MAINTENANCE • MACHINE TOOL SPINDLES
The spindle shaft, with a steel ball located in one of its centers, is then mounted on parallel V-blocks and held against a weighted end block, Figure 2. Bench centers may be used, provided the shaft centers are free of damage and true. With the dial indicator seen at the left set at zero and contacting the bearing seat, the shaft is revolved slowly to determine runout and straightness. Both seats should be true with each other within 0.0002 inch total indicator reading (TIR), and within 0.0001 inch for very high-speed spindles. The wheel-end and pulley taper of the shaft are checked, and if the runouts do not coincide with the respective bearing seats, the high points of eccentricities are marked for identification. With the dial indicator seen at the right in Figure 2, the squareness of the bearing shoul- ders on the shaft is ascertained. This runout should not exceed 0.0002 inch total indicator reading.
Each Bearing Packed with Three Grams of Grease
For this particular installation two pairs of 35mm bore, light series, angular -contact, duplexed superprecision ball bearings are used. In unpackaging, care is taken to prevent the entry of dirt or moisture into the bearings. The light protective oil put in the bearing by the manufacturer is removed with Varsol. Each bearing is carefully packed with about three grams of clean, suitable, ball bearing grease, using a flat steel scale for inserting the grease in to the bearing and around the balls. Grease voids within the bearing are eliminated, and even distribution of the grease is accomplished by revolving the bearing slowly in the hands until the torque throughout the bearing becomes noticeably constant.
The housing bore is checked with a two- or three- point hole gage which is set to the basic diameter with a master ring, or with precision blocks and a micrometer. By rocking the gage in several positions in the housing bore, any out- of-roundness and deviation of diameter is indicated. To discover any existing taper, the gage is moved axially. Housing bores are held within specified limits and checked for finish and freedom from burrs and tool marks.
Spacers, locknuts, and slingers carefully inspected, and all burrs are removed to insure proper seating of the ball bearings. Faces of spacers A and B, Figure 1, and slingers C and D must be flat and parallel within 0.0002 inch and square with their bores. The inner-ring and outer-ring spacers should be equal in length within 0.0002 inch.
Prior to assembly, all parts of the wheelhead, Figure 3, including housing bores are cleaned thoroughly with a solvent such as Varsol and then carefully wiped with a clean, lint-free cloth. All dirt must be removed from threads, keyways, grooves, and splines. To prevent corrosion, all parts, including shaft bearing seats and shoulders as well as the housing bores, are wiped with grease, leaving only a thin, protective film.
Bearings Assembled in Spindle Mounting
Proper shaft and housing mounting fits must be established and maintained in the application of ball bearings. Generally, for machine tool spindles, the ball bearing should have a line- to-line fit with the shaft bearing seat and a sliding push fit in the housing bore. Before the actual assembly operations begin, the ball bearings may be used for trial purposes to insure the desired mounting fits at final assembly. Damage to the ball bearings is avoided by taking care to mount bearings without cocking and to apply a firm hand pressure to the inner-ring face when placing the bearing on the shaft. Only thumb pressure is applied to the face of the outer ring when the bearing is pushed into the housing. Each bearing should be earmarked for its eventual position in the spindle assembly, making certain that the floating or spring-loaded bearings are slightly looser in the housing than the front or fixed bearings.
At the wheel-end or nose of the spindle, the minimum housing bore dimension for the fixed superprecision ball bearings may be established as 0.0001 inch less than the maximum outside diameter of the bearing. If the bearing outside diameter tolerance is 0.0002 inch, the maximum housing bore dimension is established at 0.0004 inch larger than the minimum housing bore.
Figure 2 – Inspection of Shaft
Figure 3 – Wheelhead Parts
MAINTENANCE MACHINE TOOL SPINDLES