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This article was first published in 2008.
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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!
Types
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.
Cracks?
If
you have any doubts as to the integrity of a grinding wheel, remove it from the
machine. Hang it from a piece of string and tap it with a wooden mallet or
wooden handle. The wheel should make a clear, ringing sound. If it makes a dull
sound, it’s cracked and should be discarded.
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Set Up
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.
Sharp
Bits...
Don’t
position a bench grinder where the abrasive particles it sheds can do damage.
Machine tools with accurately ground faces – eg lathes and mills – should never
be located in line with the ‘throw’ of the wheel. The idea of getting those
abrasive particles into an open engine should also send a shudder up your
spine...
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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.
Supercharger
Bracket
A
lot of work on this supercharger bracket was done on a bench grinder. The curved
parts of the bracket were cut from 8, 9 and 12mm steel plate by means of an oxy
torch, then the edges smoothed and shaped to the correct dimensions with the
grinder.
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Grinding
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.
Conclusion
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...
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Interested in home workshop projects and techniques? You’re sure then to be interested in the Home Workshop Sourcebook, available now.
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