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A drill bit cuts correctly only when geometry, speed and feed agree
The cutting lips shear material, the chisel edge pushes at the centre and flutes evacuate chips. Excess speed creates heat and softens the edge; insufficient feed makes it rub. Too much feed overloads the lips, machine and workholding.
Diameter changes cutting speed at the outer edge, so a larger bit normally turns more slowly in the same material.
Bit families serve different materials and hole shapes
| Bit type | Typical purpose | Strength | Main limit |
|---|---|---|---|
| HSS jobber | General metal, plastic and wood where suitable. | Versatile standard-length twist geometry. | Loses hardness if overheated. |
| Cobalt-alloy HSS | Stainless and tougher heat-producing metals. | Improved hot hardness. | Can be less forgiving of flex/impact. |
| TiN-coated HSS | Reduced friction/wear in compatible cutting. | Hard low-friction surface. | Sharpening removes coating at the edge. |
| Blacksmith/reduced-shank bit | Large hole with smaller chuck compatibility. | Large diameter without full-size shank. | Machine may lack low-speed torque/rigidity. |
| Step drill | Several clean diameters in thin sheet. | Low grabbing and good deburring. | Limited material thickness/depth. |
| Hole saw | Large circular cut with minimal material removal. | Efficient large diameter. | Needs arbor, pilot, clearance and chip control. |
| Drill/tap combination | Drill and form thread in one supported operation. | Fast alignment in suitable thin material. | Needs exact pitch, thickness and reversing control. |
| Multi-material bit | Named masonry/wood/metal combinations. | Useful where substrate is known and supported. | No universal optimum for every layer. |
Material drives point geometry and cutting conditions
| Workpiece | Useful approach | Avoid | Warning sign |
|---|---|---|---|
| Mild steel | Sharp HSS, suitable fluid, steady feed. | High-speed rubbing. | Blue chips/bit and squeal. |
| Stainless steel | Rigid setup, cobalt HSS, low speed and positive feed. | Pausing while edge rubs/work-hardens. | Glazed hard spot and no chip. |
| Aluminium | Sharp open flutes and compatible lubricant. | Built-up edge and clogged flutes. | Material welded to cutting lip. |
| Thin sheet | Step drill or supported sharp bit with backing. | Hand-holding the sheet. | Snagging and triangular hole. |
| Plastic | Low heat, suitable rake and backing. | Melting/cracking from pressure. | Stringing, white stress marks. |
| Masonry/tile | Correct dedicated bit and impact setting. | Using hammer mode on unsupported tile/glass. | Cracking or polished tip. |
HSS, cobalt and coatings
High-speed steel combines toughness and hot hardness for cutting. Cobalt-alloy grades retain hardness at higher temperature but still need correct speed and lubrication. A surface coating can reduce friction and wear, but it cannot compensate for poor geometry or a soft substrate.
Do not identify metallurgy by colour alone. Oxide, coating and temper discolouration can look similar.
Shank and chuck compatibility
Round shanks need full clean jaw engagement. Hex shanks improve quick-change retention but may be made for drill/drivers, impact drivers or both according to the bit rating. Reduced shanks let a small chuck hold a large bit but do not increase machine capacity.
Check chuck maximum diameter, spindle taper, minimum rpm and side handle. Tighten at every keyed position where the chuck instructions require it.
Impact-compatible drill bits have boundaries
A hex-shank set may tolerate an impact driver's torsional pulses, but the cutting tip still needs appropriate speed, feed and material. Impact action can chip brittle cobalt edges, enlarge holes or strip a combined tap.
Use the mode and tool specified by the bit maker. Do not confuse an impact driver with a masonry hammer drill.
Workholding prevents the most violent failures
Clamp the work to a bench or drill-press table and add backing beneath thin sheet. Position clamps clear of the bit path. A drill catching at breakthrough can spin sheet metal into a sharp blade.
For handheld work, brace the side handle and stand so a stall will not trap wrists. Never lock the trigger on where rapid release is needed.
Marking, centre punching and pilot holes
Measure twice, use a centre punch suitable for the surface and begin with a short rigid pilot when the final bit needs guidance. An oversized pilot can leave the final chisel edge unsupported and make cutting chatter.
Step drilling through several twist-bit sizes is not always necessary; follow the geometry guidance for the final bit and machine.
Speed, feed and chip evidence
| Observation | Likely condition | Adjustment | Risk if continued |
|---|---|---|---|
| No chip, polished surface | Dull edge, low feed or work hardening. | Stop, sharpen/replace and reset. | Heat and hardened spot. |
| Blue bit/chips | Excess heat/speed or poor fluid. | Reduce rpm, improve cutting/lubrication. | Lost hardness. |
| Long tangled swarf | Ductile material and poor chip break. | Use peck/geometry as supported. | Entanglement and surface damage. |
| Chatter/triangular hole | Flex, poor centring or thin sheet. | Improve support; use step drill. | Oversize hole and broken edge. |
| Sudden grab at exit | Feed too high at breakthrough. | Support backing and reduce pressure near exit. | Spun work or wrist injury. |
Cutting fluid and temperature
Use a fluid compatible with workpiece, bit, subsequent paint/welding and workplace controls. Apply enough at the cutting zone without aerosolising it. Some plastics and composites require dry methods or extraction instead.
Stop to cool naturally. Quenching an overheated bit can cause cracking or does not restore lost temper.
Step drills in sheet metal
Mark the required step, clamp the sheet and stop before the next diameter enters. The next step can lightly deburr if the tool is designed for it, but excessive pressure makes a countersink or thin edge.
Check the opposite side for burrs and treat exposed metal against corrosion.
Hole saws, arbors and pilot drills
Match saw diameter, arbor thread/pins and pilot to the material. Secure drive pins fully and use a drill with sufficient low-speed torque and side handle. Large saws produce strong reaction and a hot slug.
Clear chips with power isolated. Confirm space behind the work for pilot and slug; protect hidden components.
Combined drill-and-tap tools
These tools suit specified material thickness because the drill portion must pass through before the tap engages. Use the exact thread pitch, tapping speed and lubricant, keep square and reverse cleanly to break/remove chips.
Do not use them for deep blind holes or safety-critical threads unless the engineering procedure approves the resulting engagement and tolerance.
Sharpening and replacement
Restore geometry, not just a shiny edge
Twist-drill lips must be equal in length and angle with correct relief; an off-centre point cuts oversize. Sharpen with guarded equipment and cooling discipline, or use a professional sharpener. Coated bits lose edge coating when ground.
Retire structurally damaged tools
Replace cracked, badly overheated, bent or severely chipped tools. Never straighten a hardened drill bit for reuse.
Depth stops and breakthrough protection
Set a collar, machine stop or physical spacer when the hole must not reach hidden equipment. Verify from the bit tip and account for the full conical point, not only flute length. A tape flag can move and is only a visual guide.
Reduce feed as the point breaks through and support the exit face to limit burr, delamination and grabbing.
Vehicle drilling hazards
Inspect both sides for fuel/brake lines, looms, airbags, batteries, tanks and structural reinforcements. Isolate electrical systems where conductive swarf could short them. Shield glass and interiors from hot chips.
Do not drill chassis, restraint, wheel, brake or high-voltage structures without approved repair data. A neat hole can still make a component unsafe or illegal.
PPE and swarf control
Wear impact eye protection and manage noise; use face/respiratory controls for the material risk. Do not wear loose gloves near rotating spindles. Stop rotation before brushing chips and never clear swarf with fingers or compressed air toward others.
Hot spirals can cut shoes and ignite debris. Maintain a clean fire-controlled work area.
Practical drill-bit-and-set FAQs
Q: Is cobalt HSS only a coating?
A: No. Cobalt is alloyed into the HSS; TiN is a surface coating.
Q: Can a large bit run at the same rpm as a small one?
A: Usually no. Larger diameter needs lower rpm for the same cutting speed.
Q: Why will a bit not cut stainless?
A: It may be dull or rubbing has work-hardened the surface; use rigid low-speed positive-feed technique.
Q: Can sheet metal be held by hand?
A: No. Clamp it because a grabbing bit can spin it violently.
Q: Does a hex shank mean impact-rated?
A: No. Confirm the bit's tool and mode rating.
Q: What is a reduced-shank bit?
A: Its cutting diameter exceeds the shank, allowing a smaller chuck, but machine capacity still limits use.
Q: When is a step drill useful?
A: For clean multiple diameters in suitable thin sheet with controlled breakthrough.
Q: Can any arbor fit a hole saw?
A: No. Match thread, drive pins, pilot and diameter range.
Q: Should cutting fluid always be used?
A: Use the fluid or dry method specified for bit, material and later process.
Q: Can a blue overheated bit be quenched and reused?
A: Cooling does not restore lost hardness; inspect and replace/sharpen as appropriate.
Q: Are drill-and-tap bits suitable for deep holes?
A: They are normally intended for supported thicknesses where the drill clears before tapping.
Q: How should swarf be removed?
A: Stop and isolate rotation, then use a brush or chip tool—never bare fingers.
Q: What completes a drilled vehicle hole?
A: Deburr, remove all swarf, protect bare metal and verify no hidden system was damaged.
