In the past, traditional low-speed C620 lathes used sliding bearings for their spindle systems. Although sliding bearings have advantages such as a simple structure, easy manufacturing, low cost, smooth operation, and resistance to shocks and vibrations, they are no longer suitable for modern high-speed, high-precision, and high-efficiency machining requirements. Additionally, these bearings are difficult to repair once worn, leading to poor surface finish and accuracy of machined parts, as well as low productivity. To address these issues, the spindle bearing system of the C620 lathe was redesigned and upgraded. The following are the detailed steps involved in the modification (refer to the attached diagram):
1. **Modification of the Front Bearing Hole of the Headstock**:
The front bearing bore of the headstock is machined to Ø150K6. A hole with a diameter of Ø151 mm and a length of 55 mm is also machined at the left end of the bearing hole to facilitate the installation and removal of the bearing. In addition, a Ø16 mm hole is drilled to make it easier to fasten the screw.
1. Headstock 2. Adjusting Nut 3. Spindle 4. Nut Stop Pad 5. Fastening Screw
6. 3182120 Bearing 7. Bearing Cap Pressure Ring 8. Improved Bearing for C620 Lathe Spindle
2. **Modification of the Spindle**:
A thread of M100×1.5 mm is machined on the spindle according to the dimensions and position shown in the figure. The adjustment nut, nut stopper, and fastening screw are then installed accordingly. A taper of 1:12 is machined on the spindle to match the inner taper hole of the bearing. The bearing cover and pressure ring are also machined, and the original sliding bearing adjustment nut is re-machined to match the dimensions of the new bearing cover (unlabeled dimensions correspond to the original part).
3. **Adjustment of Axial Fluctuation and Radial Clearance**:
- The axial misalignment of the spindle is adjusted using the rear adjustment nut. The measured misalignment of the spindle shoulder support surface should be within 0.015 mm.
- The radial runout of the centering journal of the spindle is adjusted using the adjustment nut, ensuring a slight radial clearance for the bearing. The radial runout of the spindle's centering shaft head should not exceed 0.02 mm.
After the modification, the machine tool's accuracy was tested, and the results were as follows:
- Radial runout of the spindle centering shaft head: 0.01–0.03 mm
- Axial movement of the spindle: 0.015 mm
- Parallelism of the spindle axis to the slide movement: 0.02 mm over 300 mm (in both vertical and horizontal planes)
- Verticality of the cross-slide movement relative to the spindle axis: 0.02 mm (concave) over 3000 mm
- Height deviation between the headstock and tailstock tips: 0.03 mm
- On a 300 mm long shaft, the roundness was 0.02 mm, and the cylindricity was 0.03 mm
- Flatness of the finishing carriage end face: 0.02 mm (concave)
- Surface roughness of the machined part: Ra 6.3 to 3.2 mm
All these data conform to the JB2670-82 "General Rules for Inspection of Metal Cutting Machine Accuracy." As a result, the performance and quality of the modified machine significantly improved compared to the original model without spindle bearing upgrades.
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