A Holistic Approach to Machining
Difficult machining tasks can be tackled with confidence
By Jim Lorincz
Manufacturing has undergone a remarkable transformation toward productivity, quality, output, operating efficiency, and cost savings in the last decade. In this exclusive Q&A, Bert P. Erdel, manufacturing management consultant, discusses how manufacturers must adopt machining strategies that will enable them to meet the continuing challenges to their competitiveness in a global environment.
Manufacturing Engineering: Why is a new approach to machining needed today?
Bert P. Erdel: The attributes of small, fast, light, and smart have begun to permeate all facets of industrial manufacturing. As products become more complex, the way we make them demands utmost attention to every detail and element of machining processes. We must be cognizant of the fact that best practice is best process, and only a holistic approach can yield true high-performance machining.
ME: What is a holistic approach to machining?
Erdel: Products are made by processes, and the two are intertwined. The product's form, functionality, precision, size, and dimensions dictate the process, which, in turn, determines the acceptability of the finished product. Whether it is form over function or vice versa, style and looks dictate to engineering and manufacturing the end product.
ME: How do manufacturers benefit from high-performance machining?
Erdel: High-performance machining is supposed to positively affect the manufacturing bottom line of cost—effectiveness, productivity, quality, and efficiency. There are certain criteria by which performance and impact on the bottom line are measured. The effectiveness of machining processes can be determined by cycle time, price/performance ratio, part finishes, tolerances, repeatability and accuracy, process robustness, and—ultimately—cost per part. Parts production can be measured similarly by scrap rates, response time, non-conformance, delivery time, customer satisfaction, ease of assembly, and quality. All will determine the status of the manufacturer within the industry.
ME: What are the critical elements of the machining process?
Erdel: To stay within the framework of the given criteria and meet the targeted requirements, we first must have a complete understanding of the entire machining envelope. Each element has a direct impact on the machining outcome. Machining results are produced by the right combination of machine tool, cutting tool, toolholding, workholding, coolant and chip control, programming and part design, and quality control.
ME: What levels of technology are required?
Erdel: The level of technology of machine tool, cutting tool, and toolholding should be commensurate to one another. Certain process design aspects deserve special attention. They include the latest developments in linear motors, thermal control, bearing technology, and design and construction that produce dynamic stiffness for machines and spindles. Tooling interfaces need to be capable of high-speed machining—such as HSK and shrinkfit toolholders—balanced, and stiff. Cutting tools are benefiting from new materials and coatings and multipurpose designs, and can be optimized to various workpiece materials through available cutting data. Similarly, we must identify what technology-based processes are best suited with regard to parts material, end-product requirements, and equipment availability. There are several process strategies to choose from, including multifunction machining, one-pass machining, high-speed/high-feed machining, and dry/near-dry machining.
ME: How can a holistic approach affect machinability?
Erdel: Given the availability of the selected technology, it is the workpiece itself that determines the degree of machining difficulty. Some of the workpiece material is even known as difficult-to-machine material. A holistic approach with attention to technological detail can alter this to a machining-with-ease material.
ME: How does multifunctionality achieve machining objectives?
Erdel: Multifunctionality, for example, goes beyond CNC machining, and is seen in multitasking machining, where different operations, such as turning, milling, drilling, and grinding are done on one machine, saving setup time, part handling, and floor space. The objective is to produce parts complete in one fixturing. It also plays into the technologies of hybrid processes, like laser-assisted turning or ultrasonic drilling, and the challenge of designing reconfigurable machines. Designs of multifunctional cutting tools can feature the finishing of several part contours, preferably in one pass rather than through sequential operations and passes. For example, drilling and milling with one and the same tool, or interpolating bores of different diameters with one tool through helical and circular milling or plunge milling.
ME: How are developments in high-speed cutting [HSC] affecting machining results?
Erdel: Defining high cutting speed can only be done in relative terms. Machining nickel-based alloys is considerably slower than machining aluminum, for example, but recent innovations in cutting technology are pushing the envelope further. Two examples illustrate that quite dramatically. Machining titanium alloys begins with the fundamental understanding that the melting point of titanium is 1660°C, while that of carbide is roughly 1000°C. Because of its hardness, coupled with the fact that the failure of the cutting material is usually catastrophic [there are no visual wear marks], short tool life and high demands on machine spindles make this part material challenging, indeed.
First, let's look at the necessary machine tool characteristics. Since there are extraordinary demands driven by the process, structural and thermal stability and dynamic stiffness [minimal vibration] are required. Cutting forces generated and desired accuracy prescribe spindle design with angular contact ceramic ball bearings or hydrodynamic fluid bearings. Built-in HSK assures high repeatability, accuracy, and precision at elevated speeds, and vector-controlled spindles can provide fullpower, full-torque over the entire rpm range. For machining complex contours, five-axis machines with a volumetric accuracy within microns should be specified. Efficient chip discharge for high chip volume [in roughing operations], as well as increased coolant volume/pressure for cooling the cutting zone are necessary.
Because heat buildup within the machine is a given, any potential thermal drifts have to be compensated for, and spindle and axes movements must be monitored by temperature sensors. Special software algorithms can predict thermal drifts at various machining intervals and cutting parameters. Defined thermal control keeps potential drifts in the Z axis to ensure accurate machining, unaffected by internal heat buildup.
ME: How important is the cutting tool interface?
Erdel: From the machine spindle on out, the machinetool/cutting-tool interface, which includes adapters and toolholders, plays an important role in high-performance machining. Whatever decision is made about the type of spindle connection or toolholder employed, besides the expected precision, an absolutely balanced condition has to be secured. Often misunderstood, the failure at the cutting edge is preceded by microcracking of the cutting material, usually induced by vibration, and enhanced by out-of-balance tool-assembly conditions.
Maximum allowable residual balances have been empirically determined for precision machining and are classified by G grades. For precision machining, the most acceptable are G2.5 and G6.3. For medium-speed precision machining, G6.3 is acceptable, as G2.5 is for high-speed machining. Dynamic balancing is particularly important for extreme tool lengths and toolholders with asymmetric designs, long overhangs, excessive weights, and high spindle speeds.
ME: What advances in cutting materials are aiding HSC?
Erdel: Regular production machining and extensive tests in the aerospace industry have shown that carbides with elevated cobalt content of about 10% lend more desired toughness, while AlTiN coating reduces the impact of heat during cutting. The increased aluminum content creates a layer due to its oxidation, thus creating a barrier between the workpiece material and the carbide substrate. This smooth layer also makes for easier chip flow. Filling the gaps between the micrograins of AlTiN with Si3N4 gives this so-called supernitride coating much higher abrasion resistance and markedly reduces adhesion wear, since Si3N4 constitutes a diffusion barrier and delays the oxidation process. Supernitride coatings extend the usable cutting temperature to over 1100°C.
ME: How does cutting geometry affect performance?
Erdel: A stable, sharply honed cutting edge reduces stress and heat during the machining process. Maximizing the cross-section between clearance angle and rake face gives the insert more stability. For boring or reaming, separating the primary and secondary lead angles subdivides the chip loads per edge, thus reducing stress. For milling operations, tangentially arranged inserts reduce the cutting force on the insert, while helically shaped milling cutters allow for interpolating cutter paths and also reduce cutting force. Redirecting radial cutting forces to become more axial is done with 35 to 45° lead angles for face milling, and positively arranged inserts decrease cutting forces as well. The less force needed and used, the less heat is generated and the longer the tool life becomes.
No matter what the operation, machining needs to be done with cutting fluids for cooling. Two machining passes are preferable to a single pass to minimize stock removal per pass, and ramping-in of the tools must be done gradually. Sudden shifts of tool direction, speeds and feeds should be avoided. Current state of experience and successful titanium machining pegs the maximum values at: cutting speed 400-450 fpm [122-137 m/min]; stock removal of 0.5-1 mm; and feed of 0.5-1 mm per rev.
These values can be used for traditional titanium workpieces; for example TiAl6V4 or the advanced Ti5553 alloy, which includes 5% V, 5% Mo, and 3-5% Cr.
ME: How does this holistic approach apply to machining composites?
Erdel: Because of their superior strength and stiffness, and their low specific weight, combined with good formability, composites have made significant inroads into industrial manufacturing. The term composites needs some clarification, for they are not all equal. What composites have in common is that they consist of a matrix and reinforcement material that are manmade and artificially grown, whereby the matrix material gives the part its shape and the reinforcement material its physical properties.
There are three groups of composites:
- Polymer Matrix Composites [PMC], which use a matrix material of plastic and reinforcement of fiberglass, carbide, or aramid,
- Metal Matrix Composites [MMC], in which the matrix material is aluminum, magnesium, or titanium, and the reinforcement is silicon carbide, aluminum, or graphic fiber, and
- Ceramic Matrix Composites [CMC] which use Si3N4, Al2O3 as the matrix and aluminum or silicon carbide as the reinforcement.
There are differing requirements for the varying material compositions.
Polymer matrix composite's relative softness makes for easy machining, which is carried out at lower speeds to avoid excessive heat and degradation of the material. The group of MMCs is the most popular in aerospace. Complex contouring makes five-axis machines necessary. Due to their extreme hardness, CMCs are machined with nontraditional processes. Typically, composite parts are delicate in design as to their size, material thickness, and shape. Therefore, special attention must be paid to workpiece clamping, applying minimum pressure, and achieving more secure holding by design. For composite sheets, which are sensitive to compressive stress, workpiece support and workpiece clamping must be done in a planar fashion, to offset each other's forces.
Because cutting forces are generally small, there is no need for high-output machine spindles. Also, the emphasis is more on speed than feed. Toolholding, minimal runout, and balanced conditions allow machining at elevated speeds. The cutting material of choice is polycrystalline diamond [PCD] for tool life and stringent finish requirements. Carbide is the alternative choice for less demanding requirements. I suggest that manufacturers approach the finishing of ceramic composites with laser assistance or high-frequency oscillation to achieve better tool life.
As for the question of cutting fluids, in principle, the same basic rules apply as with other part material. They provide lubrication, cooling, and chip disposal. If the elimination or reduction of cutting fluids is pursued, machines have to be equipped with dust encapsulation and chip exhaust inside the machining area. For part-penetrating operations such as drilling or reaming, minimum-volume lubrication would be the substitute for regular coolant application to ensure needed process lubrication.
ME: What special strategies are used in aerospace composites machining?
Erdel: In aerospace, there is more and more stacking of dissimilar materials, for example, a combination of metal-matrix composites, aluminum, and titanium. The challenge is to finish-drill the combined holes with absolute straightness and roundness, and to ensure good tool life. Cutting forces, speeds, and feed rates can be difficult to control to make machining a robust process with a high degree of repeatability and accuracy.
One way to approach precision holemaking in such sandwiched material is to drill with subsequent reaming, using padded reamers that do not follow the previous machining pass, but cut their own hole, cleaning up tapered holes or uneven material stock. These pads absorb the cutting forces generated, foregoing deflection during cutting. Another way of tackling such stackup is through orbital boring, following a traditional drilling pass. Similar to circular milling, orbital boring creates the hole incrementally through forward drilling and a circular motion, thus substantially lowering radial cutting forces and minimizing deflection while cutting dissimilar material. The cutting material in either case should be multigrain carbide with a sharp cutting edge. Supernitride coating [AlTiN plus Si3N4] prolongs the usable cutting temperature and tool life. A totally different approach would be to machine each material individually, applying the cutting material best suited, and then stack the material up individually to each other's part contours and configurations. Only absolutely precise machining and data points during assembly make that a robust process.
Berthold P. Erdel
Berthold P. Erdel is a manufacturing management consultant and developer of advanced machining and manufacturing processes. He is an the author of several books on critical issues facing manufacturers, among them High-Speed Machining, which is published by SME. It discusses proven productivity improvements, including simplified machining operations and cost savings through practical applications of HSM. Erdel is also a frequent presenter for SME on the topic of high-performance machining and manufacturing.
This article was first published in the October 2006 edition of Manufacturing Engineering magazine.