In the experiment, ball milling was used to produce powder due to the limited amount of raw materials, but overall, the particle size of the powder obtained by ball milling is not as uniform as that obtained by airflow milling. The ball to material ratio is 10:1, the medium for ball milling is 120 # aviation gasoline, and the ball milling tank is made of stainless steel. The balls used are steel balls with four diameters, namely 2mm, 5mm, 8mm, and 10mm. Each type of steel ball has the same weight.

Pulverizing includes two processes: coarse crushing and grinding. Firstly, the oxide skin on the surface of the ingot should be removed, and then coarse crushing should be carried out. When the ingot is broken to about 20mm, hydrogen crushing method is used to improve its fragility and make ball milling easier. Grinding fine powder includes two methods: ball milling and airflow milling. Among them, the airflow mill utilizes the collision between powders under the propulsion of supersonic airflow to further refine. Unlike this, ball milling uses steel (or other) balls to break coarse powder into finer powder. Ball milling includes methods such as rolling ball milling, vibration ball milling, and high-energy ball milling. The irregular shape of powder particles produced by vibration ball milling is not conducive to magnetic field orientation. The powder particle size of high-energy ball milling is too dispersed, which is also not conducive to improving the performance of the magnet. Rolling ball milling can effectively avoid the aforementioned defects.

Rolling ball mill generally uses stainless steel tanks, which contain rolling bearing steel balls of different diameters. The grinding medium can be 120 # aviation gasoline, acetone, alcohol, or petroleum ether. The rotational speed of the ball milling tank should meet the following equation in order to improve the ball milling efficiency.

υ＝ (0.65-0.70) υ C#

In the equation, υ C - Critical speed, r/min, is the speed at which the centrifugal force of the steel ball is equal to its gravity. The calculation method is as follows:

set up υ＝υ c. Then,

M (2 π υ C/60) 2 × D/2=mg: d7 U # j% W "Q0 L * l8 v k, W0 f

In the formula, D - diameter of the ball milling tank, m; 8 M (A8 K "{9 j~

M - mass of the steel ball, g;

G - Gravitational acceleration, 9.8 g × M × S-2;

π - pi, 3.14

By equation, υ C=42.3/D1/2;