EFFECTS OF ENGINEERED MICRO-GEOMETRY ON BURR FORMATION IN PCD MILLING OF ALUMINUM

Transcription

1 EFFECTS OF ENGINEERED MICRO-GEOMETRY ON BURR FORMATION IN PCD MILLING OF ALUMINUM William R. Shaffer Conicity Technologies One Wildwood Drive Cresco, PA USA ABSTRACT In recent years, PCD tools have become the tool of choice in high-speed aluminum milling applications. Burr generation by PCD tools in aluminum milling is the most common reason for tool replacement. Burr generation is significantly impacted by flank wear on the primary cutting edge. Two mechanisms produce flank wear. First, chipping and flaking of the primary cutting edge lead to deterioration of the flank of the tool. Second, frictional rubbing created when flank wear reduces the clearance between the tool and the workpiece generates additional flank wear. Engineered Micro- Geometry (EMG) is a new cutting tool edge preparation technology, which strengthens the cutting edge, and delays flank wear by precisely correlating the size and shape of the cutting edge preparation to the uncut chip thickness. The purpose of this study was to determine empirically the effects of EMG on burr formation. This was accomplished by conducting tests in a high-speed aluminum milling application, the production of automotive cylinder heads. The study was conducted by running parallel tests on tools with sharp edges and tools prepared with EMG. Test results showed that burr formation was delayed by using EMG. Specifically, tools with EMG ran nearly four times longer than standard PCD tools before burr generation forced tool replacement. INTRODUCTION At a macro level, cutting tools produce a stream of chips that constitute the material that is being removed from the work piece. The chip thickness is determined by the penetration of the cutting tool edge into the workpiece. Regardless of whether the metalcutting application is turning, milling, or drilling, the interface of the tool and the workpiece is a narrow band. This band is dimensioned in length by the depth of cut and the width (the most critical dimension) by the forward feed rate of the tool. Depth of cut can be a fairly large amount, but feed rate normally measures only a few thousandths of an inch. The management of this microscopic interface has an enormous impact on cutting system performance. All cutting tools are produced with an edge preparation. Even up-sharp (no edge preparation) is an option. Edge preparation ranges in size from up sharp to as large as 0.008, depending on the application. The purpose of the edge preparation is to create a cutting edge that will increase the performance of the tool. Properly applied edge preparation will blend and smooth small edge defects and micro-chipping inherent in all hardmetal tooling. The most critical feature of edge preparation is that its size is the same order of magnitude as the thickness of the interface between the cutting tool and the workpiece. Hence, the ratio of the edge preparation to the thickness of the uncut chip is a significant variable in controlling cutting dynamics.

2 As anyone who has attempted to take a fine finish cut (perhaps ) with a tool having a large edge preparation (perhaps ) will attest, the relationship between a tool s edge condition and the feed rate is critical. If the cutting tool edge preparation is larger than the feed rate of the tool, several undesirable conditions will occur. These include: excessive heat, residual stresses, work hardening of the cut surface, tool failure, and burr production. Conversely, if the feed rate, the chip load on the tool, is dramatically larger than the edge preparation, the edge of the tool may chip or catastrophically fail under the heavy load. THE TOOL IN IPR (INCHES PER REVOLUTION). A CROSSECTIONAL VEIW AT B-B SHOWS A TRACE CHIP THICKNESS. THE MAGNIFIED VIEW OF THE TOOL-WORKPIECE INTERFACE AT B-B SHOWS A NEGATIVE CUTTING CONDITION CREATED BY THE EDGE PREPARATION ON THE CORNER OF THE TOOL. FIGURE 1A: TOP VIEW OF AN APPLICATION IN WHICH A TOOL IS CREATING A GROOVE IN A CYLINDRICAL WORKPIECE. FIGURE 1B: ASSUMING CLOCKWISE PART ROTATION, THE A-A CROSSECTIONAL VIEW OF THE GROOVING APPLICATION SHOWS THE CREATION OF A CHIP. THE CHIP THICKNESS AT A-A IS EQUAL TO THE FEED RATE OF The importance of managing the interface between the cutting edge of the tool and the workpiece is complicated by the fact that the width of the interface changes along the cutting edge of the tool. Figures 1A and 1B illustrate this concept in a grooving application. Although the principle is explained the context of a grooving application, the concept is equally relevant in milling, turning, and drilling. In all cases, the thickness of the chip varies from a maximum, which is equal to the feed rate of the tool, to a negligible amount on the tool s corner radius. As a result, the ideal edge preparation is different at the center of the primary cutting edge than at the tool s

3 corner radius. This concept will be discussed in greater detail in the following section. Recent technological advances have enabled the creation of edge preparations that can be precisely controlled and manipulated. Specifically, the size and shape of the edge preparations can be varied along a tool s cutting edge with precision of Tools with these edge preparations, called Engineered Micro- Geometries (EMG), have empirically demonstrated favorable performance attributes including better cutting system consistency and reduced tool wear. The purpose of this study is to determine the empirical relationship between EMG and burr generation in aluminum milling with PCD tools. This application was selected because it has the desirable characteristic that burr creation is the most common reason for tool replacement. THEORY & BACKGROUND The processes used to fabricate PCD tools inherently create microscopic defects in the cutting edges. In the case of ground tools, grinding pressures can create micro cracking as well as chipping and micro chipping. (Chips, for edge preparation purposes, can be classed as defects of to and greater. Microchipping represents defects in the tool cutting edges that are less than ). PCD tools that are produced using wire EDM contain a binderdepleted-layer on the face of the tool at the cutting edge. (This binder-depleted surface can range in thickness from to depending upon wire EDM process parameters and the grain size of the PCD material.) The diamond particles in this binder-depleted-layer are inadequately supported and comprise a cutting edge that is highly prone to micro chipping and early breakdown of the cutting edge. Due to limitations in edge preparation technology employed by tool fabricators, most PCD tools are used in the as-ground or in the case of wire EDM, as-manufactured condition. Despite their sub-optimal edge condition, these tools frequently offer significant performance advantages compared to conventional carbide tooling. Unlike carbide tooling, PCD tools do not generally fail from abrasive wear or heat. Rather, their edge condition eventually precipitates burr generation, which is the most common reason for tool replacement. In an aluminum milling application, the normal life of a PCD tool consists of the three stages summarized in Figure 2. The first stage is break-in. This period is characterized by semierratic performance with flaking and chipping of primary cutting edge. During this period, surface finish quality fluctuates and begins to improve as residual edge defects are mitigated through wear. On up-sharp PCD tools, the erosive wear that occurs during tool break-in is not controllable. Hence, the tool break-in period results in part-topart inconsistencies in surface finish, part flatness and burr generation as well as variability in tool life. Stage 1 Tool Break-In Fluctuations in surface finish are common during preliminary break-in of cutting edge. Stage 2 Consistent Operation Cutting temperature and pressure gradually increases due to flank wear. Stage 3 Advanced Flank Wear Heat from flank wear becomes severe. Burr size grows appreciably necessitating tool change. FIGURE 2. PCD TOOLS USED FOR ALUMINUM MILLING EXHIBIT THREE STAGES OF TOOL LIFE. GENERAL DESCRIPTION OF EACH STAGE IS PROVIDED ABOVE.

4 Stage Two begins when the edge has lost the inadequately supported PCD particles, or the weak, sharp corner from grinding has been microscopically eroded. The tool stabilizes producing consistent performance including minimal burr generation. (Tool performance and life will vary from batch-to-batch based on the outcome of the break-in stage.) During the second stage of stable performance, the cutting edge slowly degrades as wear on the front flank surface of the tool causes reduced clearance between the front of the tool and the workpiece. Stage Three of tool life occurs as a gradual transition while the loss of clearance between the tool and workpiece leads to increased tool pressure, increased frictional heating and subsequently increased burr size. In PCD machining, this stage normally necessitates tool change. Uniform edge preparations, that are the same size and shape along the entire cutting edge, have been tried as a means of extending PCD tool life with positive results. However, tool performance has been sub optimal due to problems at the interface of the tool and the workpiece. On the primary cutting edge, the edge preparation reduces chipping and wear caused by the interrupted cutting forces. But, on the corner radius the tool, the edge preparation becomes larger than the thickness of the uncut chip. (This is illustrated in Figure 3.) At this point, high compressive forces and frictional heating change the cutting dynamics and negatively affect burr formation. In the case of both up-sharp tools and uniformly edge-prepped tools, the primary driver of increased burr size is heat. Up-sharp tools suffer from premature flank wear, the creation of a flat on the leading edge of the tool, which increases cutting forces and heat. A uniform edge preparation forestalls this condition on the primary cutting edge, but creates a localized area on the corner of the tool where cutting forces and temperatures rise. This is shown as the Rubbing Zone in Figure 3. Elevated cutting temperatures increase the ductility of the material in front of the tool making it more difficult to shear. When the tool reaches the edge of the part, it will tend to push material over the edge rather than cut cleanly, increasing burr size. Direction of cut Uniform Edge Preparation Primary Cutting Edge Cutting zone Rubbing zone Last point of contact with the workpiece Variable Edge Preparation / EMG Direction of cut Primary Cutting Edge Cutting zone Last point of contact with the workpiece Point A FIGURE 3. UNIFORM EDGE PREPARATION IS CHARACTERIZED BY CONSISTENT SIZE ON THE PRIMARY CUTTING EDGE AS WELL AS THE ADJACENT EDGE. ON AN EMG TOOL, THE SIZE OF THE EDGE PREPARATION ON THE PRIMARY CUTTING EDGE IS DIFFERENT THAN THE EDGE PREPARATION ON THE ADJACENT EDGE. ON THE CORNER RADIUS JOINING THE TWO EDGES, THE EDGE PREPARATION TAPERS EVENLY FROM THE SIZE ON THE PRIMARY EDGE TO THE SIZE ON THE ADJACENT EDGE.

5 Based on the above, the ideal edge preparation should transition from a maximum magnitude on the primary cutting edge that is based on the feed rate of the tool to a negligible magnitude on the nose of the tool where the uncut chip thickness approaches trace thickness. An Engineered Micro-Geometry (EMG) of this design is shown in Figure 3. The following features define the EMG for PCD milling of aluminum. Primary Edge Prep Geometry: On PCD tools used in aluminum milling, an edge preparation of with a radius shape has been found to be effective at removing most defects in the cutting edge. Tool Corner Edge Geometry: In this area, the edge preparation is reduced in size from on the primary edge to at Point A. The radius shape of the edge prep is maintained. Adjacent Edge Geometry: In this area, the magnitude of edge preparation is reduced from at Point A to the cutting edge being sharp at the last point of contact with the workpiece. The fundamental principle behind the design of EMG is to strengthen the primary cutting edge while maintaining an edge preparation that does not exceed the width of the interface between the tool and the workpiece. The result is a cutting edge that cuts cleaner, with reduced compressive forces, for a longer period of time before wearinduced heat produces an unacceptable burr. TEST DESIGN The parts used in this study were automotive cylinder heads. These are cast aluminum parts that have a large number of machined surfaces and features. The particular machining operation used for this study was the face milling of the joint face of the head. The surface finish and flatness of this face are critical and burr formation on the edge of the combustion chamber is problematic. The following table contains details of the tools used in this study. Table 1 Control Tools EMG Tools Insert Type PCD Milling Cartridge PCD Milling Cartridge Insert Brand Clapp Dico Clapp Dico Inserts per Cutter Head Edge Prep Up-Sharp EMG TABLE 1. TESTS WERE PERFORMED USING EMG TOOLS AND CONTROL TOOLS. Tests were conducted in a production environment where numerous outside variables had potential impact on test results. External factors can be divided into three general groups: tool properties, workpiece properties, and equipment properties. The following is a description of the techniques that were used to mitigate the effects of these factors on the data. Tool Properties: All tools came from the same manufacturing lot and were randomly divided into two sets. Workpiece Properties: Test tools and control tools were used during the same time period in the same plant. Equipment Properties: In order to conduct both tests concurrently, it was necessary to run the test tools and the control tools on different production lines. While these lines are similar, they are not identical in all respects. To understand the impact of testing on parallel production lines, data from the control test was compared to data normally produced when the control tools were used on the control line. In the production of aluminum cylinder heads, tool changes are most commonly made due to the creation of an unacceptable burr. Under normal conditions, the burr will grow as the tools wear until an unacceptable condition is reached. The machine operator determines the acceptability of the burr condition. During the seven weeks that the tools are normally in the machine, the operator regularly monitors burr thickness and executes a tool change when burr size reaches a critical limit.

6 In this study, tool life was established as the measure of interest. Tool Life = Number of parts produced before burr size becomes unacceptable as determined by same individual Throughout the test, surface finish, part flatness and burr condition were checked in accordance with standard practice to ensure compliance with quality requirements for production parts. TEST RESULTS Two significant findings were gained from this study. First, EMG delayed burr formation. The number of parts produced before burr size became unacceptable is shown in Table 2. Table 2: Tool Life Control Tools EMG Tools 70,000 parts 275,000 parts TABLE 2. LIFE OF EMG TOOLS EXCEEDED LIFE OF CONTROL TOOLS. In this particular test, nearly 400% increase in tool life was achieved while maintaining the same failure mode (burr formation), strongly suggesting that EMG did not fundamentally change the tool wear process. Rather, EMG delayed the typical failure mode of the tool and extended tool life by eliminating edge defects and maintaining an edge preparation that did not exceed the width of the interface between the cutting tool and the workpiece. Second, tools with EMG produced a surface with finer finish and improved flatness than the upsharp tools. Figure 4 shows process control data from each production line after 65,000 parts. The data for the control tools is shown on the A-Line chart while the data for the tools with EMG is shown on the B-Line chart. Each chart shows the surface finish profile as well as a measurement of part flatness. As the data indicates, tools with EMG showed significant improvements in both metrics. Improved flatness would indicate that the tools experienced less pressure during cutting resulting in significantly flatter surface from the EMG tools when compared to the control tools. FIGURE 4. PROCESS CONTROL DATA FROM TESTS SHOWS TOOLS WITH EMG PRODUCED FINER SURFACE FINISH THAN CONTROL TOOLS. CONCLUSIONS Data collected in a controlled, production test of PCD milling of aluminum strongly suggests that EMG delays burr formation. The proposed mechanism for this delay is the ability of EMG to delay tool wear and the accompanying increases in pressure and heat. EMG reduces pressure and heat in two ways. First, it prevents defects in the primary cutting edge from manifesting themselves as rapid flank wear. Second, it maintains an edge preparation that is smaller than the width of the interface band

7 between the cutting tool and the workpiece. This allows the tool to cut more efficiently with less pressure by promoting clean cutting along the entire cutting edge. Another finding concerned the ability of EMG to improve surface finish and flatness. Data suggests that EMG can significantly improve both measures. It is hypothesized that these improvements result from reduced cutting forces at the corner radius of the tool. Specifically, tools with EMG work with less pressure because they can effectively cut the work piece material at points along the tool corner radius where the chip thickness is diminishing. In contrast, the unprotected edges on the control tools cut with higher pressure due to wear lands created during earlier stages of tool life (initially formed during the break-in stage). These wear lands create a tool-workpiece interface which is highly negative at points along the corner radius of the tool. In these areas, the wear land is wider than the uncut chip thickness and material is compressed between the workpiece and the tool causing the milling cutter to push away from the work. The magnitude of this distance is extremely small, but is detected in surface finish and flatness metrics. Endres, J.W., 2002, The Effects of Corner Radius on Tool Flank Wear, Proceedings of 2002 NSF Design, Service and Manufacturing Grantees and Research Conference, January 7-10, San Juan, Puerto Rico, pp Kennedy, B., 2004, A Better Edge, Cutting Tool Engineering, Vol. 56 / February 2004, pp Shaffer, B., 2000, Getting a Better Edge, Cutting Tool Engineering, Vol. 52 / March 2000, pp Shatla, M., Yen, Y.-C., Altan, T., 2000, Toolworkpiece interface in Orthogonal Cutting - Application of FEM Modeling, Transactions of NAMRI/SME, Vol. XXVIII, pp Yen, Y.-C., Jain, A., Altan, T., 2004, A Finite Element Analysis of Orthogonal Cutting Using Different Tool Edge Geometries, to appear in Journal of Materials Processing Technology (Special Issue of 2004). A potential related area of study exists in the in the aerospace industry. In airframe component production, PCD and carbide tools are frequently used up-sharp. This is because a uniform edge preparation can smear and burnish the workpiece surface for the same reason it produces burrs. This is problematic because such a surface condition can hide defects that can act as crack nucleation sites in fatigue failure. Because EMG allows a tool to cut cleanly along its entire cutting edge, it is conceivable that applying EMG to existing up-sharp tools may reduce/eliminate smearing and burnishing. Comparing surfaces milled by both types of tools would demonstrate whether a difference could be documented. REFERENCES Endres, J.W, Manjunathaiah, J., 2000, A New Model and Analysis of Orthogonal Machining with an Edge-Radius Tool, Journal of Manufacturing Science and Engineering, Transactions of the ASME, Vol. 122, pp