Material: carbon steel. 16Pcs drill depth stop collar, clamping device, positioner. Size: 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, 16mm. Durable alloy steel set screw type collar will stay strong when tightened and is specially designed not to damage bits. Bit limit rings are used as a shaft collar on machinery and can be used on twist drills or brad point drills or any type of drill bit to prevent over penetration of the drill bit and for any work that uses a drill bit and requires precision. The Stop Drill Bit Collet Set will quickly and easily help you drill holes to exact depths and is ideal for production work and repeat drilling by positively locking anywhere along the bit. With the set screw tightened, the collar will stay securely in place exactly where you placed them and will precisely control the depth of the hole you drill. Package includes: 2 x Allen Key Hex Wrench, 16 x Drill Stop Collars.