7 Sizes Drill Bits with Adapter: includes 7 different sizes of square hole drill bits, namely 1/4 inch, 5/16 inch, 3/8 inch, 1/2 inch, 9/16 inch, 5/8 inch, and 3/4 inch, to meet your varying needs, as well as 1 piece of square hole drill bit adapter; Use different sizes of square hole drill bits, matched with the appropriate power of the square hole drilling machine, you can drill square holes to meet your requirements Square Hole Drill Bits: crafted from quality bearing steel, these mortise chisel drill bits boast an extended core, incisive cutting edges, and continuous blade; Such construction ensures rapid cutting and a firm fixation on the wood; The set is only for open square holes and locking drills, square hole drill bits cannot be applied on electric drills and bench drills; It requires the installation of a square hole drill (mortising machine) to drill square holes Square Hole Drill Bit Adapter: this square drill bit adapter is constructed from all steel casting, ensuring durability and longevity; With its length of approx. 6.3 inches, 0.75 inch top inner diameter, and 2.17 inch bottom diameter, the adapter reduces the usage error, providing a more precise drilling performance Efficient Combination: the adapter is compatible with all the different sized mortise drill bits for wood, whether you are a beginner or an expert, each drill bit is designed to drill precise holes with minimal pressure, significantly reducing the stress on the operator and enhancing work efficiency, bringing much convenience in construction, installation, and decoration industries Widely Applied: these square mortise drill bits with adapter are Perfect for both DIY enthusiasts and skillful workers, suiting well for construction as well as woodworking; They are suitable for variety of lower hardness materials, including compressed board, plastic, density board, sandalwood, pinewood, and so on; This makes it applicable for various sectors including DIY woodworking