1、- 99FTM8 e by: H.J. Stadtfeld, The Gleason Works American Gear Manufacturers Association TECHNICAL PAPER Power-Dry-Cutting of Bevel Gears COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesPower-Dry-Cutting of Bevel Gears Dr. Hermann J. Stadtfeld, The Gle
2、ason Works The statements and opinionscontained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. Abstract High speed machining using carbide has been known for some decades for milling and turning operations.
3、The intermittent character of the gear cutingprocess has delayed the use of carbide tools in gear manufacturing. Carbide was found at first to be too brittle for interrupted cutting actions. In the meantime, however, a number of different carbide grades were developed. The first successful studies i
4、n carbide hobbing of cylindrical gears were completed in the mid gos, but stili did not lead to a breakthrough in the use of carbide cutting tools for gear production. Since the carbide was quite expensive and the tool life was too short, a TiN coated high speed steel hob was more economical than an
5、 uncoated carbide hob. Improvements in carbide grades and sintering processes in combination with new coating methods and the use of CNC machines has lead to a significant new trend in the way cylindrical gears are produced. This trend combines high speed hobbing using coated carbide tools, without
6、coolant. Provided that process parameters are set optimally, extremely short cutting times can be achieved with long tool life and high part accuracy. Process development in bevel gear cutting also benefits from the carbide and coating developments. After successful investigations of the high speed
7、carbide cutting process with coolant, the next logical step was to follow the general trend and proceed with a process development of a bevel gear dry cutting method. It was found that nearly ali geometrical and technological parameter of the carbide wet cutting method could be applied also to bevel
8、 gear dry cutting. The surface cutting speed of the newly introduced method is 1000 ft./min. which is four times the value of conventional cutting. The cutting process is conventional cutting in the continuous face hobbing method and plunge cutting in the single index face milling case. Copyright O
9、1999 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 22314 October, 1999 ISBN: 1-55589-746-0 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesDr. Hermann J. Stadtfeld Vice President Research the photo gives an i
10、mpression of the high-cutting speed. The Gleason Power-Cutting process, as opposed to the carbide rough cutting, is a finishing process, which is suitable for later short time lapping or grinding. AGMA Class Q12 and Q13 gear quality was achieved in all development studies 2. 5evel Gear-Dry-Cutting A
11、fter successful investigations of the high-speed carbide cutting process with coolant 2, the next logical step was to follow the general trend and proceed with a process development of a bevel gear dry cutting method. It was found that nearly all geometrical and technological parameters of the carbi
12、de wet cutting method could be applied also to bevel gear dry cutting. The surface cutting speed of the newly introduced method is 1000 ft./min. - which is four times the value of conventional cutting. The cutting process is conventional cutting in the continuous face hobbing method and plunge cutti
13、ng in the single index face milling case. The generating of pinions can be a combination of conventional and climb cutting. Cutting feed rates identical to conventionally applied rates were found to be optimal as a result of the investigations. Since the index speed of the singe-index face milling c
14、ycle is the same for both Power-Dry-Cutting and conventional cutting methods, cycle time reduction of 70% is possible in the case of Power-Dry-Cutting face milling. In the continuous-index face hobbing case, the indexing speed is proportional to the cutter RPM, which reduces the cutting times up to
15、80%. This, and other advantages, shows the enormous economic potential of the Power-Dry-Cutting, especially if it is applied to the continuous-indexing face hobbing process. Figure 2 shows the Power-Dry-Cutting continuous-index face hobbing process of high-speed cutting a ring gear. The thin oil fil
16、m for rust protection on the blank surface does not cause any visible smoke development. The illustrated gear is a face hobbed ring gear with 45 teeth and a module of 5.9 mm. The complete cutting time was 1.5 min. (as opposed to 5 min. conventional). The carbide stick blades were TiAIN coated. The m
17、aximal tool life corresponds to two times that of high- speed steel tools. It is expected to achieve even higher tool life in the future. Figure 3 shows chips from ring gears, which were cut in the course of the parameter studies. The feed rate used to generate the chips was 0.005 in./blade. The hig
18、hest temperatures of the chips in Figure 3 were around the tempering temperature. Only parts of the chips turned 1 blue. The chips in Figure 3 were created during the end of the plunging cycle; they are wide and u-shaped and were generated simultaneously from the cutting edge, the blade tip and the
19、clearance side of the blade. The wider chip was cut in a Formate face milling operation; the smaller chip comes from Formate face hobbing. As a plunging feed function, a single ramp is applied that decreases the feed rate with increasing depth of the cut. The chip thickness, at the beginning of the
20、plunging, is thinner than comparable HSS cutting to protect the 2 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servicescarbide at the blade tips. At the end of plunging, a thicker chip than for HSS is chosen to allow the carbide cutting edge that is less sh
21、arp than an HSS cutting edge to penetrate easily enough into the steel and shear off a chip. Also, the dwell time on the end of plunging is kept to an absolute minimum to reduce the time where cutting edges only rub on the flank surface without taking chips. This rubbing only has the risk of chippin
22、g the carbide cutting edge. The long and wide cut engagement between blade and work in case of bevel gear cutting prevents the generation of many microchips. This is the reason why, in contrast to cylindrical gear hobbing no chip welding in the root area occurs. The temperature of the work pieces am
23、ounted to 47“ above room temperature. The temperature of the cutter head stabilized at 91F (about 21 OF above room temperature). . - . Figure 3: bottom face hobbing Chips from dry cutting; top face milling, A cost comparison to evaluate the economic viability of Power-Dry-Cutting is based on the fol
24、lowing factors: J Cost of carbide blade sticks (30% higher than HSS) J Sharpening of carbide blades (same as HSS) J Coating of carbide blades (only once necessary on the face front like HSS) Building of blades in cutter head (1.5 the time of HSS) J Tool life of carbide (I .5 to 3 times of HSS) J Man
25、ufacturing time = machine occupation (20% to 30% that of HSS) J The cost of the carbide is more than compensated by fewer numbers of resharpenings, requalifications and so forth. In case of the ring gear in Figure 2, the total tool cost per manufactured gear is reduced by 60%. To apply the Power-Dry
26、-Cutting method, it is necessary to utilize bevel gear generators equipped with special high-speed spindles. Cutter spindle speeds up to 800 RPM cannot be realized with conventional machines that have the traditional cradle design. The complicated gear train in a conventional machine (with play betw
27、een each gear set) cannot produce smooth coordinated motions, and maintain stable temperatures in high cutting speeds. The six axes free-form machine, however, has the cutting spindle motor mounted directly to the same vertical slide that houses the cutter spindle. Complicated couplings and mechanis
28、ms are then not required to transmit power to a tilted cutter head, as on cradle-style machines. Power-Dry-Cutting can be applied to single index face milling or continuous index face hobbing and is always a completing operation. 3 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by I
29、nformation Handling ServicesFigure 4: Vacuum chip removal system To set up the cutting machine for the dry cutting process, a chip-guiding channel was developed which takes advantage of the kinetic energy and the aerodynamic shape of the chips to remove them out of the work zone. In spite of cylindr
30、ical cutting, the chips fly tangential to the cutter in the direction of the back guarding, where they are removed by a vacuum system that transports them to a cyclone. There are no additional means of auger or chip conveyor. Except for the 0.75 Kw vacuum unit, no energy is applied to remove the chi
31、ps and maintain a perfectly clean machine interior. The chip removal is on top of the oil-free process, an additional aspect of an environmentally friendly and energy-saving overall concept. Up to 80% of the process heat is removed from the machine, together with the chips, without any heat transfer
32、 to the machine elements. The air flow from the vacuum unit causes, as a positive side effect, a more than 10F reduced part and tool temperature. As a dry cutting option, the cutting machine has no oil tank and no chiller unit for cutting oil, as well as no chip conveyor. This results in a 30% reduc
33、ed floor space and a very compact appearance of the machine tool. Figure 4 shows a photograph of the dry cutting work zone. The chip channel is visible in the center, surrounding the complete work area of cutter head and workpiece. The 4 front part of the channel can easily swing aside and allows a
34、convenient change of the cutter head. Figure 5: Completing cutter with positive seated blades The tools for dry machining are preferably stick blade cutter heads with pentagon-shaped blade cross section and positive seating. Figure 5 shows such a completing stick blade cutter; solid carbide stick bl
35、ades are used to perform the high-speed carbide cutting. This simplifies the manufacturing of the blades, as compared to brazed- on carbide tips and allows for the highest possible number of resharpenings. The resharpening of the carbide blades is done on a 5-axes cutter grinding machine with a diam
36、ond-plated grinding wheel. 4. Tool Materials, Coatings on Wear ,I Mechanism The carbide blades are the keys to the Power-Dry- Cutting process. Best results have been conducted with fine grain K-grade carbide. From the available grades: P, M and K - P has the highest hardness and K the highest toughn
37、ess. The most commonly used ingredients are tungsten carbide (WC), tantalium carbide (Tac), titanium carbide (Tic) and cobalt (Co). The K . grade carbide uses next to the major component tungsten carbide, only cobalt. A higher percentage of COPYRIGHT American Gear Manufacturers Association, Inc.Lice
38、nsed by Information Handling Servicescobalt reduces hardness but increases toughness. The suitable range of cobalt is between 6% and 16%. Best results in Power-Dry-Cutting delivered a composition of 90% tungsten carbide and 10% cobalt. This reflects the demand in highest possible toughness combined
39、with acceptable hardness. The toughness is important because of the interrupted cut and dynamic disturbances by the fact that more than one blade is cutting at certain times. Cobalt is a critical element, which could cause a chemical reaction with oil during grinding and cutting. The so-called “coba
40、lt leeching“ can be avoided by using cutting and grinding oils with a low chemical affinity to cobalt. During sharpening of carbide, a breakdown of the carbide grid along the cutting edge takes place. The craters with a size of up to 40pm are the roots of later chippings during the cutting process.
41、Toughness of the substrate already reduces the breakdown. A further reduction happens by choosing a small carbide grain size. Grain sizes between 0.6pm and 1.2p-n were investigated, where the optimum for the particular bevel cutting process was found at approximately 0.9pm. Carbide manufacturers ref
42、er to this grain size as fine grain carbide. The remaining edge breakdown after resharpening might be in the range of 5pm to 20pm. This is still not acceptable and will end the tool life early due to cutting edge chipping. An edge rounding device, using a rotating nylon brush was developed to put a
43、constant and smooth rounding along the entire cutting edge. The nylon bristles contain 400 grid silicon carbide particles. Figure 6 shows the cross section of a blade while cutting off a chip. The size of the rounding radius is chosen to remove the small craters completely and smooth the edges of th
44、e bigger size craters. The necessary radius to achieve that is between 1Oum and 20pm. This is the smallest possible radius to eliminate an early cutting edge breakdown during production. - The wear mechanism, besides chipping, can be explained with the assistance of Figure 6. The chip, with a temper
45、ature of up to 570“F, glides with high pressure along the front face, close to the cutting edge. A front face coating reduces the friction between chip and blade and protects the carbide grid from wear and chemical reactions. The different possible coatings are: J Titanium Nitride (TiN) J Titanium C
46、arbon Nitride (TiCN) J Titanium Aluminum Nitride (TiAIN) Multi Layer J Titanium Aluminum Nitride (TiAIN) Single Layer The coating is processed according to the PVD method that requires a temperature of 500F to 950F. This is low compared to the sinter temperature of 3000F and will not negatively affe
47、ct the carbide substrate. The oxidation temperature of TiN is about 750F, which is too close to the chip temperature of 570F. The highest oxidation temperature of the single layer TiAIN coating is 1500F. 5 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Servic
48、esCarbide Substrate 10% Cobalt 90% Tungsten Carbide Figure 6: Mechanics of chip formation During the development of Power-Dry-Cutting, all the above-mentioned coatings were tried in parameter studies. The optimal coating, with respect to tool life and surface speed is the TiAIN coating. The edge rou
49、nding removes some of the coating layer on the front. The need for coating on the side relief surface is very low. The temperature of the workpiece while contacting the friction land (Figure 6) is below 250F. The radius gives a smooth, polished transition between side relief and the cutting edge. The radius, itself, is in the steady state situation (while cutting off a chip), not in contact with steel. The only critical moment for the edge rounding is the penetration of the blade into the steel to start the cut of a chip. There is no need for coating on the cutting edge roundi
