100mm or 125mm? Choosing an Angle Grinder

Angle grinders are chosen on two numbers — wheel diameter and wattage — and almost everybody chooses on the first one alone. That is how a fabrication shop ends up running 100mm machines through work that wants 125mm, burning through wheels and machines both, and how a homeowner ends up with a 180mm grinder they cannot hold steady enough to use safely.

100mm (4 inch)

The one most people should own. Light enough to hold in one hand for long stretches, small enough to get into a corner, and enough for cutting rod, angle, sheet and tile, and for cleaning up welds on light section.

Its limit is depth of cut. A 100mm wheel gives you roughly 25mm of cut before the body of the machine is in the way, and that is before the wheel wears down. If you regularly cut anything thicker, you will be approaching it from both sides and getting a poor edge.

125mm (5 inch)

The professional default for fabrication. About 30 to 35mm of cut, a wider grinding face so you cover a weld in fewer passes, and generally a bigger motor because the machine is expected to work harder.

The cost is weight and control. It is heavier, it has more torque to twist out of your hands if the wheel binds, and it is more tiring above waist height. For somebody doing a few jobs a month, a 100mm is the better tool. For somebody grinding welds all day, 125mm is the one that stops being tiring first.

180mm and 230mm

These are two-handed machines for heavy structural work, concrete and thick section. They are not a bigger version of a 100mm grinder — they are a different class of tool with a different set of risks, and kickback on one is genuinely dangerous.

If you are asking whether you need one, you do not.

Wattage matters more than people think

Two 100mm grinders at 670W and 900W are not the same machine with different stickers. Under load — the moment the wheel is actually in the steel — the lower-powered one slows, and a grinder that slows takes longer, burns wheels faster and gets hot enough to shorten its own life.

• Under 700W — occasional use, light cutting, jobs measured in minutes.
• 750–900W — the sensible working range for a 100mm machine in daily use.
• 1400W and above — 125mm and larger, continuous fabrication work.

Corded or cordless

Cordless 18V grinders have become genuinely good, and for site work — where the alternative is running a lead across a floor other people are working on — they are worth it. Two things to be honest about: a battery grinder is heavier than the corded machine it replaces, and it will not run continuously all day without several batteries.

If the machine lives on a bench in a workshop with a socket beside it, corded is cheaper, lighter and never flat. If it lives in a van, cordless.

If you go cordless, read the battery guide first — a grinder is exactly the tool where ProCORE batteries earn their extra cost, and where a 2.0Ah battery will disappoint you.

The things that actually hurt people

This is the tool that sends more people to hospital than anything else in the catalogue, and almost always for the same handful of reasons.

• The guard. Removing it is common and it is the single worst decision anybody makes with this tool. A wheel that bursts at 11,000rpm goes somewhere, and the guard decides where.
• The right wheel for the job. A cutting disc is thin and made for its edge. Grinding with the side of one is how they shatter.
• The wheel's speed rating. Every wheel is marked with a maximum rpm. It must be at least the machine's no-load speed. A 100mm wheel on a slower large machine is fine; the reverse is not.
• Eyes, always. Not sunglasses, not spectacles — proper impact-rated protection.
• Let it stop before you put it down. A coasting wheel touching a bench walks the machine off it.

Where to buy the wheels

Not from us — this shop sells machines, not consumables, and we would rather say so than sell you a wheel we cannot stand behind. Buy discs from somewhere that stocks a known brand and check the rpm marking on every one.

What we stock

Angle grinders are the largest single group in this shop. See the 100mm machines, the larger ones, or the whole price list. Prices include GST, and you get a proper tax invoice.