FEBRUARY 2008 / VOLUME 60 / NUMBER 2 ➤ BY JOHN A. WE B STER, PH.D., COOL-GRIN D T E C HNOLO G I E S LL C Directing coolant into the right place, at the right speed, in the right quantity. etter coolant application is and if that thermal energy isn’t when a more thermally conductive one of the most effective managed correctly, it can lead to CBN wheel was used under the same Bways to improve grind- workpiece surface damage and poor process conditions, Rs became 30 ing operations. The benefits of im- process economics because of in- percent, Rw 45 percent, Rch 20 per- proving coolant application include adequate material-removal rates cent and Rf 5 percent. being able to use a harder wheel and excessive wheel wear. Power Heat that enters the workpiece grade without causing grinding burn, consumed by the grinding process must be quickly cooled to prevent reduce thermal damage to the work- is partitioned into the wheel (Rs), the development of high local tem- piece and extend wheel life. One of workpiece (Rw), chip (Rch) and peratures, phase transformations, the most important and often over- coolant (Rf), the ratios of each de- cobalt leaching (in tungsten-carbide looked factors in effective coolant pending on the chip size, abrasive grinding), and high residual tem- delivery is nozzle design. Among grain type, coolant type and material. peratures after the wheel has passed the different designs, coherent-jet In shallow-cut grinding of M-50 tool by. Phase transformations are often nozzles offer superior performance steel at a material-removal rate of 20 responsible for tensile residual and improved operating efficiency. cubic mm per second, per mm WOC stresses, white layer formation, re- (Q prime), Rs was shown to be 8 per- duced fatigue life and surface and Thermal Domination cent, Rw 72 percent, Rch 10 percent subsurface cracking. Process cooling Grinding generates a lot of heat, and Rf 10 percent. By comparison, is achieved by applying a cooling and lubricating fluid, as well as by se- cating ability. Pressure controls fluid n reduced thermal damage of the lecting process parameters that reduce velocity, while flow rate and tempera- workpiece material, allowing heat generation. ture control the rate of heat transfer higher mrr, Although the coolant’s primary job into the fluid. Proper jet direction al- n increased effectiveness of the ap- is to cool the grinding process, it also lows the fluid to penetrate the air bar- plied flow rate, such that the over- must: rier that travels around the wheel. Use all applied flow rate may even be n cool the grinding wheel (impor- of coherent-jet coolant nozzles can reduced, tant with resin-bonded diamond n reduced entrained air (foaming), and CBN wheels), misting and vapor problems, n allow the lubricants in the cool- Process cooling is achieved n reduced disturbance of the jet by ant to do their jobs (especially by applying a cooling and the air barrier surrounding the with single-layer superabrasive lubricating fluid, as well wheel, wheels), n more robust and stiffer setup, and n flush chips from the machine and as by selecting process n no reduction of wheel speed or workpiece area, and parameters that reduce application of softer wheel grade n clean the wheel (especially when to alleviate burn. grinding ductile materials using heat generation. water-based coolant). Nozzle Design optimize all of these elements, leading Jet nozzles are the simplest type of One of the key elements in coolant to improved coolant application. The nozzle for grinding. Bendable plastic application is nozzle design. One de- benefits of improved coolant applica- and open metal pipe fall into this cat- sign is the coherent-jet coolant nozzle. tion include: egory. The jet coming from the ends of Effective use of this nozzle involves n reduced dressing frequency, due these nozzles can be very dispersed, many factors. Pressure, flow rate, tem- to less loading with workpiece with the jet thickness increasing by perature and direction of the jet all material and reduced abrasive more than 10 times within 12" of the influence the fluid’s cooling and lubri- grain wear, nozzle, leading to air entrainment and increased foaming. Other disadvan- damage by the wheel or fixture, tages of jet nozzles include: n the nozzle is at the compression Traditional formed and wedge noz- n lack of stiffness when subjected to fitting holding the pipe, and once zles also have disadvantages. Formed the reaction force of the jet, formed, the jet is constrained by nozzles are usually created by crush- n the nozzle must be placed close to the tube until it exits, and ing the tube in a press with an internal the grinding zone to overcome off- n nozzle tubes are often of small di- shim to control the final aperture, and set from the dispersed jet, risking ameter to be bendable and coolant often include a stabilizer (planar) sec- velocity down the tube far exceeds tion after the transition from the input Keywords the critical velocity of 20 ft./sec. tube diameter to final aperture (round for creating turbulence. or rectangular) to minimize breakup of coolant: Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid, such as soluble or chemical mixtures (semi- synthetic, synthetic), but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to- compound ratio varying with the machining task. grinding: Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound or slurry. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets and undercuts); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding. grinding ratio: Ratio of workpiece material removed to grinding wheel material lost. grinding wheel: Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding. —CTE Metalworking Glossary the jet as it exits. Wedge nozzles are the actual dimension of the jet is a lot often fabricated from welded metal less than the geometric aperture of the sheet and taper down from the inlet nozzle lead to a vena contracta and pipe diameter to the desired exit aper- D subsequent air entrainment. ture. With this design, the exiting jet Coherent-jet nozzles enable better can disperse at the same angle as the grinding performance because they wedge, putting much of the flow rate concentrate the coolant into the grind- Vena contracta, Dj where it is not needed, thereby reduc- ing zone, allowing the wheel pores to ing cooling effectiveness. be fully filled, thereby minimizing ap- In contrast, coherent-jet nozzles use plied flow rate, required coolant tank a precisely machined internal profile to size and pumping energy. However, produce a high-quality jet. An example 30˚ coherent-jet nozzles need to be more of coherent-jet nozzle geometry is the accurately targeted than dispersed-jet D ¾ D Rouse profile. This design requires a Dr nozzles so they reach their intended contraction ratio (inlet to exit diameter 1.5 D target, or zone. A coherent-jet nozzle ratio) of at least 2:1. A definition of co- also concentrates its kinetic energy to herency is a dispersion of two to three break through the air barrier and wet times the exit diameter, or thickness, at a Figure 1: Nozzle geometry based on the wheel. distance of 12" from the nozzle. Figure crushed pipe (upper) and Rouse design In contrast, the entrainment associ- (lower). 1 shows the Rouse profile underneath ated with dispersed-jet nozzles forces a traditional crushed pipe, or wedge, efficiency of open-pipe, wedge and air and coolant into the grinding wheel, nozzle schematic. The efficiency of the crushed pipe nozzles can range from displacing some of the fluid with the Rouse design is 95 percent, as defined 70 percent to 90 percent, depending on air. In some cases, dispersion is so high by the coefficient of discharge. The the internal transitions. Nozzles where that much of the coolant hits the work- CERATIZIT USA INC. 800-245-6880 l www.ceratizit.com/usa l 847-498-9100 www.ctemag.com ® HAMILTON TOOL CO. INC. 800-724-1799 www.jawsware.com/hamilton Dealing with the air barrier on grinding wheels any researchers have shown that the boundary layer of the wheel and enter the grinding zone, and the coolant was Mair surrounding the grinding wheel, which travels at the reflected back. In the same experiment, when the nozzle jet same speed as the periphery of the wheel, can disrupt the was aimed at the wheel a few millimeters ahead of the grind- flow of coolant into the grinding zone. The depth of the air ing zone and the air was peeled off, the coolant attached to barrier increases with abrasive grit size and porosity of the the wheel and was transported into the grinding zone. This wheel, but is difficult to measure. work was first done using conventional nozzles, but is now The energy associated with the air barrier should not be being done with coherent-jet nozzles supplied by Cool-Grind underestimated.