This Mortising Chisel is great for drilling square holes in wood with a mortising machine or a mortising attachment. Square hole drill can not be used on an electric drill, drill square holes need to fit a square hole drill drilling (mortise and tenon machine) to use. Mortise and tenon square hole drilled by the terrible + outer sleeve in the middle of the combination. Reduce labor intensity of construction workers, speed up the construction speed, reduce construction costs for construction, installation, renovation, decoration industries. Size: 1/4 inch (635mm), 5/16 inch (794mm), 3/8 inch (95mm), 1/2 inch (127mm) Material: high-speed steel Hardness: HRC 48-50 Color: silver Clamping diameter: 19mm (Round) Length: 210mm Package Contents: 4 x Drill Bit Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.