Over the years I’ve used bench grinders many times. But the most vivid memory of using a bench grinder is actually quite old. Maybe 15 years ago I was using a large, pedestal mounted grinder. I can’t even remember what I was grinding, but whatever it was, it slipped. My finger nail came into momentary – almost glancing – contact with the spinning grinding stone, but it was long enough to grind straight through the nail and into the quick.
It hurt – it hurt quite a lot. But what I wasn’t prepared for was the faintness that followed. I reeled around a bit in a daze, but luckily there was another bloke in the workshop who could physically hold me up and lead me to a seat. The injury was quite minor – the hole through the nail was perhaps only 5mm in diameter, and its depth only a few millimetres. But the devastating effect it had when so many nerves were raggedly ground into was a very interesting experience...
Like all machine tools, bench grinders can be very dangerous.... But, on a more positive note, they can also be extremely useful!
Most amateur workshop bench grinders are fitted with 4, 6 or 8 inch diameter wheels. Typically a pair of wheels is used, one coarse and one fine, with each mounted at opposite ends of a centrally-positioned electric motor. The motors have especially long shafts, and each wheel is held in place between two compressible washers by nuts located on this threaded shaft.
In most home workshop applications, the grinding wheels will rarely - if ever - need to be changed, but if the wheels do need to be replaced, take care to mount the new wheels exactly as the original wheels were mounted. That means getting the orientation of the washers correct, and re-using any paper or rubber washers that are included in the sandwich. Note that normally the left-hand wheel will be retained by a nut with a left-hand (ie backwards) thread.
Why the need for such care when replacing wheels? Primarily because the wheels spin so fast. A large diameter grinding wheel might have a surface speed of 23 metres/second – that’s nearly 83 km/h! The idea of a wheel coming off the machine (or shattering – see the breakout below) doesn’t bear thinking about.
When selecting a bench grinder, buy the largest you can afford. Large machines are more powerful, they have larger diameter wheels that present a flatter surface to the work being ground, and the wheels last much longer. However, with prices of bench grinders having come down a long way in recent years, I wouldn’t necessarily select the most expensive. Also be aware of new prices when looking second-hand – unless the machine is a very large one, you’ll normally do better buying new.
A bench grinder should be securely mounted. It can be bolted to a bench or it can be placed on a steel or wooden stand that is bolted to the floor. A grinder that is free to move when pressure is applied to the work is very dangerous – the grinder must be held securely in place.
The grinder’s work-rest should be adjusted so that it is as close to the wheel as possible. Under no circumstances should it be more than 2mm from the wheel – a small work piece that momentarily becomes jammed between the wheel and the rest can instantly become a dangerous missile. A new bench grinder is likely to have its work-rest correctly adjusted; be vigilant with second-hand machines, and as the wheels wear down.
The wheel-guards (arrowed) that cover most of the grinding wheels should always be left in place; they prevent pieces of wheel being sprayed all over the workshop should the worst happen and the wheel shatter.
However, the other type of guard – the clear fold-down guard fitted to many machines – is much less of a requirement. Huh? He doesn’t want the clear guards fitted?
Firstly, you MUST – absolutely MUST – wear eye protection when using a bench grinder. No ‘ifs’, no ‘buts’. With personal eye protection in place, the clear guards that can be swung down over the work often become an unneeded nuisance. They get dirty, reflect light and – with some work pieces – get in the way. Leave them off.
Ear protection, especially when grinding thin sheet, is also very important.
Make sure that the work-rests are well illuminated – a reflector type bulb aimed correctly from above works well. Go brighter with the lighting than you think will be needed – you can always switch it off when the grinder isn’t being used.
Problems When Using the Grinder
There are some common difficulties that can occur when using a bench grinder.
When grinding a narrow work-piece, make sure that you move the work-piece laterally back and forth, so that you use the full width of the wheel. Grinding in one place – usually the centre – will result in the wearing of a grove circumferentially around the wheel. In turn that makes accurate grinding of tools like chisels very difficult.
Sometimes a grinding wheel will become loaded with metal. That’s most often the case when grinding soft materials like aluminium. The metal will be held in the wheel, taking on the appearance of long shiny streaks. Typically this occurs when an overly fine wheel is being used – made even more likely because the natural tendency is to use a fine wheel with soft materials.
Wheels can also become glazed – usually for just the opposite reason that loading occurs. If the work-piece material is too hard, the wheel will develop a shiny, glassy appearance and the cutting action will be much reduced. A wheel with a softer bond between the particles will overcome this problem, but if no such wheel is available, applying less pressure also often improves things.
If the grinding wheel becomes grooved, loaded or glazed, you can use a wheel dresser to return it to as-new condition (as-new but smaller in diameter!). A wheel dresser is a handheld tool that has cutting discs positioned at one end. The tool-rest is adjusted backwards to engage in a lug on the dresser, then the tool is brought into contact with the wheel by raising the handle, so moving the toothed wheels of the dresser into contact with the grinding wheel.
If sparks occur, the pressure with which the dresser is being applied should be increased. The dresser can be carefully move back and forth across the width of the wheel until the wheel is true and a new grinding surface has been created. Always wear eye protection when using a dresser.
Bench grinders can be used for many tasks – sharpening tools like chisels and drill-bits, removing material much more accurately than can be achieved by (say) a hand-held angle grinder, grinding castings and sometimes on small items, cleaning-up welds.
The most difficult items to grind are those that are small and those that are thin.
Work-pieces must always be supported on the tool-rest, with the work positioned so that it is in contact with the tool-rest as close to the grinding wheel as possible.
If the work is supported far from the grinding wheel, chatter will result. In addition to it being difficult to control the grinding, the work may be flung from your hands. (However, note that when sharpening some tools, the work will in fact be supported on the outer edge of the tool-rest by your left hand.)
Angling the work-piece too high will tend to drag the material into the gap between the wheel and the rest. The material will quickly overheat and control will be difficult. In the worst case, the work will be snatched from you.
Grinding small items is difficult. Gloves should not be worn when using grinding machines, especially if your hands are to be close to the wheels. This is because it is possible for a glove to be caught-up by the wheel, resulting in a loss of fingers or a hand.
Holding small work-pieces in pliers can be very dangerous – better to use vice grips that can apply a continuously strong grip. If such a strong grip will mark the work-piece and this is not desired, consider removing material from the work-piece in other ways rather than using the grinder.
When sharpening tools such as centre punches and chisels, angle them as shown here. The developed edge will be free of burrs and the direction of airflow being dragged along with the motion of the wheel will aid cooling.
Also have a container of water nearby and frequently cool the work-piece by dipping it in the water. In no case when grinding is excessive heat desirable. If the work-piece is too hot to hold, it is not being cooled by the water frequently enough.
Bench grinders are now so cheap that every home workshop should have one. In fact, once you have a bench grinder, you’ll wonder why you spent so long without one! But remember to always wear safety glasses, and to watch where your fingers go, especially when grinding small objects...