The Secret to Effective Centerless Grinding - Wheel Set-Up
The use of superabrasive (diamond and cubic boron nitride/cBN) wheels to perform centerless grinding has existed for nearly 50 years – it’s a relatively simple process of following the directions of the machine tool manufacturer and the wheel manufacturer. However, having spent some 35 years in this industry, I have all too often seen an operator relying heavily on the wheel to solve challenges associated with the grinding process. Operators should also pay special attention to machine set-up since proper set-up can help cure many of the most common grinding issues.
Below you’ll find some fundamental set-up procedures for the average centerless grinding machine. I will cover the topics broadly - if a more detailed answer is required, please leave a comment in the section below.
Trueing the Regulating Wheel: The function of the standard regulating wheel trueing unit is to make the diamond trueing nib follow the same line of contact with the wheel as that of the work.
Factors to consider:
1.) Angle of inclination of the regulating wheel.
2.) Location of the center of the work in relation to the regulating wheel and the diamond wheel.
3.) Angle to which the regulating wheel is swiveled.
4.) Amount the diamond nib is “set over.”
5.) Using the highest wheel RPM of the regulating wheel during trueing to create a smooth, consistent feed surface.
Work Blades: When grinding with diamond or cBN wheels, the most common work rest blades are made of steel with a carbide blade insert attached. PCD work rest blades are also available, but are quite expensive when compared to carbide. Under most normal grinding circumstances, the centerline of the work piece should be located above the centerline of the grinding and regulating wheels. If the centerline is set too high, the work piece may exhibit chatter, and if set too low, the work piece may exhibit an out of round condition with three high lobes.
Choosing the correct work rest blade angle is also important to the set-up process. For example, when grinding with a 4” wide superabrasive wheel, the work rest blade should work well at 30 degrees. However, once the width of the wheel is changed to 6” or 8” wide, the 30 degree work rest angle may generate too much pressure and cause unwanted chatter. Changing the work rest blade angle to 20 or 25 degrees will reduce the pressure toward the grinding wheel and eliminate the chatter on the part.
When regrinding work rest blades, it is important to not burn the blade. For high speed steel, aluminum oxide wheels will be effective if infeed rates do not exceed .0005” per pass. If a very high quality tolerance is required on the work rest blade, a cBN wheel should be used. When re-sharpening carbide work rest blades, a medium hardness 120 grit diamond wheel should be used. Performing light passes between .0005”- .001” with spark out passes in between is recommended to ensure maximum flatness of the blade.
Coolant & Coolant Flow: A common misconception I encounter deals with coolant flow and the direction of coolant. Many people believe coolant is used solely to keep the grinding wheel cool. Underestimating the importance of the placement of the coolant at the interface between the grinding wheel and work piece can be a critical mistake. Coolant removes heat from the grinding zone where the workpiece actually makes contact with the grinding wheel. A failure in alignment of the coolant will cause grinding heat to return to the work piece and/or the grinding wheel. Once heat enters the work piece, the operators’ ability to hold tolerances for roundness and straightness becomes substantially more difficult. In extreme heat situations, the diamond wheel can be damaged thermally, exhibiting blisters and cracks.
During the grinding process, the air barrier created by the grinding wheel and work piece must be overcome to allow coolant to interface between the two. Properly pressurized coolant will remove heat from the grinding zone, allowing a free cutting grind process and ensuring optimum performance capability. Using coolant nozzles that enhance the flow of coolant pressure will further help to increase wheel life and facilitate the free cutting nature of the diamond wheel.
I also see more coolant scrubbing systems being added to the back side of the wheel. This practice utilizes a high pressure nozzle system (with pressures exceeding 600 PSI) to “scrub” the diamond wheel removing material, or “swarf,” that has been ground from the surface of the grinding wheel. Customers often believe a wheel loaded with swarf creates a better surface finish on the work piece. However, performance of the grinding can be reduced by 50% from a clogged grinding wheel. Additionally, increased pressure generated during grinding with a clogged grinding wheel can reduce overall wheel life by 20-30%.
Trueing a Superabrasive Wheel - Trueing a superabrasive wheel on a centerless grinder can be a time consuming process. Here are a few tips to make the truing process easier:
1.) Never use diamond nibs when trueing a superabrasive wheel. Nibs will generate flats on the tips of the abrasive and may generate too much heat, potentially damaging the bonding agents.
2.) The superabrasive wheel should be mounted on the machine’s wheel adaptor, which adapts the wheel to fit. Always balance the wheel assembly before operating in the grinding mode. Balancing the wheel/adaptor assembly prevents premature wheel breakdown and the potential risk of a wheel explosion during the grinding process.
3.) Most commonly, soft carbon steel rod can be used to safely true the wheel, removing run-out from the outside diameter of the wheel. Alternating the use of the steel rods and a good sized cleaning stick (dressing stick) will help speed up the process of trueing. A properly trued and conditioned centerless diamond wheel is critical to attain optimum wheel performance.
4.) Once the diamond wheel appears to have obtained total concentricity to its bore, use a grease pen, crayon or china marker and draw lines on the outside diameter of the wheel. Re-run a few steel rods - taking light passes of .0002” to .0005” - if the wheel is true the lines on the wheel will disappear. Use at least one cleaning stick to condition the wheel, exposing the superabrasive crystals to grind the material efficiently. Depending on the size of the wheel it can take several cleaning sticks to achieve proper grinding efficiency.
Once the wheel has been completely trued and dressed, it is important to verify that the entire surface of the diamond wheel is in use. Setting the angle of the regulating wheel too acutely will cause the work piece to enter too far into the grinding zone and cause uneven wear and, most important, taper. This will lead to reduced wheel life. Setting the regulating wheel angle too close to parallel with the diamond wheel will cause stalling of the parts between the regulating wheel and diamond wheel. In extreme cases it can cause a work piece/wheel crash at the entry side of the diamond wheel.
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