How to drill through metal
Drilling metal fails in predictable ways: the wrong bit for the steel, too much speed, no lubricant, or a bit that rubs instead of cuts. This page covers the four decisions that prevent all of them, with the stainless-specific rules that make the difference between a clean hole and a ruined workpiece. Every claim cites two or more named sources.
The four decisions, in order.
- Bit: cobalt (M35 or M42) for stainless and hardened steel, titanium-coated HSS acceptable for mild steel [1] [2].
- Speed: slower than you think. Mild steel around 1,200 to 1,500 RPM at 1/4 in; stainless roughly half that [3] [4].
- Lubricant: cutting oil on steel always; stainless never runs dry [5] [6].
- Feed: firm and constant. A rubbing bit work-hardens stainless in seconds [7] [5].
Step 1: pick the right bit
Two bit families matter for metal. Cobalt bits are high-speed steel alloyed with 5 percent cobalt (M35) or 8 percent (M42); the cobalt is in the whole bit, not a coating, so the bit stays hard even after sharpening, and it is the standing recommendation for stainless and hardened steel [1] [2]. Titanium-nitride coated HSS is a surface layer that suits wood, plastic, aluminum, and mild steel; the coating wears off in hard service and when sharpened, which returns the bit to plain HSS [1] [2].
The full coatings comparison, including black oxide and what the coatings actually do, is our drill bit coatings page. For drilling different metals generally, including aluminum and brass, see drill bits by material.
Step 2: run the right speed
Speed comes from one formula: RPM equals surface speed (SFM) times 3.82, divided by bit diameter in inches [3] [4]. The surface speed depends on the material and the bit grade; everything else is arithmetic. Worked numbers for common cases:
| Material and bit | 1/8 in bit | 1/4 in bit | 1/2 in bit | Sources |
|---|---|---|---|---|
| Mild steel, HSS (about 80-100 SFM) | about 2,400-3,000 RPM | about 1,200-1,500 RPM | about 600-750 RPM | [3] [4] |
| 304 stainless, HSS (about 30-50 SFM) | about 900-1,500 RPM | about 450-750 RPM | about 230-380 RPM | [4] [8] |
| 304 stainless, M35 cobalt (39-59 SFM) | about 1,200-1,800 RPM | about 590-900 RPM | about 300-450 RPM | [8] [3] |
Row sources: [3] WorkshopCalc's drill speed chart (formula RPM = SFM x 3.82 / diameter; mild steel 1/4 in about 1,200 RPM, 1/2 in about 600 RPM; stainless about half), [4] Test Talk's drill speeds by material chart (mild steel HSS 80-100 SFM giving 1,464 RPM at 1/4 in and 732 at 1/2 in; stainless 30-50 SFM), [8] Slugger Tools' drill speed chart (SFM by grade: stainless M2 26-46, M35 39-59, M42 46-72; mild steel M2 66-98, M35 82-125, M42 92-138). RPM values in the table are computed from those published SFM bands with the formula, then rounded; they are consistent with the worked examples both sources print.
Two reading notes on the table. The SFM bands are honest ranges, not points, and machinists argue inside them; treat any single RPM as a starting point to adjust from, not a target to hold. And the grade progression is real money: cobalt lets you run roughly 25 to 50 percent faster than plain HSS in the same stainless, or run the same speed and live longer [8] [1]. The complete version of this table, including aluminum and wood, is our drill speed chart.
Step 3: lubricate
Cutting oil is not optional on steel. It carries heat away, flushes chips out of the flutes, and prevents the bit losing temper; Bosch's own drilling charts specify drill emulsion or cutting oil for steels [6] [5]. On stainless the rule is absolute: sulfurized cutting oil or tapping fluid, generous and continuous, because dry drilling hardens the surface in seconds [5] [1]. WD-40 is a penetrant, not a cutting oil, and performs like one [5] [9].
Where oil runs off, on vertical or overhead work, wax-style lubricant sticks melt onto the heated tip and stay; drill-bit lubricant guides list them as the no-mess alternative [9]. That claim is single-sourced here, so treat it as a convenience option rather than a performance recommendation.
Step 4: feed it like you mean it
The metal-drilling mistake that ruins workpieces is subtle speed, not subtle pressure. A twist bit cuts with its edges; if the feed is too light, the edges rub, friction builds heat, and in stainless the rubbed surface work-hardens into a layer the bit can no longer penetrate [7] [5]. The British Stainless Steel Association's drilling guidance is blunt about it: maintain the feed, avoid dwell and rubbing, and re-enter a hole at full speed and feed, never by easing the bit back in while it spins [7].
The feedback loop tells you which regime you are in: long curled chips mean cutting; fine dust or no chips means rubbing, and rubbing in stainless means the hole is hardening under you [5] [7]. Slow to your drill's lowest range if the bit squeals or stalls, keep the pressure up, and let the oil carry the heat away.
Stainless steel: the full section
Stainless deserves its own rules because austenitic grades (304, 316) work-harden, and every failure traces back to that one property.
- Bit: M35 or M42 cobalt, sharp. A dull bit rubs, and rubbing is the work-hardening trigger [1] [5].
- Speed: slow. 304 with HSS runs 26 to 46 SFM; even M42 cobalt only reaches 46 to 72 SFM [8] [4]. That is roughly a quarter to half of mild-steel speed.
- Lubricant: sulfurized cutting oil or tapping fluid, continuous, from first contact [5] [6].
- Feed: firm and constant; never dwell with the spindle turning [7] [5].
- Punching: mark the hole with a light punch only. Heavy conical punching can locally work-harden the surface; BSSA recommends a light three-cornered pyramid-tip punch mark instead [7].
Sheet metal: use a step bit
Thin stock defeats twist bits at breakthrough: the bit grabs, the sheet spins or tears, and the hole burrs. Step drill bits (unibits) solve this by cutting one small step at a time. The Irwin Unibit #3 covers 1/4 to 3/4 in in 1/16 in increments with a stated maximum material thickness of 1/8 in [10] [11], and DeWalt's titanium step sets span 1/8 to 1/2 in and larger on 1/4 in hex shanks [11] [12]. Full step-bit size charts, inch and metric, are our step drill bit sizes page.
Bigger holes: pilot, then step up
Above about 1/4 in, drill a pilot first. The pilot's job is to give the big bit's chisel edge somewhere to go: size it just above the final bit's web thickness, or follow the simple progression of 1/8 in pilot, then a bit about half the final diameter, then the final size [13] [14]. Center-punch every hole location before the pilot; the punch dent restrains the bit's chisel edge and stops it walking [15] [7]. For screw threads rather than clearance holes, the bit-to-screw pairing tables live in our pilot hole chart and tap drill chart.
The method, start to finish
- Mark and punch. Center-punch the spot; light mark only on stainless [15] [7].
- Clamp the work. To the bench, not held by hand; a spinning bit can wind a handheld sheet up fast.
- Set the speed. From the table above; when in doubt, slower [3] [4].
- Oil the spot. Before contact, and again mid-hole on anything deep [6] [5].
- Drill the pilot for holes above 1/4 in [13] [14].
- Feed firm and constant. Chips should come out curled; dust means slow down and press harder [5] [7].
- Ease the breakthrough. Back the feed off as the bit breaks through, to keep the exit side from burring and the bit from grabbing.
- Deburr. A countersink or a larger bit twisted by hand takes the exit burr off.
Related guides
Also on this site: the drill speed chart for the full RPM tables across every material, drill bit coatings compared for the cobalt question in depth, step drill bit sizes for sheet metal work, the pilot hole chart and tap drill chart for screw threading, drill bits by material for the wider picture, and drill bit sharpening for bringing dull bits back.
Sources
- AIMS Industrial, Cobalt Drill Bit Guide (M35 5 percent and M42 8 percent cobalt, solid alloy, recommended for stainless and hardened steel): aimsindustrial.com.au
- Tool Army, Cobalt vs Titanium Drill Bits (cobalt for stainless and hardened steel; TiN coating for mild steel, wears off when sharpened): toolarmycompany.com
- WorkshopCalc, Drill Speed Chart reference (RPM = SFM x 3.82 / diameter; mild steel about 1,200 RPM at 1/4 in, 600 at 1/2 in; stainless about half): workshopcalc.com
- Test Talk, Drill Speeds by Material Chart (mild steel HSS 80-100 SFM: 1,464 RPM at 1/4 in, 732 at 1/2 in; stainless 30-50 SFM): testtalkhq.com
- Drillbitsworld, How To Drill Stainless Steel (sulfurized cutting oil; dry drilling hardens the surface in seconds; dust means rubbing): drillbitsworld.com
- Bosch Power Tools, Accessories Drilling Overview catalog PDF (cooling recommendation: drill emulsion or cutting oil for steels): bosch-pt.com
- British Stainless Steel Association, Speeds and feeds for drilling and reaming stainless steels (maintain feed, avoid dwell and rubbing, re-enter at full speed and feed; light pyramid-tip punch): bssa.org.uk
- Slugger Tools, Drill Speed Chart (SFM by material and grade: stainless M2 26-46, M35 39-59, M42 46-72; mild steel M2 66-98, M35 82-125, M42 92-138): sluggertool.com
- Journeyman HQ, Best Drill Bit Lubricants (wax sticks including Boelube as the no-mess alternative for portable work): journeymanhq.com
- Allen Frame, IRWIN Unibit #3 manufacturer product data (1/4 to 3/4 in in 1/16 in steps, 9 hole sizes, maximum material thickness 1/8 in): allenframe.com
- TopTenPick, step bit specs roundup (DeWalt DWA1790IR titanium set 1/8 to 1/2 in on 1/4 in hex; IRWIN Unibit #3T 1/4 to 3/4 in): toptenpick.com
- IRWIN Tools, UNIBIT Step Drill Bits product page (single- and double-flute Unibits for thin materials including stainless, copper, brass, aluminum): irwintools.ca
- Engineer Fix, How to Use Pilot Drill Bits for Accurate Holes (pilot sized just above the main bit's web for larger holes): engineerfix.com
- Drillbitsworld, metal drilling guidance (start with a 1/8 in pilot, step to about half the final diameter, then final size): drillbitsworld.com
- Engineer Fix, How to Prevent Drill Drift (center punch creates the indentation that restrains the chisel edge): engineerfix.com