Faithfull HSS Drill Bits for UK Electricians and Plumbers — Pro Jobber Drill Bits and Cobalt Impact Sets

Whether you're drilling cable entry holes through steel back box knockouts, routing pipes through timber joists, or chasing through mild steel unistrut, a sharp HSS drill bit is one of the most-reached-for tools in a sparky or plumber's van. Faithfull's Pro HSS jobber range covers individual sizes from 1mm right up to 13mm, alongside complete sets and cobalt quick-change impact bits for cordless drill-driver work. All in stock at APM Electricals.

Faithfull Pro HSS Drill Bit 4.2mm (2pcs)

£4.99 inc. VAT

4.2mm is the electrician's go-to size for #8 wood screws and back-box pilot holes. Comes as a two-pack so you have a spare when the first blunts on site.

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Faithfull HSSE M35 Cobalt Quick-Change Impact Drill Bit Set (5 Piece, 2–6mm)

£13.50 inc. VAT

M35 cobalt-alloyed HSS bits with 1/4in hex quick-change shanks rated for impact drivers. Cuts harder steels, stainless fixings and abrasive board materials that blunt standard M2 bits. Sizes: 2, 3, 4, 5, 6mm.

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Faithfull Pro HSS Drill Bit 6mm (2 Pack)

£3.11 inc. VAT

Twin-pack of 6mm M2 HSS jobber bits — a workhorse size for pilot holes, M6 bolt clearance and lighting back-box knockouts. Ideal for keeping spares in the van when you burn through one on site.

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Faithfull Pro HSS Drill Bit 10mm

£8.57 inc. VAT

Single 10mm M2 HSS bit for larger fixings, M10 clearance holes and breaking out steel back-box corners. The larger flute geometry sheds swarf cleanly through steel and aluminium profiles.

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Why Electricians and Plumbers Reach for HSS Drill Bits Every Day

Masonry bits get the headlines, but HSS jobber bits do the quiet grind: drilling out knockout slugs from metal consumer unit enclosures, opening cable entry holes in steel trunking, piloting M5 and M6 fixing bolts, boring through central-heating pump brackets, and clearing burrs from copper tube cut with a wheel slice. On any given install day, a decent set of HSS bits will be used more than almost anything else in the bag.

Faithfull Pro HSS bits are ground from M2 high-speed steel — the same alloy used in most professional British and European tooling — and finished with a 118° split point for clean self-starting on metal without centre-punching every location. They run cooler than cheaper M1 equivalents and hold an edge noticeably longer when cutting mild steel electrical boxes.

M2 HSS vs M35 Cobalt — Which Do You Need?

For everyday mild steel (back boxes, trunking, conduit brackets, unistrut), M2 HSS bits are perfectly adequate and cost-effective. They sharpen easily with a bench grinder or diamond wheel if needed and are inexpensive enough to replace rather than fettle.

Step up to M35 cobalt (HSSE) when you're cutting hardened or stainless steel — think old cast-iron pipe brackets, stainless fixings in marine-grade installations, or work-hardened steel enclosures from certain continental manufacturers. The 5% cobalt alloying raises the red-hardness of the steel, allowing the bit to maintain its cutting edge at the higher temperatures generated by harder workpieces. Faithfull's M35 set also comes with 1/4in hex quick-change shanks, making them a direct fit in cordless impact drivers and driver-drill bit holders without needing a separate chuck — handy when you're balancing up a ladder.

Choosing the Right Drill Speed for Steel

Too fast and you'll burn the bit; too slow and you'll work-harden the cut face, making it progressively harder to break through. A useful rule of thumb for HSS on mild steel:

• Up to 3mm: 3,000–5,000 RPM — high speed, light pressure
• 4–6mm: 1,500–2,500 RPM — moderate speed, steady feed
• 8–10mm: 600–1,200 RPM — slow speed, firm consistent pressure
• 12mm+: 300–600 RPM — pilot-drill first with a 5–6mm bit, then open out

Apply a drop of cutting oil or even a smear of petroleum jelly when drilling thicker steel sections — this keeps the bit cool and reduces work-hardening on the cut face. For steel up to 3mm thick (like standard back-box steel), dry drilling at the right speed is usually fine.

The Essential Sizes for Electrical and Plumbing Work

A well-stocked sparky's or plumber's van will typically carry the following HSS sizes for day-to-day use:

• 3mm — M3 clearance, electrical terminal pilot holes, general sheet metal
• 4mm — M4 clearance, conduit fixing bolts, SWA gland entry holes (alongside a step drill)
• 5mm — M5 clearance, standard back-box fixing screw pilots, copper pipe bracket pilots
• 6mm — M6 clearance, consumer unit base fixing, busbar bolt clearance, larger bracket work
• 8mm — M8 clearance, larger SWA gland prep, pipe flange fixing
• 10mm — M10 clearance, heavy bracket and containment fixing, incoming cable entry prep

For a complete range of sizes, Faithfull Pro HSS bits are available from 1mm upward — pick the individual sizes your work demands most, from pilot holes at 2mm through to heavy clearance at 10mm and beyond hole. For the larger 7–13mm sizes, individual bits are often more economical than buying a complete range you'll use once a year.

Quick-Change Impact-Rated Cobalt Bits — Why They Matter on the Tools

Traditional round-shank HSS bits can't be used in impact drivers — the shank will round off in the chuck jaws under the hammer blows. Faithfull's M35 cobalt quick-change set solves this with a standard 1/4in hex shank that locks directly into any bit-holder or impact-driver chuck. On a busy second-fix day drilling through dozens of steel back boxes, swapping from a screw bit to a drill bit and back in under a second is a genuine time saver.

The M35 cobalt alloy also handles the higher cutting temperatures that impact drilling generates — standard M2 bits can soften and lose their edge faster when subjected to the intermittent impact loads from a hammer-action driver.

Sharpening and Looking After Your HSS Bits

A dull HSS bit is more dangerous and less controllable than a sharp one — it requires harder pressure, wanders more on entry, and generates more heat. Signs a bit needs attention: it's squealing rather than cutting cleanly, leaving a blue heat-discoloured ring around the exit hole, or requiring noticeably more pressure than when new.

For bits up to around 6mm, a diamond bit-sharpening tool (available for a few pounds) is sufficient for touching up the 118° point angle. Larger bits can be reground on a bench grinder with practice, or replaced given the low cost of individual sizes.

Using HSS Bits on Wood and Plastics

HSS jobber bits work perfectly on timber, MDF, and most engineering plastics — they'll drill clean through timber joist noggins when running cable, open up back-box cut-outs in consumer unit enclosure bases, and bore through junction box sides. The flute geometry is designed for metal so chip clearance isn't as aggressive as a dedicated wood auger or spade bit, but for holes up to 10mm in timber the result is entirely adequate on a building site.

For larger holes in wood (20mm+) or for fast drilling through deep timber sections, a dedicated flat bit or auger will be quicker. But for the everyday mix of metal, wood, and plastic encountered on an install, a good HSS set does the job without swapping tooling constantly.

Stock Up on the Sizes You Burn Through

The twin-packs — like the 6mm (2pcs) option — exist specifically for the bits that get burned through fastest on regular metalwork. Keeping a spare 6mm or 3mm bit in the bag means a blunted bit on a Friday afternoon doesn't mean a trip to the merchant before you can finish the job.

All Faithfull HSS drill bits listed here are in stock at APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or order online at apmi.uk. Dispatched the same working day on orders placed before 3pm. Browse the full range of Faithfull tools including holesaws, screwdriver sets and more.

Frequently Asked Questions

Can I use HSS drill bits on masonry or brick?

No — for masonry, brick, block, and concrete you need carbide-tipped masonry drill bits or SDS bits. HSS bits will blunt immediately on hard masonry and may shatter. HSS is for metal, wood, and plastic only.

What's the difference between HSS and HSCO (cobalt) drill bits?

HSS (M2) is standard high-speed steel — good for mild steel, aluminium, wood, and plastic. HSCO or HSSE (M35) adds 5% cobalt to the alloy, raising the cutting temperature it can withstand. Use cobalt for stainless steel, hardened steel, and hard alloys.

Do I need cutting oil when drilling steel?

For thin steel (under 3mm) at the right speed, usually not. For anything thicker — heating pump brackets, unistrut sections, steel enclosures — a drop of cutting oil significantly extends bit life and reduces heat discolouration of the workpiece.

What size HSS bit do I need for an M6 machine screw clearance hole?

6.5mm gives a medium clearance for M6. If you need a loose clearance (for thermal expansion or easier assembly), 7mm is better. For a close fit, 6.2mm works. Faithfull Pro HSS bits are available in 6mm and 6.5mm individually, both covering M6 clearance directly.

Can the cobalt impact bits be used in a regular drill?

Yes — the 1/4in hex shank fits any drill with a standard keyless chuck or 1/4in bit holder. They don't need to be used in an impact driver; they simply also work there, unlike round-shank bits.