Grinder buying guide

An electric angle grinder is probably what springs to mind when you think of grinders. But depending on what you plan to use the tool for, a different type of grinder or cutter may be better suited to your task. There are several types of grinder which vary in terms of features, the functions they perform and how they are powered. They also differ with regards to the position of their rotating disc. On an angle grinder, the cutting or grinding disc is mounted onto a spindle that is positioned at a right angle to the motor shaft (forming an L shape); on a straight die grinder, the spindle and motor shaft are in-line with each other (forming a straight line).

115mm and 125mm angle grinders

Popular among DIYers, corded angle grinders can be used to cut through metal pipes and sections, screws and thin-gauge materials. With the right range of accessories, they can also be used to grind, strip, sand and polish.

230 mm angle grinders

Heavy-duty and powerful, these handheld tools are great at handling brickwork and tiles and cutting chases into walls. They are mainly recommended for structural work.

Straight die grinders

Straight die grinders are designed for use in awkward spots that an angle grinder may struggle to reach. Used for metalworking and finishing work, the disc of a straight grinder is mounted in the opposite way to that of an angle grinder.

Cut-off saws

Cut-off saws, or power cutters (with 2-stroke engines), equipped with 350 or 400mm discs are best for large-scale professional tasks. Often used for roadworks, such as cutting through concrete or asphalt, power cutters also serve as rescue tools for firefighters.

Bench top cut-off saws

Bench top cut-off saws are stationary metal cutter machines. They feature an iron base plate that is placed on a table top while the cutting action is performed by a moving arm. These machines are similar to mitre saws but cannot be used to make mitre cuts.

Air die grinders

Air die grinders are powerful and lightweight but can be difficult to manoeuvre due to the presence of an air hose. Widely used by welders and metalworkers, these tools are powered by air supplied by an on-site air compressor.

Angle grinders are multi-purpose electric or pneumatic power tools that function by means of a gearbox. This gearbox operates a spindle which is positioned at a right-angle to the motor shaft. When a thicker disc is threaded onto this spindle, angle grinders can be used for grinding materials. Used with a thin disc or a diamond disc, angle grinders are able to cut through materials. As both tasks require the same amount of power, one machine can be used for both grinding and cutting.

Angle grinders are categorised by the diameter of their discs. There are three different options which correspond to three different types of angle grinder:

• Ø 230 mm for larger metal-cutting tasks;

• Ø 125 mm for large-scale and lighter work (albeit with less power and a lower maximum cutting depth);

• Ø 115 mm for finishing work.

Angle grinders come with the following components:

• a handle equipped with a switch and a paddle switch (dead-man);

• a motor (power ratings may vary);

• a gearbox equipped with a lock button for securing the disc and two or three holes for threading on the side handle;

• a side, or auxiliary, handle, that can be placed to the left or right of the gearbox;

• a threaded spindle, or drive shaft, for connecting discs (standard M14 fitting);

• a clamping nut with two holes that correspond to the holes in the pin wrench;

• an adjustable wheel guard made from sheet metal (must always be attached);

• a pin spanner for changing discs;

• a power cord, lithium-ion battery or air compressor hose (for air die grinders).

Precautions for use

When using an angle grinder, it is essential to follow the manufacturer’s recommendations and only use discs as they are intended to be used. Do not attempt to grind using a cutting disc. You run the risk of shattering it as only the edges of these discs are designed for cutting. A thick grinding disc, on the other hand, can be used across its entire surface.

Grinding

Used with a thick disc appropriate for the workpiece material, 230mm angle grinders can deburr, sand, grind and thin down sheet metals. You can also use these tools to resize metal sections or grind chamfers. They have a maximum cutting depth of 70 mm.

Cutting

Used with a diamond cutting disc or a thin cutting disc, 230mm angle grinders can be used to cut rebar, separate structural materials (brick, tile, breezeblock, etc.) or cut down metal for scrap.

When choosing between 115mm or 125mm angle grinders, it is, once again, important to consider what you intend to use the tool for. It is quite possible to buy two separate 125mm angle grinders for different purposes. You won’t break the bank and you’ll save time not having to swap your discs every time you change task. Most importantly, you will avoid the temptation of trying to grind with a cutting disc!

In addition to the functions offered by larger angle grinders, these smaller tools can also be used for tasks like cleaning and polishing. Self-gripping sanding discs, flap discs and felt polishing discs are just some of the accessories DIYers can add to their arsenal.

All in all, 125mm angle grinders are extremely versatile tools despite their limited cutting depth of just 38 mm. While 115mm angle grinders offer a maximum cutting depth of only 30 mm, they are precise, easy to handle and lighter than larger versions.

If you tend to believe the bigger the better, this really isn’t the case with grinders. A 230mm grinder is heavy (upwards of 5 kg on average), powerful (requires a heavy-duty handle to hold on to) and bulky (no chance of using it in a tight space). If you are a frequent DIYer, go for a 125mm grinder and you may find yourself saving time and money!

Straight die grinders are made up of the following components:

• a handle with a power switch;

• a motor (power ratings may vary);

• a spindle, or drive shaft, positioned parallel to the body;

• a collet for tool attachments (diameter 6 or 10 mm);

• a power cable or an air compressor hose.

Straight die grinders are designed for use with mounted points, wire brushes or even carbide grinding bits. These tools are often used to clean, deburr and widen drilled holes that are slightly too narrow. They can also be used to remove excess material on metal workpieces.

The design of straight die grinders makes them ideal for use in difficult-to-reach spots. They are also used in metalworking.

Electric grinders

The vast majority of grinders, used by DIYers and professionals alike, are corded electric tools running off mains power. Their power rating is measured in watts (W) and is usually specified in terms of power consumption (what the tool draws from the mains) and power output (the power actually sent to the tool minus losses). If you do not have mains power on your worksite, you can get battery-powered cordless angle grinders which are more portable. Nonetheless, you will have to compromise on power and duration of use. What’s more, these are only available as 115 or 125mm grinders. The li-ion battery should be chosen according to voltage (V) and current (Ah) characteristics. The higher the numbers, the more efficient the tool!

Pneumatic grinders

Pneumatic grinders are popular among solders and metalworkers for a number of reasons. Lighter than electric grinders, pneumatic grinders are safe to use when there is moisture present. Most importantly, these machines offer an instant start and stop function.

Electric grinders can take up to five times longer than a pneumatic grinder to stop. Considering welders are often paid per inch, it’s easy to understand the appeal of a pneumatic tool. Of course you will also require a compressor with the right air flow rate but building sites and workshops are usually set up to allow this.

Petrol-powered grinders

Fuel-powered grinders (or cutters) are very powerful cutting machines used only for large-scale tasks such as cutting concrete or asphalt. Power cutters can have a cutting diameter of up to 400 mm and a cutting depth of around 125 to 145 mm. They vary in terms of engine capacity and output power. Two- or four-stroke engines are typical for these machines and they usually power up with the use of a starter cord.

Depending on the disc size of your grinder, the power will range from 500 to 2500W for disc speeds of 5,000 to 11,000 rpm. To choose the right power rating, consider your needs and how often you intend to use the power tool.

• Regular professional use, 230mm = 2500W;

• Occasional large-scale use, 230mm = 2000W;

• Regular small-scale use, 125mm = 1200W;

• Occasional household use, 125mm = 750W.

As mentioned above, grinder discs rotate at high speeds of up to 11,000 rpm. These tools require constant vigilance, as well as physical strength to handle recoil in case the disc jams.

Several safety options are available to facilitate use and minimise risks.

• A kill switch can stop the disc in under 2 seconds;

• A circuit breaker will trigger in the event of power failure;

• A disengaging system is used in case of disc jamming.

When choosing your grinder, don't overlook the possibility of add-on features that could make your DIY project run so much more smoothly.

• Electronic speed controller for gradual start-up and variable disc speed for different materials;

• Lock/unlock function for easier and safer disc changes;

• Anti-vibration or non-slip handle to improve comfort on the job;

• A built-in damping system and extra handle to reduce vibration and protect joints;

• Weight: remember, the lighter your model, the easier it will be to handle;

• A vacuum attachment to keep your workplace clean (well, more so, anyway!);

• Quick-change disc feature;

• An LED light signal to indicate circuit breaker trigger or overheating;

• Additional three-position handles: come with almost all models;

• Protective swivel guard;

• Optimised coolant system;

• Carbon brushes for motor protection.

Prices do vary from one grinder to the next and this tends to be based on the following factors:

• quality of finish and motor quality;

• resistance to overheating for more comfortable use;

• the quality of the tool's components;

• handle comfort and the level of vibration absorption;

• on-off switch responsiveness;

• security features (as noted above);

• rate of power loss (the difference between power consumption and output);

• carbon brushes;

• brand and warranty terms.

Remember, it's not all about the rpm for 230mm grinders: the more powerful they are, the slower they generally operate. And if you’re concerned about losing your pin spanner, you should know that replacements can be bought separately. But before you get to that stage, you can always do as you would with a drill and attach the spanner to your power cable!

Whether you are grinding, cutting, deburring or sanding, no material is able to stand up to a grinder. But, of course, that’s what makes them so dangerous. You must always put safety first by wearing appropriate protective gear. Gloves, safety glasses, a respiratory mask, a helmet and, most importantly, safety shoes must be worn at all times when using these powerful tools. Finally, please remember to take care at all times.