Cobalt vs HSS Drill Bits for Hardened Mill and Lathe Work
By Products for bench top machinists | LittleMachineShop.com | Published: 2026-10-16
Category: Comparisons
Cobalt bits keep cutting hard, abrasive steel when HSS dulls fast, but they cost more and stay brittle. Here's how to choose for bench mill and lathe work.
Quick Answer
Reach for cobalt when you're drilling hard or abrasive steel, stainless, or work that has work-hardened on the bench mill or lathe. Cobalt steel displays higher red hardness, so it holds an edge when the tip gets hot, while plain HSS dulls faster and needs more resharpening. Stay with HSS for soft aluminum, brass, and plastic, plus everyday holes where the lower price and tougher edge matter more than heat resistance. If most of your work is hard steel, a broad cobalt set such as the 115 Piece Jobber Length Cobalt Steel set covers fractional, letter, and number sizes in one index.
What Actually Separates Cobalt From HSS
The difference is heat resistance, not magic sharpness. Cobalt steel displays higher red hardness, which means it keeps cutting when the drill tip is hot from hard or abrasive material. Plain HSS loses hardness at lower temperatures, so the cutting edge rounds over sooner and you push harder to keep the hole going. That extra pressure is what dulls the bit and can work-harden stainless or hard steel ahead of the cut.
For a bench machinist, that shows up as fewer resharpening sessions and less spindle pressure on a small mill or lathe. It does not mean cobalt cuts through fully hardened tool steel. If the material is harder than the drill, neither alloy will help; you need an abrasive or a different process.
Where HSS Still Wins
HSS is tougher and cheaper, and it handles interrupted cuts and hand-fed drilling with less risk of chipping. On aluminum, brass, and plastics, the heat never builds enough to need cobalt, so you would be paying more for an advantage you cannot use. HSS also resharpens easily on a bench grinder, which matters if you drill a lot of holes in mild steel and expect to touch up bits often.
The practical rule: match the bit to the material and the machine. On a small bench mill with limited rigidity, a sharp HSS bit in soft material often drills cleaner than a dull cobalt bit pushed too hard.
Reading a Large Set Before You Buy
A 115 piece drill bit set is usually a range purchase, not a single drill. The 115 Piece Jobber Length Cobalt Steel set covers 1/16 in. to 1/2 in. by 64ths, A through Z, and No. 1 through 60, with a metal index for storage. That spread matters because tapping, clearance, and reaming jobs often call for letter or number sizes you cannot buy one at a time without a trip to the store.
Jobber length is the normal-length drill, so it reaches most bench mill and lathe work without a stub or long-series compromise. Straight shanks fit common drill chucks, and the cobalt construction is the reason the set is priced above a comparable HSS index. Check your chuck capacity before buying: the largest bit in the set has to fit the chuck you actually own.
Tool Holding and Setup Details That Decide Results
Cobalt bits reward a rigid setup. If your drill chuck arbor is worn or mismatched, the bit runs out and the corner chips before the coating or alloy ever gets a chance to work. The Arbor, Drill Chuck 3/8 in. Straight to 0JT connects a drill chuck with a 0 Jacobs taper to a 3/8 in. straight shank, which is the kind of small interface that keeps a drill running true in a chuck or holder.
On lathe work, a solid compound rest helps you feed consistently instead of forcing the bit. The Compound Rest Assembly for SIEG C0 lathes gives 40 mm of travel and holds tools up to a 1/4 in. shank, so you can set up boring and drilling operations without fighting a loose slide. Neither item changes drill alloy, but both reduce the runout and chatter that kill cobalt edges early.
Trade-Offs to Weigh Before You Buy
- Heat resistance vs. cost: Cobalt steel displays higher red hardness for hard or abrasive materials, but a 115 piece cobalt set costs more than a comparable HSS index. Buy cobalt if hard steel is routine; stay HSS if you mostly drill aluminum and mild steel.
- Edge life vs. toughness: Cobalt holds an edge longer under heat but is more brittle, so it chips more easily under hand-fed or interrupted cuts. Use a rigid setup and steady feed, or keep HSS for rough hand drilling.
- Set coverage vs. single-bit spending: A 115 piece set covers fractional, letter, and number sizes in one index, while buying individual cobalt bits adds up fast. If you regularly need tap and clearance sizes, the set is the cheaper long-run path.
- Alloy vs. setup: Runout from a worn arbor or a loose compound rest dulls a cobalt edge as fast as drilling soft material with HSS. Fix tool holding and rigidity before blaming the bit.
Products That Support This Comparison
- Drill Set, 115 Piece Jobber Length Cobalt Steel: 115 Piece cobalt steel drill set covering 1/16 in. to 1/2 in. by 64ths, A through Z, and No. 1 through 60, with jobber length straight shanks and a metal index. One indexed set covers fractional, letter, and number sizes, so you are not buying single cobalt bits every time a job calls for a tap drill or clearance size.
- Arbor, Drill Chuck 3/8" Straight to 0JT: Drill chuck arbor connecting a 0 Jacobs taper chuck to a 3/8 in. straight shank. A correct arbor keeps the drill running true, which protects the cutting corners on cobalt bits and improves hole size and finish.
- Compound Rest Assembly SIEG C0 Lathes: Compound rest assembly for SIEG C0 lathes and Emco Unimat 4 mini lathes, plus Grizzly G0745 and Harbor Freight Central Machinery 95012 micro lathes, with 40 mm of travel and a two-sided tool post holding up to a 1/4 in. shank. A rigid, smoothly feeding compound lets you advance a drill steadily on the lathe instead of forcing it, which is how cobalt edges last longer.
- Motor, Z-Axis Power Lift, HiTorque Bench Mill Deluxe: Z-Axis power lift motor for the HiTorque deluxe large bench mill and SIEG SX3.5 large bench mills with a power lift on the Z-Axis drive. Easier head positioning means less time cranking and more consistent setup when you switch between drilling and milling hard material.
Which Should You Choose?
Choose cobalt if you:
- Drill hard, abrasive, or stainless steel on a bench mill or lathe
- Want one indexed set covering fractional, letter, and number sizes
- Can keep the setup rigid and feed steadily
Stay with HSS if you:
- Mostly drill aluminum, brass, or plastic
- Hand-feed with less rigid setups and expect occasional chipping
- Want the lowest upfront cost and easy bench-grinder resharpening
