Application of Cubic Boron Nitride Tool in Machining Wear-resistant Castings

1. Difficulties in processing wear-resistant castings

At present, wear-resistant castings have been widely used in mining, cement, electric power, road construction machinery and refractory materials. Wear-resistant castings are difficult to process due to irregular surface, uneven hardness, and a large number of hard spots and pores inside. In addition, when the wear-resistant parts are processed, large cutting force and cutting heat are generated, and large vibration and impact are generated, which will accelerate the wear of the tool. When ordinary carbide tools are used for processing, they often chip or burn, resulting in increased processing costs.

2. Application of cubic boron nitride tool in the processing of wear-resistant castings

(1) Tool geometry parameter selection

Cubic boron nitride material is brittle and low in strength, so the rake angle of the tool is usually γo=-5°～-0°, and the relief angle αo=-6°～-15°. In order to improve the strength of the cutting edge, the tip angle is generally ≥90°, and a negative chamfer must be ground on the cutting edge. The chamfer width can be b=0.1～0.3mm, and the chamfer rake angle=-20°～-6°, Tool tip rounding r=0.2～0.6mm. Tool tip rounding radius and negative chamfering are too large, which will increase the chance of chattering. Therefore, when the system rigidity of the machine tool, fixture, tool and workpiece is insufficient, especially when processing slender workpieces, it is not suitable to use too large tool nose radius and negative chamfer.

(2) Selection of feed

Cubic boron nitride tools are mainly used for finishing and semi-finishing, so the bite is generally small, usually below 2mm, and the feed rate is 0.05-0.4mm/r. Because the tool has a negative chamfer, in order to give full play to the characteristics of the cubic boron nitride tool material, and to meet the processing requirements more economically, the feed rate is generally selected to be greater than the width of the negative chamfer.

(3) Matters needing attention in processing. Before cubic boron nitride tools are used, the front and flank surfaces must be ground to reduce friction during the cutting process, while also improving the straightness of the cutting edge and preventing abnormal wear of the tool. In addition, use a magnifying glass of more than 50 times to check the quality of the cutting edge before use to control the serrated edge of the cutting edge, so as not to affect the tool life and processing quality.

In addition, although cubic boron nitride tools can withstand cutting temperatures of 1,250 to 1,350 °C, CBN tools will react with water vapor and oxygen in the air at high temperatures (about 1000 °C) to generate ammonia and boric acid, which accelerate the wear of cutting tools. Therefore, in wet cutting, avoid using aqueous solutions as cutting fluids, and use aqueous solutions with extreme pressure additives or extreme pressure cutting oils to reduce hydrolysis.