1、I 1 99FTM9 Dry Hobbing Process Technology Road Map by: G. Schlarb and K. Switzer, Gleason Pfauter Hurth Cutting Tools Corporation American Gear TECHNICAL PAPER COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services,- Dry Hobbing Process Technology Road Map G
2、lenn Schlarb and Kurt Switzer, Gleason Pfauter Hurth Cutting Tools Corporation The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. J Abstract With todays advances in gear man
3、ufacturing equipment, there is a necessity to advance the capabilities of tools. in order to exploit new machine potential, tool development with regard to new coatings, new materials, and new design methods has taken place. The difficulty now is to determine the best combination possible for a give
4、n application taking into consideration the specific gear manufacturers expectations. A systematic approach to identifying the right combination of substrate material, coating, and application technology in order to meet those expectations is discussed in this paper. The intent is to create a “road
5、map” to minimize risk of failure while maximizing the potential return for a given application. An explanation of physical properties of todays materials and coatings is presented, as well as a summary of the results from test applications. Copyright O 1999 American Gear Manufacturers Association 15
6、00 King Street, Suite 201 Alexandria, Virginia, 22314 October, 1999 ISBN: 1-55589-747-9 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesDry Hobbing Process Technology Road Map Glenn Schlarb and Kurt Switzer Gleason Pfauter Hurth Cutting Tools Corporati
7、on Introduction Recent trends in gear cutting technology have left process engineers searching for direction on which combination of cutting tool material, coating, and process technology will afford the best quality at the lowest total cost. Applying these new technologies can have associated risks
8、 that may override the potential cost savings. The many interrelated variables to be considered and evaluated tend to cloud the issue and make hobbing process development more difficult. Considerable work has been done, cooperatively between the tool manufacturers and material vendors, to improve th
9、e capabilities of the substrates being used. Efforts by both High- Speed Steel and Carbide manufacturers are yielding materials that allow a continuous expansion of the envelope of productivity gains in gear production. 0 With todays advances in gear manufacturing equipment, there is a necessity to
10、advance the Capabilities of tools. In order to exploit new machine potential, extensive tool developments have taken place in recent years. Building on the successes (and failures) of earlier efforts, there has been an explosion of new technology with both new coatings as well as new materials. The
11、days of having one broad range coating and limited material selection are long gone. The difficulty now is to determine the best combination possible for a given application, taking into consideration the specific gear manufacturers expectations. The purpose of this paper is to: 0 Describe current t
12、echnologies of gear cutting tool materials - specifically the relative properties of High-speed Steels (HSS) and Carbide grades. Describe thin film coating technologies used for both wet (water-soluble or oil) and dry 0 e cutting processes and discuss the properties and merits of these coatings. Dis
13、cuss tool configuration requirements necessary for higher material removal rates and for dry cutting. Present application parameters for the use of tools under dry cutting conditions and results of successful and failed applications. Discuss the evaluation of the failure modes most common to dry cut
14、ting processes. Present a systematic approach to aid in the application of these technologies. By evaluating costs and risks associated with various processes for applications, the process engineer can implement new technologies where the savings/risk factor is most favorable . The scope of this pap
15、er is limited to application of tools in the 10 to 20 NDP range. However, the concepts presented can be modified and applied to other applications. Systematic Approach 1. The first step, before making any changes to optimize an existing process, is to fully understand the current process parameters,
16、 costs and failure modes. Define the variables such as part data, material, hardness, machinability, machine capacity and restrictions, tooling rigidity, chip removal issues, speed, feed, number of cuts and shift strategy. Tool design characteristics, material properties, and coatings must be define
17、d. Define the measurables of the present process such as cycle time, part change time, parts per hour, and downtime for hob change. Costs such as tool price, sharpening costs and recoating costs should be known. How much wear is generated for current number of parts produced? Is the failure mode pur
18、e flank wear, or is chipping or cratering also causing tooth damage? Without a firm understanding of the present costs, how can the best potential option offering the greatest 1 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services2. 3. 4. 5. 6. 7. 8. Rex 7
19、6 Rex 121 chance for improvement with the least risk be identified? 1.5 3.8 10.0 5.3 3.1 9.0 32.7 3.4 4.0 10.0 5.0 9.5 9.0 40.9 Perform theoretical evaluations of cycle time possibilities at various hob diameters, and numbers of threads and gashes. Hob speed, chip load, feed scallop size will be the
20、 limiting factors, within the constraints of machine speed and horsepower capacity. Look at material options such as Carbide Dry vs. High Speed Steel (HSS) Dry vs. Wet Cutting Tools. Look at coating options for Wet vs. Dry application. Look at cost per part (CPP) evaluations of the best options from
21、 the above choices. Develop a test matrix to try one or two of the choices that show the best cost predictions. Test tools for initial use and throughout sharpening and recoating, evaluating wear performance, part quality, performance through subsequent operations, etc. Compare actual results to est
22、imates. Tool Materials The following paragraphs provide a brief summary of commercially available substrate materials and coatings commonly used in the gear cutting industry. Although some of this information may seem academic, it is essential to have a good understanding of the characteristics of t
23、ool materials and coatings in order to maximize efficiency of the application. Far from the days when conventional (cast) M2, M42 and T15 alloys were the predominant materials in gear cutting tools, the tool designer now has an extensive selection of quality high speed steel materials from which to
24、select. In the United States, the newest O generations of materials we manufactured by Particle Metallurgy (PM) for improved manufacturability, toughness and general cutting performance. Due to the prevalence of vacuum hardening and tempering, many of these alloys have evolved over recent years to o
25、ptimize their heat treatment response. Although there are a multitude of high speed steels available worldwide, they can be generally categorized into one of several groups based on their physical properties. In order to limit the scope of this discussion, only those materials readily available in t
26、he United States are covered. The most common material in domestic gear cutting tools today is CPM M4 (Crucible Particle Metallurgy). Typically used in the hardness range of 64-66 HRc, it has very good wear resistance, excellent edge toughness and is generally applied on a wide range of applications
27、 cutting workpiece materials with hardnesses up to 38 HRc. Since M4 contains no cobalt, it has a relatively low red (hot) hardness. Table #1 shows a comparison of some common gear cutting tool steels. Upgrade considerations to the base M4 material might be considered to take two paths - increased we
28、ar resistance or increased red hardness. CPM 54 is a fairly new alloy based on the M4 grade but with slightly higher carbon for higher hardness and with 5% cobalt for improved red hardness. At the high end of abrasion resistance is CPM T15 with an attainable hardness of 66-68 HRc and applicability t
29、o workpiece materials up to 48 HRc. - More aggressive applications (e.g. harder workpiece, faster cutting speeds) may require tools with even higher red hardness. CPM Rex 45, with 8% cobalt, is often recommended as an upgrade from CPM M4. Both alloys contain some amount of cobalt (Co) for red hardne
30、ss as shown 2 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicesin Table #1. Rex 45 might be selected where red where Rex 76 is performing without chipping but hardness is more critical than wear resistance and with excessive wear, and in carbide tool Re
31、x 54 where additional red hardness and high applications where chipping is uncontrollable abrasion resistance are required. (perhaps due to workpiece material or design considerations). Applications that generate high heat due to very hard or very abrasive workpiece materials can Coatimg Technologie
32、s benefit from high petformance steels like Rex 76 and T15. Again, the selection criteria is based on The advent of commercial Physical Vapor whether wear resistance is most important (T15) Deposition (PVD) coatings in the early 1980s had or wear resistance and excellent red hardness are a tremendou
33、s impact on the gear cutting industry. most important (Rex 76). Titanium Nitride (TiN), the first and still most common tool coating, resulted in significant Cemented carbides are also good candidate performance gains that allowed tools to run for materials for gear cutting applications, and as two
34、to ten times their normal life. Soon with the high speed steels, there are a wide applications were being designed around the variety of carbide grades from which to choose. expected performance from coated tools. That is, Typical grades for hob manufacture shown in where a coated tool historically
35、gave additional Table #2, are mid-range in cobalt concentration performance gains, the coating was now and are often fine-grained or ultra-fine-grained for necessary for the application to work at all. the highest possible toughness (transverse rupture streng th). Today there is a vast array of PVD
36、coatings appropriate for gear cutting applications. Many of these coatings are unique in both composition and marketing name, but the most common tool coatings can generally be classified as Titanium Nitride (TiN), Titanium Carbo-Nitride (TiCN), Titanium Aluminum Nitride (TiAIN-X or TiAIN-F) and Har
37、d/Soft combination coatings like TiAIN-WC/C (TiAIN-H). In some cases, high speed steels are not wear resistant enough and carbides are too brittle to Table #3 shows some physical properties of these satisfy the application requirements. This chasm coatings. Oxidation onset is a measure of the in the
38、 road of application development has led to the development of very specialized high speed steel “bridge” materials. Rex 121, shown in Table #1, is a good example of this type of material. By comparing the total amount of alloy content (right column), it is clear that this alloy falls into a class o
39、f its own. Rex 121 is hardenable to over 70 HRc, and has wear resistance and red hardness levels unprecedented in the high speed steel family. Figure #1 compares Rex 121 hardness at elevated temperatures to Rex 76 and to C4 cemented carbide. Rex 121 is recommended in steel tool applications 1550 135
40、0 1150 I 950 e al x 750 550 350 u) : .- Hot hardness of Rex 76, Rex 121 and C4 Carbide _ -_ - _ 850 900 950 1000 1050 1100 1150 1200 1250 Temperature (deg. F) Figure 1. Hot Hardness of HSS and Carbide Materials. 3 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handlin
41、g Servicesthermal stability of the coating, which determines its ability to withstand the high temperatures encountered at the cutting edge. With the exception of TiAIN coatings, it is desirable to maintain a cutting edge temperature below the oxidation onset temperature in order to obtain the most
42、benefit from the coating. See below for a more detailed explanation of the thermal Table #3. Properties of Common Physical Vapor Deposition (PVD) Coatings wear and also acts as an insulating layer to aid in keeping the frictional heat in the chip. In many cases a decline in coating performance has b
43、een observed when the cutting edge temperature is not sufficient to oxidize the coating. applications. TiAIN-X is a single layer coating with a very high hardness that is typically recommended in dry cutting or very high Hardness (HV .05) Coefficient of Friction 2200 I 1 TiN 2800 0.4 2800 2600 2500
44、0.4 0.4 0.2 G Pa -3.5 400 Oxidation onset Coating Color -3.5 -2.5 -2.3 800 800 800 characteristics of TiAIN coatings. Blue gray I “let I Violet gray gray TiCN I TiAIN-X I TiAIN-F I TiAIN-H I Dark gray The choice of coating material was much simpler before the advent of commercial multi-layer and hyb
45、rid coating combinations. Coatings can be applied in discrete or graded chemistries and in almost any combination of layers and layer thicknesses. By way of example consider the TiAINiTiN multiple-layer coating system TiAIN-F, shown in Figure #2 below. Here The hardness value generally correlates wi
46、th the abrasion resistance of the coating, with higher hardness providing better wear resistance. Titanium Nitride - This gold colored coating accounts for the vast majority of coated gear cutting tools due to its proven performance base and its relatively low cost. TiN exhibits good thermal stabili
47、ty and can be used in a wide range of applications Titanium Carbo-Nitride (TiCN) is a specialized coating for cutting abrasive workpiece materials in applications that liave a relatively low temperature at the cutting edge. Successfu I Figure W2. Magnification of ground spherical crater in High Spee
48、d Steel sample showing multi-layered coating. applications might include cast iron, alloy steels and fiber reinforced polymers. -. I itanium Aluminum Nitride (TiAIN) coatings are typically used in high-heat generating applications including dry cutting. Here the intent is to cause the aluminum compo
49、nent of the coating to oxidize, resulting in a thin layer of Al2O3 at the surface that is constantly replenished as it is worn away. The Al2O3 provides resistance to adhesive the two component layers, TiAIN and TiN, are deposited as discrete layers and the total layer thickness is on the order of 4 to 6 microns. There can be multiple advantages in coating combination systems such as this. First, the coating has a higher toughness due to the inhibition of crack propagation through the layers. Toughness is also enhanced by the distribution of internal compressive stresses due to th
