Essentially, bench grinders are just tools for spinning grindstones, and these grindstones are for gently shaping metal. They’re commonly used for getting blunt tools back to full sharpness, but they’re not a one-stop-shop for instantly putting an edge on a tool.
When using a cutting blade, the edge gradually dulls. When an edge is slightly dull, it can be honed back to full sharpness using either a very fine grit or a sharpening steel. This works by making fine adjustments to the very tip of the blade. However, at a certain point, this just won’t work. If the blade has become too dull, these fine adjustments won’t be enough to put a sharp edge on. Also, if there’s a chip in the edge, then you’ll need to remove this chip before putting the edge back on.
In both these cases, you need to grind a larger amount of metal off the tool in order to get a sharp edge. It’s not essential to use a bench grinder to do this, but bench grinders are an easy and accurate method for doing this. Obviously, the power tool aspect of them means that the electrical motors are doing the hard work, but this also means that you can keep much greater control over the grinding. Using a proper tool rest and an angle-setting gauge, you can put a precisely angled bevel into a piece of metal much more easily than doing it by hand.
The actual process of sharpening a tool depends a little on the type of tool that you’re sharpening, so look up the proper process for working with your tool before starting.
One key thing in sharpening all tools on a bench grinder is not to overheat the metal. Most cutting tools are made of steel that’s hardened by heat treatment. If you heat it up too much, this can undo the heat treatment and leave you with softer metal. In order to keep the temperature down, only apply light pressure to the metal being ground, and dip occasionally in water to keep it cool.
Grindstone wheels come in different levels of coarseness. In general, bench grinders are for removing quite a bit of material quickly, so coarser stones are often the most useful. As you use your grindstones, the front edge of them becomes worn and clogged with metal, so it’s important to periodically dress your grindstone. This means using a dressing tool to wear away the grit, to expose new areas. Some people dress the front of their grindstones flat, while others prefer a slight convex curve.
In addition to grindstones, you can get wire brush wheels for removing paint and rust. These are particularly useful for preparing small areas for welding, or other adhesions. Polishing wheels can help you achieve a fine finish on your metals.
As well as the actual spinning wheel, your bench grinder will probably come with some accessories. A tool rest is the most important of these. If you’re going to be sharpening tools regularly, you may find that the rest supplied with your grinder is insufficient and aftermarket tool rests can provide much more control.
Grinders sometimes have additional attachments to make them more useful – particularly in workshops with limited space. Some come with sanders, while others, such as the one we’ve been testing out, come with a flexible shaft that can be used as a Dremel-like rotary tool.
We’ve been testing out the Scheppach HG 34, which is a small bench grinder for hobbyist use. It lacks the oomph of larger grinding wheels more suitable for heavy-duty work, but it is perfectly capable of light workshop tasks. The grinding wheels are quite a bit smaller than many workshop grinders (75 mm), and there’s not much room around them for manoeuvring large tools, but this means it takes up less bench space for confined workshops. The rotary tool is a complementary addition, as the sanding and grinding bits can get into places that the main grinding wheel can’t. It’s slower than some tools, so not well-suited for cutting hard materials, but fine for other sanding and polishing.
The spinning action of a grinding wheel means that bits are going to fly off, and you have to protect yourself – especially your eyes – accordingly. Make sure that you’ve got no loose clothing that could get caught in the wheel, and you may need ear protection if things get loud.
Not all materials are suitable for grinding – most notably, aluminium. The basic problem is that it’s quite soft, so gets stuck in the grit on the wheel clogging it up. The wheel then stops grinding properly and can get hot. If the wheel overheats, it can shatter. Avoid aluminium, and if you notice the grit getting clogged, dress the wheel to remove this, and never let the wheel get too hot.
Grinding wheels are rated for a particular maximum speed, so when getting a new wheel, check this against your grinder’s speed and never overspeed your wheel.
As always, follow the manufacturer’s advice. The UK’s Health and Safety Executive also has a free-to-download booklet on the safe use of abrasive wheels, available from hsmag.cc/jeObic.
Tools provided by Screwfix – thanks!