The Planet Forstner Extension Bars are designed for use with the Planet Forstner bits. They are supplied with 3 grub screws and allen key. Thanks to the very high-quality strong build and design, Forstner bits, even up to 4" in diameter, can drill deeper holes in your workpieces. The Planet Forstner Extension Bars are machined from high-quality Steel. The one-piece design ensures the shank cant work loose from the socket. Made in the UK by Planet Plus Ltd For Forstner Bit sizes from 2 1/4 to 4?, use the1/2? Shank Extension Bar (11?/280mm Long): FE28012