Magnetic grinding technology has not yet been widely adopted in our country. The key to its popularization and application lies in the development of practical machine tools—specifically magnetic grinders. This paper presents the structural principles, technical parameters, and design and calculation methods of a magnetic lapping machine modified from a conventional lathe.
1. Electrical device 2. Additional pad 3. Centripetal bearing 4. Flywheel 5. Bearing 6. Additional mandrel 7. Solenoid 8. Rear tip 9. Rear tailstock 10. Additional pad 11. Handwheel 12. Electromagnetic Induction 13. Drive Belt 14. Motor
Figure 1 Lathe modified magnetic grinder structure
1. Base plate 2. Sliding base 3. Magnetic conductor 4. Solenoid coil 5. Pole head 8. Feed plate 9. Hand wheel 11. Screw
Figure 2 electromagnetic sensor structure
The structure of the magnetic grinder modified from a CA6140 lathe is shown in Figure 1. The workpiece is fixed on the top and back tips (8) of the additional mandrel (6). The mandrel is mounted on support (5) using a sliding bearing, with one end splined to the machine tool's spindle and the other to the machine’s top. A motor (14) mounted on the bed drives the workpiece via an additional mandrel, transmitting motion through a belt (13) to the radial bearing (3), then to the flywheel (4). The oscillating movement is restored by the rear top spring. The electromagnetic sensor (12) is mounted on a horizontal slide, moving in sync with the slide to achieve longitudinal feed during grinding. An electrical device (1) controls the sensor solenoid (7) and feed motion. The knife holder’s longitudinal feed is hydraulically driven. To install additional pads (2 and 10), the front center and rear tailstock (9) must be lifted due to the higher center height of the magnetic sensor. During magnetic grinding, the sensor generates a magnetic field that carries magnetic abrasives. The electromagnetic inductor structure is shown in Figure 2, consisting of a base plate (1), a slider (2) with an L-shaped magnetic conductor (3), an electromagnetic coil (4), and a pole tip (5). A tray (8) collects cooling liquid, and a screw (11) adjusts the base plate. All components are made of non-magnetic, wear-resistant materials. When the coil is energized, magnetic abrasive fills the gap between the two pole heads, forming a brush-like arrangement that presses against the workpiece. The workpiece rotates, and the sensor moves axially along the workpiece for full-length grinding.
**Calculation of Magnetic Sensor Parameters**
Assuming the magnetic flux passes completely through the magnetic circuit without leakage, and all air gaps are filled with air. The magnetic conductor size is shown in Figure 3. The working gap area S = mA = 8 × 230 = 1.84 × 10â»Â³ m², where the pole head width A = 230 mm, the chord length m = 8 mm, and the working gap is 2 mm. The steel saturation magnetic induction B = 2 T. The magnetic flux F = BS = 2 × 1.84 × 10â»Â³ = 3.68 × 10â»Â³ Wb. The dimensions of the magnetic conductor are as follows: Lâ‚ = 25 mm, Lâ‚‚ = 165 mm, L₃ = 190 mm, Lâ‚„ = 395 mm, with a cross-section of 230 mm × 30 mm. The workpiece diameter is 11 mm.
The magnetic field strength in the working gap is Hâ‚€ = B / μ = 2 / (4Ï€ × 10â»â·) = 0.1595 × 10â· A/m. The magnetic potential in the working gap is Fâ‚€ = 2Hâ‚€Lâ‚€ = 2 × 0.1595 × 10ⷠ× 2 × 10â»Â³ = 6400 A. The magnetic pole area S = (m + 30)A / 2 = 4.37 × 10â»Â³ m². The magnetic induction of the pole head is B = F / S = 3.68 × 10â»Â³ / 4.37 × 10â»Â³ = 0.84 T. The magnetic field strength of the pole head is H = 940 A/m (from the magnetization curve of steel). The magnetic potential of the pole head is Fâ‚ = 2HLâ‚ = 2 × 0.94 × 10³ × 25 × 10â»Â³ = 47 A. The remaining magnetic conductor cross-sectional area is S = 30 × 230 × 10â»â¶ = 6.9 × 10â»Â³ m². The magnetic induction of the remaining conductor is B = 3.68 × 10â»Â³ / 6.9 × 10â»Â³ = 0.534 T. The magnetic field strength is H = 680 A/m (from the magnetization curve of steel). The magnetic potentials of Lâ‚‚, L₃, and Lâ‚„ are Fâ‚‚,₃,â‚„ = 2HLâ‚‚ + 2HL₃ + 2HLâ‚„ = 683.4 A. The workpiece cross-sectional area is S = 8 × 230 × 10â»Â³ m². The magnetic induction of the workpiece is B = 3.68 / 1.84 = 2 T. The magnetic field strength of the workpiece is H = 50,000 A/m (from the magnetization curve of 45 steel). The magnetic potential of the workpiece is F = 50,000 × 11 × 10â»Â³ = 550 A. The total magnetic potential of all magnetic sections is F_total = Fâ‚€ + Fâ‚ + Fâ‚‚,₃,â‚„ + F = 7680 A.
After determining the magnetic potential, the magnetic sensor coil parameters are calculated. The wire diameter d is calculated using the formula:
d = 0.2(rqINLP/pCU)^½
where rq is the specific resistance of the wire at q°C, IN is the magnetic potential (A), LP is the average coil length (cm), and C is a voltage drop coefficient (C = 0.85). With U = 220 V and rq = 0.0196 at q = 50°C, and LP = 760 mm, the wire diameter is d = 0.88 mm, rounded to 1 mm.
The number of turns N is calculated using:
N = 4fLe × 10² / pd²
where L and e are the length and width of the coil (cm), d is the bare wire diameter (mm), f is the fill factor, and SK is the cross-sectional area of the wire. With dJ = 1.23 mm and f = 0.5, the number of turns N = 3846. The total number of turns for two coils is 7692. Other parameters such as resistance, power consumption, electromagnetic suction force, copper consumption, and temperature rise can also be calculated based on these values.
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