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Arianna Smith, Corporate Application Engineer, Norton | Saint-Gobain Abrasives
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C
ylindrical parts, such as bearings, races and bushings require grinding of the inside diameter to obtain a precise surface finish. This operation, known as internal diameter (ID) grinding, involves turning of the workpiece in one direction, while turning the grinding wheel in the opposite direction (Figure 1).
| There are two types of ID grinding operations: (1) traverse grinding, where the grinding wheel is moved back and forth along the axis of the workpiece until the grinding operation is completed, and (2) plunge grinding, where the position of the grinding wheel remains stationary. Plunge grinding is commonly used if the grinding wheel has a profile that must be imparted on the inside diameter of the workpiece.
ID grinding operations present several challenges, making it one of the most difficult grinding processes in the industry. This article reviews some of these challenges and describes how the Norton IDeal-Prime grinding wheels with a new ceramic grain enhance manufacturing performance, reduce operational costs, and improve product quality. |
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ID grinding challenges |
| There are numerous challenges associated with ID grinding that can result in poor surface finish, excessive friction and heat build-up or excessive grinding tool wear. |
Lack of rigidity |
| One of the most important factors in ID grinding concerns the rigidity of the shaft or arbor — commonly called the quill — which holds the grinding wheel. The arbor must be sufficiently long to allow the grinding wheel to be inserted into the workpiece. If the arbor is too long or lacks rigidity, it can flex during machining, causing deformation and resulting in an uneven surface finish, changed part geometry, or uneven wear of the grinding wheel. In some cases, the lack of rigidity can cause vibration or chattering of the grinding wheel against the workpiece, resulting in wavy patterns on the workpiece. These problems can be solved by changing the grinding wheel specification to something that is more friable and requires less pressure during grinding, changing the quill dimensions (i.e. shorter or thicker), changing the quill material to something more rigid, tightening the flange bolts, or changing the grinding wheel infeed, which will decrease forces put on the quill. |
Swarf build-up |
| The abrasive particles on the grinding wheel that are exposed to the workpiece remove material through shear deformation. Voids between abrasive particles are prone to loading by the removed material (swarf), causing a layer of metal forming onto the grinding wheel, reducing cutting ability, and generating excessive heat. Swarf is typically removed with coolants or lubrication during the grinding process. Excessive swarf build-up can be removed by dressing the wheel, whereby material is removed from the grinding wheel, effectively sharpening it and exposing the workpiece to sharper grains. |
Excessive heat/wear |
| The high surface contact area between the grinding wheel and workpiece exacerbates the build-up of heat, since there is very little clearance for coolant to enter the grinding zone, which removes the swarf and cools the grind zone. Excessive heat can also result in burning or burnishing of the workpiece, whereby repetitive sliding of the grinding wheel against the workpiece can result in grooves on the surface or sub-surface cracks that cause spalling of the workpiece material. Excessive swarf build-up can also reduce the efficiency of the grinding operation and result in poor surface finish. |
Norton IDeal-Prime grinding wheels |
| The Norton IDeal-Prime grinding wheels use Norton Quantum Prime, a new nano-crystalline ceramic grain from Saint-Gobain, which is embedded in an optimized Vitrium 3/VS3 bonding matrix. The unique micro-fracture properties of this new ceramic grain together with the retention capability of the bonding matrix enables Norton IDeal-Prime grinding wheels to deliver grinding efficiency and significantly longer life, while ensuring outstanding part quality over time. |
| One of the key features of the new grain is that it is friable — that is, it can re-sharpen itself, reducing the build-up of swarf. A friable grinding wheel can address several of the special challenges posed by ID grinding. Sharper grains allow material to be removed more efficiently from the workpiece using less force throughout the grinding process. This reduces potential flexing of the arbor, reducing the potential for deformations that cause uneven surface finish. Furthermore, the reduction of swarf build-up on the higher surface contact area required by ID grinding translates into lower heat build-up and reduces the potential for burning or burnishing of the part surface. |  |
| The Norton IDeal-Prime grinding wheels provide several advantages. The self-sharpening grain technology increases material removal rates and reduces the need for dressing, thereby cutting down on overall cycle times, which translates into lower cost per part. Reductions in dressing requirements also significantly improve the grinding wheel life, leading to lower maintenance costs without sacrificing work piece quality. The innovative new grain technology creates grinding wheels with unparalleled sharpness and improved cutting efficiency that reduces spindle power requirements even at increased material removal rates. This results in lower mechanical stresses, improved part geometry and lower operating (energy) costs.
The Norton IDeal-Prime grinding wheels are available in a variety of sizes and blends to optimize cost and performance, including grain size, grade, structure, and speed. |
Grit sizes |
| The Federation of European Producers of Abrasives (FEPA) ranks abrasives according to grit size, where the FEPA F scale is appropriate for grinding wheels. The new Norton grain is available over a wide range, from the relatively coarse FEPA F46 (370 micron grain) to the relatively fine FEPA F150 (82 micron grain). The coarse grain provides aggressive cutting action, enabling higher material removal rates, and the fine grain can provide more precise and improved surface finishes. |
Grades |
| Grinding wheels are also rated according to grades, which indicate the hardness or strength with which the bonding matrix holds the abrasive grains in place. Grade A denotes the softest grade, and Grade Z denotes the hardest grade. The Norton IDeal-Prime grinding wheels are available from the relatively soft Grade G to the harder Grade Q. The softer grade wheels are appropriate for harder materials because as the abrasive grains dull quicker from harder materials, the softer bond will release the dull grains to prevent friction (metallurgical damage) and allow new sharper points to be used, whereas the harder grades are used for improved form hold and longer wheel life. Furthermore, the Norton Vitrium 3/VS3 bonding matrix substantially increases the elastic modulus and strength of bonds, resulting in cooler cutting, precise part geometries, and higher operating speeds. |
Structures |
| The structure of the grinding wheel refers to the spacing between the abrasive grains in the grinding wheel, denoted by the number of cutting edges per unit area of wheel face and the size of void spaces between the grains. A large number of cutting edges per unit area is a dense structure and a smaller number of cutting edges is an open structure. The Norton IDeal-Prime grinding wheels are available from the relatively dense structure rating of 6 to the relatively open structure rating of 10. The dense structures improve form hold and wheel wear, whereas the open structures enable higher material removal rates and more efficient heat dissipation. |
Speeds |
| The recommended speed of the grinding operation depends on a variety of factors, including the grade; bond type; grinding wheel diameter; workpiece material; the material removal rate; the contact area between the grinding wheel and workpiece; and the grade, structure, and grit size of the wheel. Improper speeds can cause a variety of problems that can adversely affect the part surface quality, reduce grinding wheel life, and increase operational costs. The Norton IDeal-Prime grinding wheels can operate up to speeds of 63 m/s, and higher speeds may be available upon request.
The numerous advantages and wide product availability of the Norton IDeal-Prime grinding wheels make them an ideal choice to obtain unparalleled product consistency and stable surface finishes while increasing manufacturing throughput and reducing manufacturing costs.
For more information about Norton IDeal-Prime, download the brochure and case study. |