BACK TO' BAS,i'CS ••• --- ----- - Cutting Fluid Selection and Process Controls for the Gear Manufacturing Industry Ike Tripp, Jr. Etna Products, Inc. Chagrin Falls, Ohio Introduction a helical thread corresponding to the The last decade has been a period of thread of a worm. As the hob rotates in far-reaching change for the metal work- timed relationship with the gear blank, ing industry. The effect of higher lubri- each row of teeth successively cuts the cant costs, technical advances in machine next portion of the gear tooth-space. The design and increasing competition are cutting action is continuous in one direc- making it essential that manufacturers of tion until the gear is completed. gears pay more attention to testing, Conversely, the gear shaping method selecting and controlling cutting fluid operates on the principle of two gears systems. lubricant costs are not a large Fig. 1- Gear hcbbing rolling in mesh. In molding generating percentage of the process cost relative to processes, a gear-like cutter called a items such as raw materials, equipment shaper tool is rotated and reciprocated and labor, and this small relative cost has in the correct ratio with a gear blank. tended to reduce the economic incentive The cutting tool used in gear hobbing (See Fig. 2.) The gear blank rotates while to evaluate and to change cutting fluids. is called the "hob". (See Fig. 1.) The ma- the cutter rotates and reciprocates to pro- Nevertheless, one of the largest factors jority of hobs are cylindrical in form and vide the cutting action. The shaper only in lost production during gear manufac- greater in length than in diameter. The cuts in one direction so relief is provided turing is excess tool wear, tool failure and cutting teeth on the hob are arranged in for the return stroke. subsequent product rejection. In this day and age of economic war of survival, it has become essential to consider and to evaluate new cutting fluids with an eye towards increasing tool life. improving overall productivity and product quality and lowering costs. Gear Cutting and Finishing A wide variety of manufacturing techniques are used to manufacture gears. Specifically, this article addresses the selection and process controls for the fluids used for gear hobbing, gear shap- ing and hard gear finishing. AUTHOR: MR. lKE TRIPP, JR. IS vice presiden: and gfflerai manager of £fnIl Products. Inc He has been with the company since 1974. and is the auihor of over 30 techmcal papers on industrial metalworking compounds He IS a member of Fig. 2.-Gear Shaping ASLE. ASM. the Wire Association and SME. .July/August 1987 37 Hard gear finishing is a process which a two dimensional view of meta] cutting. the environment of deformation. uses either generative grindmg or fanned Of the heat generated, about two-thirds wheel grinding to finish the flanks of is expended in sticking friction in the Types of Cutting Fluids parallel axis spur or helical gear~ after shearing zone, and one-third is expended A number of gear cutting and grinding they have been hardened by heat treat- in sliding friction at. the tool/chip and fluids meet the requirement of providing ment, The hard gear finishing technique tool/flank interfaces. The action of the adequate lubrication. This range ,of usua1ly removes ,.0021 .0025" from each fluid is ito [ower the heat generated in availability was not always present, as side of the gear teeth, these two zones, and the lubricant por- lubricant research and development was In the form grinding process, the tion of the fluid reduces friction at the oncea black art with few practitioners, grinding wheel has a profile mirroring the 'tool/chip andtool/flank interface. Now, through scientific research and the tooth space between two adjacent gear A fluid used in hard gear grinding cooperative efforts of vendors and teeth. As the formed wheel moves be- operation functions very much like a cut- buyers, lubricant development, applica- tween the teeth of the gear, it removes ting fluid, but there are very pronounced tion and behavior is becoming a 'science. excess stock. differenoes between the dynamics of the For the purposes of this article, There are several types of machines processes, Gear grinding involves lubricating fluids have been classified as and tool configurations used in the negative rake tool angles and random either oil-based or water emulsifiable. generative grinding process. These pro- orientation of cutting surfaces. The Oil-Based Fluids.Oil-based fluids are cesses either use a cutting tool designed temperatures and surface feeds are also used for gear cutting and hard gear as at. spur or helical gear, which grinds the higher. Most of the heat of deformation finishing where water emulsifiable com- gear using the shaving or gear shaping is carried into the workpiece soa gear pounds do not have the film strength or principle, ora. shaper cutter tool that uses grinding fluid must act to reduce grinding wetabili.ty to produce acceptable tool life a skiving principle or a worm type forces, which reduces heat generation. or surface finish. Oil-based fluids are .finishing tool similar to a worm type gear The cooling function of the fluid is con- gen rally compounded with the follow- grinding wheel. sidered secondary, but it is still impor- ing items: Recently, grinding wheels plated with tant to the success of a hard gear 1, Mineral oils, either napthenic grade cubic boron nitride (CBN) have been finishing operation, that have a saturated ring type used in place of dressable, conventional Fluids used for gear cutting and grind- structure, or paraffinic grade, abrasive grinding wheels. CBN is a syn- ing must exhibit a number of other pro- which have a straight or branched thetic crystalline material that is very perties. They must not be adverselyaf- chain structure. wear resistant, and it has pronounced fected by metallic contaminants or tramp cutting edges because of its cubic shape. oils that can enter a lubrication system. .2. Mineral oils blended with polar ad- CBN applied by the electroplating pro- They must not leave excessive resldue on ditives, as the oils themselves ..are cess is considered the best for form gear the surface of a gear to be subsequently nonpolar. The function of the polar grinding. CBN is very expensive, and its heat treated, and they should aid in the additive is to affect the wetting of use dictates the need for improved production of a gear that has, the desired the metal surface at the coolants delivered at higher flow rates properties - surface finish,runout, etc. tool/workpiece interface by reduc- than used with conventional wheels. The study of the subject of wear be- ing the interfacial tension between tween two materials in motion relative the mineral oi1 carrier and the gear Th.eory of Lubrication to ·oneanother is very complex, A blank. A polar additive has a sort The aim of fluids used in cutting and number 'of parameters influence wear. of magnetism for the metal due to grinding operations is to provide cooling Some of these include the shape of the its molecular structure. and lubrication. contacting bodies, applied load, relative Polar active additives come from Gear hobbing and gear shaping are velocity between the surfaces, surface several. sources. Animal. fats and oils are metal. cutting operations that generate roughness, the elastic and plastic proper- derived by rendering the f.atty tissues of chips. More than 97% of the cutting ties of the contacting materials (par- animals such as cattle, pigs and sheep. work appears as heat. Fig. 3 illustrates ticularly those of the surface layers), and Vegetable fats and oils are derived by fig. 3- Two-dimensional metal cutting diagram Table 1 - Coolants Grades by Contents FERROUS NON-FERROUS 2 SlVHUR ANDIOR HEAVY DUTY 10.1 CHL.ORINE WITH~FAT SHEAR ANGLE FAT SlMSYNTHETIC SEE MEDIUM DUTY SEE MEDIUM DUTY WORKPIECE SYNTHETIC NEW E P ADDITIVES NEW E P ADDITIVES 38 Gear Tecl1nology crushing and rendering the fruits of active agents and chemicals used in the The formulation of a comme:rcially ac- plants such as palm or coconut trees. formulation of the product are small ceptable product depends on in- Marine fats and oils are derived from enough to transmit almost all incident vestigating hu_ndredS ofemul ifiers which crushing and rendering the fatty tissues light. True synthetics contain no mineral can be classed as: of fish. oil, and each component used in the 1. Anionic- These are emulsifiers The mineral oils and polaradditives fonnulation is soluble in water on its whose electrolytic properties are are then often compounded with sup- own. The particles are smaller than H based on the formation of anionic plemental polar additives, which act as .00000000 in diameter and cannot be ions. Examples include sulfonated extreme pressure agents. The primary ex- viewed under any microscope. polyester or sulfonated castor oils. treme pressure agents utilized are sulfur. GeneraUy most .of th water chlorine or phosphorous. When sub- emulsifiable fluids available contain a 2. Cationic - These are emulsi iers jected to the elevated temper.atUf'esat the napthenic oil made soluble through the whose electrolytiC properttes are tool/workpiece interface, these 'extreme use of emulsifiers or surface-active based on the Iormation of cationic pressure additives react to form an agents. ions. Examples .indude organic sal,t5 organo-metallic film which minimizes friction and lowers heat generation. Water Emulsifiable Fluids. Water emulsifiable fluids are defined as those where wate:r is the continuous phase .. Basically. water emulsifiable fluids com- bine the cooling properties of water with the lubricating properties of anand/or various chemicals. Over the years, a good deal of jargon has evolved in the industry to describe water emulsifiable fluids for the gear manubcturing industry, Water miscible er ,emulsifiable lubricants are available in.