High-speed Machining Applications Research Articles

Investigation into the use of gaseous and liquid nitrogen as a cutting fluid when turning BS 970-80A15 (En32b) plain carbon steel using WC–Co uncoated tooling

Cutting fluids play an important role in metal cutting, but have considerable environmental impact. There is a need, therefore, to understand the role of cutting fluids within the cutting process and in this context to evaluate possible environmentally friendly alternatives. To achieve this, the cutting environment and the consequences of removing cutting fluid from the process altogether have to be assessed. This work investigates the role of cutting fluids and the implications of their use. Industrially reproducible cutting tests were devised, where BS 970-080A15 (En32b) plain carbon mild steel was turned in various cutting environments that included dry cutting. In particular, tool life is measured for both ambient and cryogenic temperature nitrogen gaseous environments and results then compared with more traditional cutting conditions. The results indicate that a 55% reduction in flank wear is achievable when turning BS 970-080A15 (En32b) steel using WC–Co uncoated tooling in a nitrogen environment, and thereby establish the suitability of nitrogen atmospheres in high speed machining applications. This work indicates that alternative, non-polluting environments should be investigated further in order to determine their viability in high-speed machining applications.

Cr 1− xAl xN coatings deposited by lateral rotating cathode arc for high speed machining applications

In this work, a series of Cr 1− x Al x N (0 ≤ x ≤ 0.7) coatings were deposited on high speed steel substrates by a vacuum arc reactive deposition process from two lateral rotating elemental chromium and aluminum cathodes in a flowing pure nitrogen atmosphere. The composition, structural, mechanical, and tribological properties of the as-deposited coatings were systematically characterized by energy dispersive analysis of X-rays, X-ray diffraction, nanoindentation, and ball-on-disc tribometer experiments. All of the as-deposited CrAlN coatings exhibited a higher hardness than CrN, showing a maximum hardness of about 40 GPa (at around X = 0.63) which is twice higher than that of the CrN. The wear performance under ambient conditions of the CrAlN coatings was found much better, with both lower friction coefficient and wear rate, than TiAlN coatings deposited by the same technique. The wear rate of the CrAlN coatings against alumina counterpart was about 2–3 orders in magnitude lower than that of the TiAlN coatings. Selected CrAlN coatings with the highest hardness were also deposited on some WC-based end-mills. An evident better performance of the CrAlN-coated end-mills was observed than the TiAlN-coated ones for cutting a hardened tool steel material under high speed machining conditions.

Investigation of tool path interpolation on the manufacturing of die and molds with HSC technology

The application of high speed machining of molds and dies with their high complex geometry makes the traditional methods of tool path interpolation normally used by CAM systems, i.e., linear and circular interpolations, the bottle neck of the whole process. These “old” interpolation methods increase the machining time, have a negative influence in the final quality of the surfaces and are a technological limitation for the full application of HSC. As a result the study of new methodologies to the tool path interpolation is becoming one of the main areas of research in manufacturing of molds by HSM. Among the different methodologies of tool path interpolation exist linear, circular and polynomial. The milling experiments were made using work pieces of Steel AISI SAE P20, the geometry from the NC Gesellschaft test part and a ROMI machining center with 10,000 min −1 spindle and 5000 mm/min of feedrate. The Unigraphics NX CAD/CAM system was used to generate the NC program with different values for the CAM tolerance to check its influence on the methods for the tool path interpolation.

Prediction of Surface Quality from Ball-Nose Milling in High-Speed Machining Applications

Many of the finishing operations in the manufacture of dies and moulds are dependent on ball-nose milling cutters to produce the required geometry and surface quality. These cutters (high cost, carbide coated) work best in situations where the load on the cutter is constant and rapid changes in direction are kept to a minimum. The majority of computer aided manufacturing (CAM) systems provide a number of alternative machining strategies to support this approach, but they do not provide an accurate prediction of the surface quality in the finished product. This paper outlines a predictive theory for the surface quality. The ball-nose cutting mechanism has been modelled and simulated to generate the surface topography expected in the actual operation. The results match those from the experimental machining trials and enable the prediction and determination of the surface quality in other applications prior to the machining operations.

The influence of friction on contouring accuracy of a Cartesian guided tripod machine tool

This paper is focused on the friction-induced contouring errors of a three-axis parallel kinematic machine (PKM) called a Cartesian-guided tripod (CGT). A dynamic model of the CGT including the nonlinear dynamic, mechanical compliance and mechanical friction has been formulated. Servo control characteristics of the formulated CGT dynamic model are analyzed based on the conventional P-PI controller and feedforward friction compensation implemented in the joint level control scheme. Quantitative analysis and comparison of the various friction sources from the actuated joints (ballscrew/nut) and passive joints (balljoint and guideway) have been conducted. Analysis results show that the ballscrew/nut friction produces the most significant friction-induced contouring errors. The ball-joint friction and linear-guide friction induced contouring errors are two-order and one-order, respectively, lower than the errors by the ballscrew friction. The main reason is that the equivalent friction torques of these two friction sources coupled into the servo drive loop are reduced by the leverage effect of the driving leg length. It was also found that the conventional feedforward friction compensation can effectively reduce the ballscrew-friction-induced contouring errors. However, if not properly compensated, ballscrew friction could excite significant platform-leg structural vibration because the tripod-based PKM usually has a high compliance vibration mode in the horizontal platform-leg direction. The friction-excited structural vibration can be reduced by putting vibration absorbers on the guideways of the passive leg to damp out the platform-leg vibration. From the primary analysis results, it can be seen that the proposed tripod based PKM has high potential for high-speed 5-axis machining applications.

Three-Component Receptance Coupling Substructure Analysis for Tool Point Dynamics Prediction

In this paper we present the second generation receptance coupling substructure analysis (RCSA) method, which is used to predict the tool point response for high-speed machining applications. This method divides the spindle-holder-tool assembly into three substructures: the spindle-holder base; the extended holder; and the tool. The tool and extended holder receptances are modeled, while the spindle-holder base subassembly receptances are measured using a “standard” test holder and finite difference calculations. To predict the tool point dynamics, RCSA is used to couple the three substructures. Experimental validation is provided.

Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications

The main advantage of the filtered arc deposition (FAD) technique is a significant grain refinement that leads to the formation of nano-crystalline (grain size approx. 60–80 nm) PVD coatings. This technique improves the wear resistance of FAD TiAlN coatings under high-speed machining conditions when cutting tool oxidation wear is dominant. A study of the surface structure characteristics of the FAD TiAlN coatings using SEM, EDS, TEM, AES, SIMS and EELFS was performed. The microhardness of the coatings were measured. The microstructure of the chips was analyzed. The compression of the chips as well as shear angle was measured. The friction parameter on the rake surface of the tools was determined in-situ. It was shown that the major cause of the high wear resistance of the FAD coatings during high-speed machining was the formation of the thin protective oxide films on the cutting tool surface. The grain size refinement of the coating promotes the formation of protective alumina films. These films are formed at the surface of nano-crystalline FAD TiAlN coating during high-speed machining. They mainly consist of protective alumina, whereas the films that are forming on the surface of TiAlN commercial coatings with coarser grains consist only of non-protective titania. The formation of the protective alumina films, composed of complicated amorphous–crystalline structures, significantly improves the friction and wear performance of coated tools. The tendency of the work piece material to adhere is reduced and considerably more heat is dissipated via chip removal. This is a major cause of the enhanced wear resistance of the FAD coating technique. Tribo-oxidation during cutting is a typical process of the self-organization of TiAlN coatings that results in a quasi-stabilization of cutting tool wear. The self-organization and friction control for coated cutting tools is considered based on a non-equilibrium thermodynamic approach.

INVESTIGATION OF WEAR PROCESSES ON WORN TOOLS USING RAMAN MICROSCOPY

In order to address the industry driven interest in wear and associated mechanisms of coated tools during dry high speed machining applications, two commercially available physical vapour deposited (PVD) coatings were studied. These were TiAlN based and had multilayered architectures deposited on to cemented carbide 8 mm ball-nosed two flute end mills. Raman microscopy and SEM-EDX analysis were used to characterise the molecular and elemental species, respectively, present on the worn surfaces of tools after controlled lifetime tests milling into A2 HRC58 die steel. This was performed to gain a better understanding of tribochemical reactions that occur in the highly complex wear environment during milling. Cutting temperatures were estimated as ≥950°C, indicated by oxide phase transformations and corroborated by static oxidation studies. In addition, transfer layers of iron/steel and iron oxide species (FeO, Fe3O4, and Fe2O3), oxidation products of the coating (TiO2 and α-Al2O3), and spectral changes owing to deformation and stress state were observed as a consequence of wear. The high spatial resolution (2 μm), sensitivity to structural/chemical changes, and the non-destructive nature of the technique make Raman microscopy ideal for the study of wear on tools with complex geometry that cannot be readily studied by other techniques without difficult sample preparation.

Inverse dynamics of hexapods using the natural orthogonal complement method

The hexapods under study are parallel mechanisms made of fixed-length legs, as opposed to conventional hexapods with extensible legs. The former are becoming popular in developing machine tools for high-speed machining applications. In this paper, the inverse dynamics of hexapods with fixed-length legs is analyzed using the natural orthogonal complement method. This analysis includes the mass of the moving platform as well as those of the legs. The leg masses become important for high-speed applications. In this development, the Newton-Euler formulation is used to model the dynamics of each individual body, including the moving platform and the legs. This is followed by assembling the individual dynamics equations to form the global dynamics equations. Based on the complete kinematics model developed in the paper, an explicit expression is derived for the natural orthogonal complement that effectively eliminates the constraint forces in the global dynamics equations. This leads to the inverse dynamics equations of the hexapods that can be used to compute required actuator forces for given motions. Simulations are provided to demonstrate the developed method.

Contour offset approach to spiral toolpath generation with constant scallop height

Recently, the application of high-speed machining (HSM) is recognized as an economically viable manufacturing technology. Even though more HSM centers have increasingly been utilized, the conventional toolpath generation methods are usually employed in practice. But the conventional methods have inherent limitations for the HSM application. This paper presents a new toolpath generation algorithm for high-speed finish cutting process. In order to minimize the fluctuation of cutting load and the possibility of chipping on the cutting edge in HSM, a spiral topology toolpath that is to cut continuously with the minimum number of cutter retractions during the cutting operations is developed. This algorithm begins with the contour offset procedure along the boundary curve of the sculptured surface being machined. In the offset procedure, the offset distance is determined such that the scallop height maintains a constant roughness to ensure higher levels of efficiency and quality in high-speed finish machining. Then, the spiral path is generated as a kind of the diagonal curve between the offset curves. This path strategy is able to connect to a neighbor path without a cutter retraction. Therefore, the minimum tool retraction toolpath can be generated. And, it allows the sculptured surface incorporating both steeper and flatter areas to be high-speed machined. Based on these techniques, experimental results are given to verify the proposed approach.

Multilayered (Ti, Al) ceramic coating for high-speed machining applications

A multilayered (Ti, Al) ceramic hard coating was deposited on tunsten-carbide ball-nose end mills for high-speed machining using an unbalanced magnetron-sputtering system. The process parameter dependence of the coating properties was studied. X-ray diffractometry, x-ray photoelectron spectroscopy, nanoindentation, and scratch tests were used to characterize the structural, compositional, and mechanical properties of the coatings. High hardness, up to 40 GPa; good adhesion strength, up to 100 N in scratch critical load; and high-oxidation resistance were achieved, leading to excellent performance in high-speed milling on hardened tool steel at a speed of 260 m min−1. The results show that the tool life with this coating is improved by a factor of 4 or better under the testing conditions used compared to the uncoated tools. The surface finish of the machined steel achieved with this coating is also significantly better.

Performance analysis of CNC interpolators for time-dependent feedrates along PH curves

Virtually any desired time variation V(t) of the feedrate along a curved path can be realized by real-time CNC interpolator algorithms for the Pythagorean-hodograph (PH) curves: the only stipulation is that V(t) should admit a closed-form indefinite integral F(t). Different feedrate variations V(t) can be accommodated with minimal modifications to the core algorithm by employing a modular function call to define F(t). Such time-dependent feedrate capability is invaluable in high-speed machining applications, where it can ensure smooth acceleration and deceleration control of a CNC machine under extreme torque or power demands on the axis drives. Empirical results from implementations of these time-dependent feedrate interpolators on an open-architecture 3-axis mill are presented, highlighting their exceptional performance and versatility. These results also furnish a compelling illustration of a basic defect of “traditional” (piecewise-linear/circular) G code path descriptions—namely, the fundamental conflict between geometrical path accuracy (which entails use of many short G code segments) and the accurate and smooth maintenance of rapid commanded feedrates.

The application of high-speed CNC machining to prototype production

This paper describes the application of high-speed milling to the production of monolithic, metallic, functional prototypes, with special emphasis placed on the applicable process times. An example component is selected and the relevant process times are presented. Fundamental requirements for the use of high-speed milling to produce prototypes in a timely manner are identified. These requirements include: 1) high speed/high power spindles, 2) proper spindle speed selection based on the system dynamics, 3) machining parameter definition based on tool wear, 4) high feed/high acceleration machine drives, 5) intelligent path generation, and 6) pre-process verification of arbitrary three-dimensional CNC part paths. The implementation of the Simultaneous Trilateration Laser Ball Bar (STLBB) system to measure the CNC part paths is described and the device verification procedure is outlined. Example two and three-dimensional path measurements are also shown.

High speed machining of moulds and dies for net shape manufacture

High speed machining (HSM) generally refers to end milling or ball nose end milling at high rotational speeds (10 000–100 000 rev./min). The paper initially reviews the development and application of HSM for the manufacture of hardened moulds and dies. Following on from this, experimental work is detailed relating to the use of indexable insert ball nose end mills to machine hardened (∼52 HRC) AISI H13 hot work tool steel. Tool wear, workpiece surface roughness and cutting force data are presented. The effects of tool material (TiCN and TiAlCrYN coated carbide and uncoated cermet) and cutting speed (150–350 m/min) were investigated using a full factorial experiment incorporating test replications. Limited performance data for solid carbide tooling are also presented. The use of indexable insert cutters, generally resulted in lower tool life values (approx. 50% length cut at a cutting speed of 250 m/min) compared to equivalent solid carbide. The highest tool life values were obtained with TiCN coated inserts and TiAlN coated solid carbide cutters. Workpiece surface roughness was typically 1–4 μm R a , with slightly lower values for insert cutters. Cutting forces were relatively high (up to ∼1300 N) with the indexable cutters, as a result of a relatively large axial depth of cut (1 mm), a negative rake angle (−10°) and a blunt insert edge condition.