Introduction
Molecular decomposition process is an electrochemical grinding process that has been refined to enable:
• The removal of increased amount of material while maintaining workpiece temperature to within 1ºF.
• Reduced mechanical force to the workpiece.
• From roughing operations surface finishes achieved measure at 6 to 8 Ra µin.
• Finishing operations are able to achieve surface finishes of 1 Ra µin or less.
The MDP can be outlined as follows:
• Isolation of workpiece and spindle from balance of equipment.
• Power supply design to address power surges or brown outs from external power sources.
• Electrolyte management system that cleans particulate (anode mud) separates particulate from the balance of electrolyte through controlled filtration maintaining uniform electrolyte conductivity. This filtration process provides a valuable benefit in the elimination of heavy metals such as arsenic and hexavalent chrome (making the designed system environmentally friendly).
• Electrolyte formulation specifically maintained to assist the total system.
• Controls of perishables through specific wheel formulation designed to address, conductivity, resistance, abrasive type and concentrations of abrasives.
• Within the operation of the MDP system, wheel life is increased requiring less time in dressing particular forms within the perishables thereby increasing the spindle time in the area of part production.
• Combining all of the actions within the system is done through controlled algorithms that enable operation of the MDP equipment to happen in the background. This permits operation of the MDP equipment from any level of expertise.
Within the MDP system the workpiece is isolated from the balance of the equipment and power is passed through the conductive workpiece making the workpiece the anode within the MDP system. The isolation of the spindle and conductive “Voltron” grinding wheel are employed to enable the grinding wheel to be the cathode within our electrochemical cell. See Figure 1.
Through the anode to cathode relationship a defined material removal through a deplating action is present within the electrochemical cell which seems to soften the material for stock removal and at the same time prevents softer alloys from adhering to the cathode (conductive grinding wheel) allowing for a clean free cut of material without mechanical or thermal damage.
Surface finishes resulting from the MDP process are improved through the ability to maintain uniform cutting action. Within conventional grinding and machining processes the tools utilized to perform the work introduce the largest potentially uncontrolled variable. Within a machining operation tools wear from the initial moment the cut edges are exposed to the alloy being removed. Efforts to enhance cut edge life require extensive engineering for cutter material, coatings, rake and cutter geometry all efforts to enable longer life through the cuts. Similar engineering efforts are performed for abrasives utilized within grinding wheel production to enable open clean cuts through porosity of the wheel or high pressure coolant systems that assist in keeping the abrasive wheel free of alloys or debris being ground. In either case, a dulling of the abrasive or cut edge translates to mechanical observations at the surface of the alloy being exposed to the stock removal process that would typically be referred to as tearing plowing smearing, or sliding of material. Figure 2 illustrates many of the conditions which can directly impact the final as machined / ground surfaces.
The cutting process being the desired action between an abrasive and a work piece (1.1) Balance of conditions illustrates a level of mechanical deformation at the surface that leads to increased friction (heat into the work piece). Abrasive stock removal reflected within (1.3 through 4) are conditions that are least desired that prohibit dimensionally accurate stock removal. MDP prevents these types of conditions.
The dulling effect of the cutter or abrasive is best illustrated as reviewed through a scanning electron microscope (SEM) of the surface of the alloys. Figure 3 and Figure 4 are SEM images of conventional ground verses MDP ground product. (Note: alloy illustrated within Figure 3 and Figure 4 maintain the same certifications as they are both from the same bar of stock).
Figure 3 is conventional ground alloy with a 180 grit diamond abrasive. Ripping tearing and smearing are all present within produced sample. Figure 4 is MDP ground alloy with a 180 grit diamond abrasive. Linear lines present are direct translations of geometry that is present on the face of the 180 grit diamond grinding wheel (cathode).
Benefits of the MDP system are that of true geometry and material characteristics are present at the surface of the work piece. Any geometry present on the surface of the perishable wheel directly translates to the surface condition on the work piece. When super finishing of specific geometry is being requested, the perishable wheel is the first item that must be produced with a uniform desirable surface finish and geometry. The perishable wheel and the grind approach must be one that will permit the removal or uniform blending of the surfaces through mechanical action or geometry. It is fair to state that if the perishable utilized within the MDP system has a measured Ra this will directly impact the final Ra of the work piece. Within conventional grinding, this level of detail is easily discarded as the change of cut condition and the presence of smearing and tearing are ongoing through the process.
Adapting the MDP process to gear grinding
Achievable surface finishes of a system are a result of multiple inputs including, accuracies of equipment (mechanically and thermally), life of the perishable tool, and repeatability through the entire geometry generation or linear inches of grind.
Gear production is typically performed by a roughing process (broaching or machining) heat treatment, grinding operations, gear form grinding, when required, super-finishing is achieved by rotary tumbling with some media and chemistry (mechanical polishing). The value added process of rotary tumbling requires dimensional planning with specific focus on the amount of material removed by that value added process, where the material is removed from and which zones are subjected to more material removal during the polishing process in order to generate the required surface finish. Once defined, these processes are predictable and repeatable and become part of the process flow.
Wheels/perishables
Cubic Boron Nitride wheels within conventional grinding systems require precision made hubs that are manufactured to matching geometries for the desired profile. These metal hubs are then plated and the super abrasive is layered onto the precision hub which is subsequently dressed and measured to ensure uniform geometries are achieved.
Aluminum oxide and silicon carbide wheels can be dressed within the gear grinding equipment utilizing programmable paths or formed dressing wheels. Dressing of these wheels within the gear grinding equipment delivers the desired profile with the geometric accuracies of the equipment directly translated to the perishable and subsequently to the product being ground. When grinding multiple teeth, multiple passes and multiple dresses of the perishable may be required within the conventional grinding system to produce uniform geometry.
Whatever the abrasive selection within the MDP system, the benefits of producing the conductive wheel is the first item that increases the overall ability of the perishable to maintain uniform geometry through the required linear inches of grind. Abrasive selection is more to enhance the overall life of the perishable to the number of form dresses that would be required for a desired form. Although wheel life is product specific, testing conducted has illustrated 5 to 10 times longer wheel life within the MDP systems as compared to conventional grinding systems.
Applied MDP process benefits
Increased wheel life is a benefit from the applied MDP technology. This increased wheel life enables uniform geometry for roughing to finishing with less expensive types of abrasives while maintaining required dimensional attributes. The increased wheel life and less expensive abrasives has an added benefit of enabling a gear grinding system to rough stock from a solid form within a single setup of the MDP interfaced equipment. Conventional grinding system utilizing a plated CBN wheel is less desirable for this approach due to the resultant mechanical and thermal stresses that would be generated within the product, along with the expense of the plated CBN wheel.
Accuracy and repeatability are achieved within the MDP perishables through the use of accurate and repeatable tool mounts. These accurate tool mounts enable the offline dressing of precision forms and provide repeatable accurate geometries. This method of form generation consistently provides uniform surfaces onto the form geometry of the perishable that would normally be viewed as grinding lay that subsequently affect the surface finish of the work piece.
Controls of the perishables through abrasive size, conductivity, and concentration further enhance the MDP process. As the grinding wheel is the cathode within our electrochemical cell, the abrasives are present to assist in the removal of non-conductive elements. The electrochemical action grinding swarf is a fine particulate (anode mud) that does not adhere to the perishable during the grinds. This enables finer abrasives to be utilized for increased levels of stock removal. Due to the finer meshed abrasives and refined dressing techniques, a uniform and consistent surface is generated onto the form of the perishable wheel. The extended wheel life gained from the applied MDP results in an improved surface of the form being ground with the applied MDP process, see Figure 5.
The blank:
Material = Alloy steel: AISI 4140, shown in Figure 6,
• Other steels ground with the MDP technology that required super-finishing of 1 Ra µin or better include: Stainless steel, 303, 304, 420, 440C; Tungsten carbide 12% cobalt binder; cast iron; Inconel 617, 625; Carpenter Pyrowear® 53; CPM10V and D2
Reheated to 845ºC (1550ºF), oil quenched, 425ºC (800ºF) double temper, see Figure 6.
Roughing the blank:
Utilizing a 100 grit aluminum oxide abrasive with form dressed wheel, stock removal rates from the solid blank grinding 41 teeth achieving a Q’ = 30.3 mm3/mm/s with the applied MDP technology, leaving 0.010” per side stock for finishing passes to simulate a standard production gear product. The applied MDP grinding was conducted with CNC grinder manufactured by Chevalier and integrated with MDP by Oberg Industries. Auxiliary fourth axis employed is a “Hardinge DD100” direct drive rotary indexer with repeatability of 2-3 arc seconds.
Figure 7 shows a roughed gear after running the aluminum oxide abrasive wheel leaving the 0.01” per side stock for subsequent finishing.
Upon completion of the roughing process, a CBN (cubic boron nitride) abrasive for the finishing process was implemented. Grinding full depth with applied MDP removing 0.005 per side for measurement review and final finish passes.
Upon completion of grinds, dimensional results are verified utilizing measurement over calibrated precision rolls online within the grinding equipment (see Figure 8). Further dimensional checks are performed first with Zeiss coordinate measurement machine matching to modeled geometry and Zeiss gear inspection equipment for verification of profile, lead, pitch, and radial runout (see Figure 9). Surface analysis checks of the as MDP ground surfaces are performed offline with a Mitutoyo SJ400 profilometer. Surface finish data (Figure 10) resultant data recorded within Table 1.
Superior surface condition is achieved through reduced mechanical forces, control of thermal properties and the ability to produce repeatable perishable tooling for use within the MDP system. Allowing the surface integrity from the MDP grinding wheel to translate directly to the work piece is a notable benefit of applying the MDP grinding process to gears.
In comparison to an MDP ground gear, a conventional ground gear had a relatively rougher 17.9 Ra µin surface finish when utilizing the same measurement equipment (see Figure 11 and Table 2).
Summary
By the means of applied MDP grinding we are able to illustrate, high stock removal rates, improved perishable life measured by repeatable geometry and surface finishes as MDP ground to a 1 Ra µin or better. The equipment utilized for these tests does not represent the geometric stability of a precision gear grinding machine or work holding devices specifically designed for holding and grinding of gears. Illustrating further that the MDP addition to a grinding system reduces mechanical and thermal forces required for stock removal in roughing and finishing operations.
MDP grinding eliminates the need for subsequent value added processes that add additional cost or cycle time to the gear production. Implementation of the MDP technology for these test grinds where performed on a CNC grinding machine produced by Chevalier equipment not specific to gear grinding processes. Adding the benefits of MDP to a precision gear grinding system would add the benefits of achieving super finishes to the “as ground” gear component and add to the types of gear geometries that could be produced with the applied MDP technology.
References
1. Marinescu, I.D., Hitchiner, M., Uhlmann, E., Rowe, W.B., and Inasaki, I., Handbook of Machining with Grinding Wheels
2. Korn, D., Low-Force, Low-Heat Grinding of Tough Materials, Modern Machine Shop
3. Ungureanu, C., and Cozminca, I., About the wear of abrasive tools in electrochemical grinding
4. Reitz, E., Surface Finishes: Methods and Metrics for Production, MDDI Medical Device and Diagnostic Industry News Products and Suppliers
** Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the Authors and may not represent the position or opinion of the AMERICAN GEAR MANUFACTURERS ASSOCIATION.
