Causes of centering errors in three-jaw chucks and corresponding methods

　The reasons for the centering error of the three-jaw chuck were analyzed. The good results achieved by using cast iron ferrules and elastic rings in actual work are explained. The adjusted and processed parts meet not only the size requirements but also the performance, thereby extending the service life of the mechanical parts.

　　The reason for the centering error of the three-jaw chuck——

　　1. The manufacturing errors of the positioning parts, guide parts and tool counters on the three-claw chuck and the amount of wear during use may change the correct positional relationship between the machine tool, the tool and the workpiece, and may cause processing errors.

　　The size of these machining errors can be determined based on the manufacturing tolerance calculations of the main parts of the fixture. For an existing set of fixtures or a set of fixtures that have been used and worn, the actual error can be measured. According to the measurement results, the centering error of a general three-jaw chuck can reach 0.1~0.15mm.

　　2. From the structural analysis of the three-jaw self-centering chuck, almost all its parts have clearance fit. There are certain gaps between each part. For example, there is a certain gap when the I-shaped groove of the jaw and the chuck housing slide back and forth; the flat thread is sleeved on the mandrel of the housing and needs to be rotated, so there is a gap; there is also radial drift when the chuck is fastened to the flange. These three types of errors together form an accumulated error, which is concentratedly reflected in the radial circular runout of the three pieces. As wear increases, the gaps in various parts also increase significantly, which is why the centering error becomes larger.

　　Solution——

　　1. When processing a large number of workpieces with high concentricity, piece-by-piece correction will add a lot of auxiliary time and affect production efficiency. The solution in the past was to add a set of rings, machine a cast iron ring to fit the workpiece inside the ring, and use a chuck to tighten the ring.

　　2. Due to the wear and tear of the three-jaw chuck during use, three types of accumulated errors occur. In order to reduce costs, they are generally used improvised. The cast iron ferrule is modified and a Ф85 elastic ring is added.

　　When the elastic ring is manufactured, the geometric tolerance must meet the design requirements of the drawing. An opening parallel to the axis is cut on the elastic ring, with a width of about 5mm. The material is spring steel, with a hardness of HRC35~45 after quenching and tempering, because the elasticity is better in this hardness range.

　　When installing the elastic ring, it must be placed correctly to prevent it from skewing. Only then can the centering accuracy of the grinding chuck be good, and the chuck should be cleaned before grinding.