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:: Fig. 1 - This drawing is an example of a shoe grinding fixture. ID and OD Shoe Grinding for Thin-Walled Workpieces By Barbara Schulz F inish grinding close tolerances for roundness and concentricity of thin-walled rings and sleeves or a rolling element such as a bearing raceway poses tricky workholding problems. e surface-profile shape must be extremely precise. Ideally, to achieve these close toler- ances, the process should allow machining of both external and internal surfaces in a single clamping. It's economical to complete the workpiece in a single handling. At its Motion Conference at the company's headquar- ters in un, Switzerland, earlier this year, Studer (United Grinding North America) demonstrated its solution to this manufacturing process. One of the demonstrations showed grinding of both the ID and OD of a thin walled blank supported axially and radially by a shoe grinding fixture. is process is called shoe grinding and because it has nothing to do with footwear, the process is also known as microcentric grinding. With this method, the company achieves the customer specified accuracies with throughput that allows medium to high volume produc- tion rates, providing an economy of scale that is difficult to achieve with the conventional grinding process. The Principle Shoe grinding is a variation of centerless grinding. Using a magnetic plate to support the wokpiece axially while driving the blank along with a set of precision rollers (shoes) that support the wokpiece radially, shoe grinding enables both ID and OD surfaces to be ground in a single setup. Using this workholding system, thin-walled, circular workpieces, such as roller bearing races, can be secured so that it cannot be deformed and so that absolute concentricity from OD to ID is already assured by the workholding system. ese requirements are not met by a jaw chuck (three-, six-jaw chuck), which tend to deform the thin-walled bland, allowing it to "spring back" upon unclamping, which can cause out of spec roundness tolerances. Concentricity between the OD and ID is achieved by machining both the external contour and the internal contour in one clamping. Likewise, using a magnetic chuck alone for supporting and driving the workpiece blank generally means that each individual workpiece must be centrically aligned manually using a dial indicator, which not only takes a lot of time, but also makes automatic loading impossible. is last point proves to be a major obstacle in the mass production required in the roller bearing industry. The best way to hold thin-walled rings is to use a method that completely separates the workpiece support from the rotary drive (workpiece drive). The shoe grinding fixture supports the workpiece in two axes; the shoe supports it axially, and the magnetic plate holds it radially and drives its rotation . Fixture Design Figure 1 shows a universal shoe grinding fixture. e workpiece has been intentionally omitted to make the design clearer. In the photo: e shoe grinding fixture is clamped to the workpiece table and is movable in Z direction. e horizontal supporting shoe is in an oscillating shoe design, with universal sliding inserts with fine adjustment. e vertical supporting shoe is in a fixed shoe design, with universal sliding inserts with fine adjustment. e electromagnetic chuck is for introduction of the workpiece rotary movement and for fixing the workpiece has radial pole pitch. ese magnet pole boosters are regularly overground in assembled state, to guarantee the flatness and perpendicu- larity of the magnet contact surface. ey are radially adjust- able and clamped in the T-slot. Studer universal shoe grinding fixtures are available in TECH BRIEF 28 PRODUCTION MACHINING :: SEPTEMBER 2017