dimensions, it's necessary that key processing parameters be carefully defined and closely controlled. Quality production starts with a quality machine tool of sufficient stiffness for the precision needed. If the cutting tool is not selected properly, if the coolant is not presented effectively, and if the workpiece is not held properly, the required accuracy in the production of parts for these materials is not achievable. Accuracy is still attainable, even when some of the parameters are not effectively controlled, but production will take considerably longer, making the manufacturing process uneconomic. There are always tradeoffs, but to produce precision components from hard materials, these key processing parameters have to be defined and closely monitored.
This article was first published in the May 2005 edition of Manufacturing Engineering magazine.