What you receive: the package includes 1 square drill bit adapter and 4 square drill bits. The drill diameter includes 1/4 inch (6.4 mm), 5/16 inch (8 mm), 3/8 inch (9.5 mm), 1/2 inch (12.7 mm). The diameter of the hand drill clamp is 38 to 55 mm/1.5 to 2.2 inches. Please note that it is only suitable for bench drills and hand drills with clamps. High-quality carbide material: the square hole drill adapter is made of carbide, sturdy and durable, which can firmly hold wood. It measures approximately 16 cm in length, with an upper inner diameter of about 0.75 inches/1.9 cm and a lower diameter of about 2.17 inches/5.5 cm, reducing usage error. The drill bit will not be blocked by sawdust, has high hardness, does not roll, and has a fast cutting speed. It is sharp and can be safely removed. Easy to use: the square drill adapter and bit set is easy to use. The precision design of the drill press mortising attachment ensures you can drill the correct mortises and tenons in the fastest way. It is more convenient and faster than a mortising machine. Suitable for bench drills and hand drills: the dual-support stand is reliable and practical, suitable for bench drills and hand drills. It is ideal for woodworking, furniture making, and other related industries. Multiple uses: you can combine the square drill bit adapter with 4 square drill bits, simplifying the process of drilling square holes in wooden boards. It is suitable for construction, woodworking, decoration industries, and materials with lower hardness such as compressed board, plastic, density board, sandalwood, pine, ash, etc.