4 different sizes: 6.4mm, 8mm, 9.5mm, 12.7mm. nbsp; Made of high speed steel, high hardness, durable and practical, heat treatment, long service life. nbsp; Square hole drill bit cannot be used on electric drill, square hole drilling need to fit square hole drill bit (tenon and mortise machine) to use. nbsp; Fits all mortising machines and drill attachments. The square hole drill bit is mainly used for woodworking. nbsp; Reduces labor intensity of construction workers, accelerates construction speed, reduces construction costs in construction, installation, renovation and decoration industries. Protective Design: The design of the mortise drill sleeve can be effectively protected against the burst situation at the beginning of use. nbsp; Size - 4 sizes in total including 6mm / 8mm / 10mm / 12.5mm. This mortise chisel set is a must have for home woodworkers. Features: Material: high speed steel. Color: silver. Dimensions: 6.4mm / 8mm / 9.5mm / 12.7mm. Content: 1 drill bit with square hole of 6.4 mm. 1 square hole drill bit of 8 mm. 1 square hole drill bit of 9.5 mm. 1 square hole drill bit of 12.7 mm.