Angle Grinders: The Definitive Guide for UK Extension and Renovation Work

You order a steel lintel for the new opening. It arrives 50mm too long. Your builder quotes a half-day call-out to come back and cut it. A 4.5" angle grinder, a thin metal cutting disc, and ten minutes of your time would have done the same job for pennies in electricity. That's the case for owning one. The case against is that they are among the most dangerous handheld power tools on site, according to RoSPA. Both things are true, and neither cancels the other. Learn the tool properly and it becomes one of the most useful things in your kit.

What it is and when you need one

An angle grinder is a handheld power tool with a spinning abrasive or cutting disc mounted at 90 degrees to the motor shaft. The motor drives the disc at between 6,000 and 12,000 RPM (revolutions per minute) depending on the disc size and model.

Construction work throws up constant need for it. Steel lintels rarely arrive pre-cut to the exact length your opening needs. Bricks at returns and reveals need trimming. Mortar needs grinding back. Metal brackets need angle-cutting. Rebar protruding from poured concrete needs shortening. Wall chases for pipes in solid masonry go faster with a diamond disc than with a chisel alone. If your project involves any steelwork or dense masonry, an angle grinder will earn its keep in the first week.

The two sizes you'll encounter are 115mm (4.5 inch) and 230mm (9 inch). The 115mm grinder is lighter, faster-spinning, and easier to control. It handles the vast majority of extension tasks. The 230mm is heavier, slower-spinning, and designed for cutting thicker material like RSJs (rolled steel joists, the I-section steel beams used to carry loads over openings). At 4 to 5.5 kg, a 9" grinder takes confidence to use safely. Start with a 4.5" and hire a 9" for the day if and when you need to cut heavy steel.

115mm vs 230mm: which size for which task

The disc diameter determines the maximum cutting depth and the material type where each grinder shines.

The 115mm disc spins faster than the 230mm, which matters for cutting thin metal cleanly. The 230mm disc runs slower but at far greater torque (rotational force), which is what you need to push through a 100mm RSJ flange. They're different tools sharing the same basic mechanism.

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Disc types: the decision that actually matters

The disc is where the money is. A budget grinder with a quality disc outperforms a mid-range grinder with a cheap one, every time. Cheap discs heat up, wear fast, wobble on the arbor (the central mounting spindle), and in the worst case shatter at speed. Always match the disc to the material and verify the disc's maximum RPM rating exceeds your grinder's no-load speed before fitting.

Here's what each disc type does and when you'd use it on an extension project:

The practical starter set for an extension: one pack of thin metal cutting discs (£5 – £12), a segmented diamond blade for masonry (£10 – £50), and a pack of flap discs for finishing metalwork (£6 – £15).

The RPM rule: Every disc has a maximum operating speed printed on its face. That number must exceed your grinder's no-load RPM. A disc rated at 6,650 RPM fitted on a grinder running at 8,500 RPM will eventually fail. The disc doesn't care that you've only used it for "one quick cut." The failure, when it comes, puts a fragment of abrasive wheel into whatever is in its path at 180 mph.

How to use it properly

An angle grinder is not difficult to use correctly. It becomes dangerous when people skip the steps they consider optional.

Before you start

Check the disc. Hold it up to the light and inspect for cracks, chips, or damage. Spin it by hand: it should run true, without wobble on the arbor. A disc that wobbles when mounted must be discarded. Spin up the grinder away from any work for 30 seconds before making contact. Any defects will reveal themselves in that run-up.

Fit the guard. The guard is not decorative. It deflects sparks and disc fragments away from your face. Adjust it to suit your angle of attack (most guards rotate to six or more positions). Leaving the guard in the wrong position doesn't protect you; adjust it before you cut.

Clamp your workpiece. Cutting with one hand while holding the work with the other is how people lose fingers. Steel pipe goes into a vice. Bricks go on a stable flat surface. RSJs get clamped to a work stand or wedged against a fixed surface. If you can't clamp it, solve that before you touch the grinder.

Fit the side handle. Always. An angle grinder's torque, particularly at startup when the motor is accelerating to full speed, can wrench the tool unexpectedly. The side handle gives you control of that startup torque and keeps you in control if kickback occurs.

The cut

Position yourself so the open face of the guard faces toward you, not away. This way, if the disc shatters, fragments are deflected away from your body. Your cutting arm should be extended, not bent close to your body.

Let the disc do the cutting. Apply steady, light pressure and let the abrasive do the work. Pressing hard into the cut overheats the disc, accelerates wear, and increases the risk of the disc binding. For cutting steel, keep the disc moving along the cut line rather than dwelling in one spot.

Work at waist height where possible. Overhead grinding puts your face in the line of sparks and reduces control. If overhead work is unavoidable, change your body position to keep the sparks going away from you rather than down onto your face.

Kickback

Kickback is the violent rotation of the grinder backward toward the operator that occurs when the disc binds in the cut. It happens when:

• the workpiece pinches the disc (the cut closes under the weight of the material)
• the disc catches on an edge or projection
• you're cutting in the wrong direction (the disc should rotate down into the cut from the top face, not up from underneath)

The kickback force is not small. It throws a running power tool toward you. The guard stops the disc from contacting your body directly, but the throw itself can cause you to cut yourself on surrounding obstacles or lose control of the tool entirely.

Prevent it by always supporting the workpiece so the cut cannot close. For steel, the section being cut off must be free to drop away, not pinch the disc from below.

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What to buy

Three tiers, real models, real prices from UK retailers.

Budget: £25 – £40

Titan TTB878GRD and Erbauer ERB977GRD (both at Screwfix, £25 – £40) are the workable budget options. Both are 750W, both do the job for occasional use. The Erbauer is the better buy of the two: it uses a slightly higher quality motor winding and the plastic body feels less hollow. The Titan is genuinely borderline on sustained use: it runs hot after a few minutes of continuous cutting.

Neither is recommended for a full extension project if you expect to be cutting regularly. Buy budget here only if you'll use the grinder for three or four specific cuts and then shelve it.

Mid-range: £50 – £90

This is where most homeowners doing an extension should buy.

Makita GA4530R (Screwfix, £50 – £90). The community consensus pick. It is the most frequently cited recommendation across r/DIYUK, DIYnot, and trade forums by a clear margin. 720W, 11,000 RPM no-load, 1.8kg. Lightweight enough for extended use, robust enough for sustained cutting. If you buy one grinder for a whole extension project, this is the one.

Bosch GWS 9-115 S (£75 – £95). 900W, 11,000 RPM. Heavier than the Makita at 1.9kg but more power for sustained masonry cutting. The soft-start (the motor ramps up to speed gradually rather than jumping to full RPM immediately) reduces the startup jerk that catches beginners out. Worthwhile if you're planning wall chasing or heavy blockwork cutting.

DeWalt DWE4206K-GB (Toolstation, £50 – £90). The upper end of mid-range. 710W, paddle switch (releases automatically when you release your grip, the safest switch type), carry case included. DeWalt's build quality is excellent and the paddle switch is a meaningful safety upgrade over a standard toggle.

Professional / 9-inch: £100 – £125

The Makita GA9020S (Screwfix, £100 – £125) is the standard corded 9" for trade use in the UK. 2,200W, 6,600 RPM, 5.2kg. The right tool if you need to cut several RSJ sections on site. It is not a beginner tool. At 5kg-plus running at full torque, losing control of a 9" grinder causes serious injuries. If you need one for a single day of steel cutting, hire it.

Cordless 9" grinders (Milwaukee M18 FHSAG230XB, DeWalt DCG460NK) are available but cost £200 – £300 for the bare body, before batteries. They exist for trade operators who can't run cables on site. For an extension homeowner, the corded 9" delivers more power for less money and the cable is not a meaningful constraint.

The disc budget matters as much as the grinder budget. Spend at least as much on quality discs as you spend on the grinder. The community position on this is unanimous: Bosch, DeWalt, Norton, and Metabo discs are worth the premium. Cheap discs overheat, wear to nothing in minutes, and wobble as the abrasive grains release. The grinder body fails slowly over years. A cheap disc fails fast, and sometimes dramatically.

Corded vs cordless for extension work

Cordless 4.5" grinders (Makita DGA452Z, DeWalt DCG405N, Milwaukee M18 FSAG115XB) have genuine advantages: no cable to snag, useful on upper floors or outside where running a lead is awkward, and the same battery as other tools if you're already on one platform.

The cost is real: a cordless kit with two batteries runs £150 – £220 versus £50 – £90 for the Makita GA4530R. If you already own 18V batteries from a drill or impact driver on the same brand platform, the body-only (bare tool) option makes economic sense. Body-only 4.5" cordless grinders run £70 – £140 depending on brand and spec. If you don't already have batteries, buy corded.

One practical note: cordless grinders on a single battery will stop mid-cut when the battery runs flat, often at an inconvenient moment. If you go cordless for sustained cutting work (chasing a wall, cutting multiple lintels), have a charged second battery ready.

Hire vs buy

For a single day of heavy steel cutting (an RSJ installation, for example), hire a 9" grinder from a local hire centre rather than buying. Hire costs £12 – £20 per day, and you return it when the job is done.

For the 4.5" grinder, buying makes more sense than hiring even for a single project. Hire for a 115mm grinder runs £9 – £15 per day, and you'll reach that cost within a week on a typical extension. Buy the mid-range Makita at £50 – £90, use it through the project, and sell it or keep it at the end.

Construction site hire from national chains (Speedy, HSS, Jewson Plant) often requires 110v tools for site safety reasons. Domestic 240v grinders are fine when working on your own home, but confirm with the hire centre before turning up with a 240v lead.

Alternatives

For tile cutting, an electric tile cutter (wet saw) is safer, more precise, and produces far less dust than using a diamond disc on an angle grinder. If tiling is the main job, buy or hire a wet tile saw instead. The grinder is a fallback for awkward cuts or when the tile cutter isn't on site.

For cutting copper or plastic pipe, a pipe cutter or hacksaw is the right tool. An angle grinder will work, but it's overkill and harder to control on small-diameter material.

For heavy demolition (breaking up concrete slabs, removing internal walls), an SDS-Max breaker is more appropriate than an angle grinder. The grinder cuts; the breaker smashes. Different mechanisms for different outcomes.

Safety: what actually matters

Angle grinders cause 5,400 recorded injuries per year in the UK. Most are preventable and most involve one of five failures: no face protection, no guard, wrong disc for the material, workpiece not clamped, or the operator setting the running grinder down before it has stopped.

PPE is not optional here. The minimum for any grinding or cutting work:

• Face shield, not just safety glasses. Grinding sparks travel 18 to 21 feet. A face shield protects your whole face; glasses protect your eyes. Flying wire fragments from a wire brush wheel need a full face shield. For anything that generates flying particles, wear the shield.
• Ear protection. Angle grinders run at 102–105 dB(A). The safe exposure limit for eight hours is 85dB. Ten minutes at 102 dB(A) approaches the daily dose limit. Sustained exposure damages hearing permanently.
• FFP3 dust mask for masonry cutting. Cutting bricks and blocks with a masonry or diamond disc generates silica dust. The HSE estimates silica-related lung disease kills around 800 construction workers per year in the UK. FFP2 is the minimum; FFP3 gives better protection, and the price difference is trivial. For wall chasing, add a dust extraction vacuum at the disc if working indoors.
• Leather or reinforced gloves. Not knitted garden gloves. Cut-resistant gloves that protect your hands from sparks and edge contact.

The five things that cause most accidents:

1. Removing the guard ("just for this one cut"). The guard is adjustable; there is no legitimate cut that requires removing it.
2. Using a cutting disc sideways (applying side-load to a disc rated for cutting only). Thin cutting discs can only take force in the direction of travel. Side pressure shatters them.
3. Setting the grinder down while the disc is still running. The disc takes several seconds to spin down after you release the trigger. Put it down face-up on a flat surface well away from your feet, or hold it until it stops.
4. Pinching the disc by not supporting the cut-off section. Support the work so the cut opens, not closes.
5. Fitting a disc rated for a lower RPM than the grinder's no-load speed. Check every time you change discs.

Where you'll need this

Angle grinders appear across multiple phases of any extension or renovation project:

• Steels and lintels - cutting RSJs and steel angle sections to length, grinding weld splatter off bearing plates
• Walls and blockwork - trimming bricks and blocks at reveals and corners where standard sizes don't fit
• First fix plumbing - cutting through existing masonry for pipe routes in solid-wall construction
• Tiling - cutting tiles to fit with a diamond disc where a wet tile saw isn't available or the cut is awkward
• Roof structure - grinding rebar ends, cutting metalwork on structural frames and connections