Metallics HSSCN21E Drill Bit HD Cobalt St 135d SplPt #21 3-1/4" YLW (1pc)

The Metallics HSSCN21E is a number 21 wire gauge bit, which cuts 0.159 inches. That is the standard tap drill for a 10-32 thread, which is why electricians keep it in the pouch for threading strut, enclosure walls, and box ears for 10-32 ground and device screws. If you want a clearance hole for a 10-32 screw rather than a tapped hole, step up to a letter or fractional bit instead.

Cobalt is the reason to pick this one. The HSSCN21E is a heavy duty cobalt high speed steel bit, so the alloy runs hotter without losing its edge the way plain HSS does. That is what makes it workable in stainless, strut channel, and tool steel. Keep the speed low and use cutting fluid in stainless, since cobalt tolerates heat but still work hardens the material if you dwell.

A 135 degree split point starts on its own without a center punch and resists walking on round or slick surfaces, which is the difference that matters when you are drilling a strut channel face or a painted enclosure overhead on a ladder. A standard 118 degree point needs a punch mark or a pilot to stay put. The flatter 135 degree geometry also cuts with less thrust pressure, so it suits cordless drills.

Overall length is 3-1/4 inches. That is a jobber length bit, so the flute portion is well under the overall figure once the shank is in the chuck. It handles normal sheet steel, strut, box walls, and panel flanges, but it is not a long reach bit for drilling deep into a blind pocket or through stacked material.

One bit per pouch. The HSSCN21E is a single piece item in a pouch pack, not a multi size index, so if you are stocking a van or a shop crib you order the quantity of number 21 bits you want rather than getting a range of sizes. The yellow finish on the bit makes it easy to pick out of a pouch of mixed bits.