How to Change a Bench Grinder Wheel Safely

Changing a bench grinder wheel takes about ten minutes, but it is one of the few workshop jobs where getting it wrong can genuinely hurt you. The wheel has to match the machine on bore, diameter and maximum speed, it needs checking for cracks before it goes on, and the spindle nut on the left hand side is almost always a reverse thread. Get those three right and the rest is ordinary spanner work.

When a wheel actually needs replacing

Not every tired looking wheel is finished, since a glazed or loaded face usually comes back with a dressing stick or star dresser. Replace it when it is cracked or chipped, when it has been dropped, when it runs so far out of true that dressing would eat most of what is left, or when it has worn down close to the diameter of the flanges. Check the manual for the minimum diameter rather than guessing.

Choosing the replacement wheel

Four things have to line up:

• Diameter and width, taken from the old wheel or the machine plate. A 150mm grinder takes a 150mm wheel, not the 200mm one that was on offer.
• Bore size. Common UK bores are 12.7mm, 16mm and 20mm. Many wheels ship with reducer bushings, so use the one supplied and nothing improvised.
• Maximum operating speed, printed on the blotter or label in RPM or m/s. It must equal or exceed the no load speed of your grinder. Never fit a wheel rated below the machine.
• Grit and bond. Grey aluminium oxide around 36 grit for stock removal, 60 to 80 grit for shaping, white aluminium oxide for hardened steel where you want to keep the temper.

Check the new wheel before it goes anywhere near the spindle

Vitrified wheels can crack in transit without showing anything obvious, and the ring test takes fifteen seconds. With the wheel dry, balance it loosely on a finger or a dowel through the bore, then tap it about 45 degrees either side of vertical with the wooden handle of a screwdriver. A sound wheel gives a clear ring. A dull thud means a crack inside, and that wheel goes in the bin. Rotate it a quarter turn and repeat to cover all four quadrants.

Changing the wheel step by step

You need a spanner to fit the spindle nut, usually 17mm to 19mm, a screwdriver or hex key for the guard fixings, a block of scrap wood, and safety glasses.

1. Unplug the grinder. Not switched off, unplugged, with the lead pulled where you can see it. Let the wheels stop on their own rather than braking them by hand.
2. Remove the eye shield, spark arrestor and outer guard cover. Keep the fixings in a pot and note how the tool rest bracket sits.
3. Hold the wheel and undo the spindle nut. Wedge softwood between the wheel and the frame to stop it spinning. This is where people get caught: the nut on the left hand side normally has a left hand thread and undoes clockwise, while the right hand side is conventional and undoes anticlockwise. Both are arranged so rotation tightens them in use. If a nut feels immovable, check the direction before reaching for a longer bar.
4. Slide off the outer flange, blotter and old wheel. The inner flange usually stays put. Wipe the spindle and both flange faces clean, because grit trapped on a flange face is a common cause of a wheel that runs out of true.
5. Fit the new wheel with paper blotters between it and each flange. If blotters came bonded to the wheel, leave them. Without them the flange bears on bare abrasive and creates stress points.
6. Refit the outer flange and nut. The wheel should slide on without being forced. If you have to tap it, the bore is wrong. Tighten the nut firmly by hand, just enough that the wheel cannot slip under load, because overtightening is what cracks new wheels.
7. Refit the guard, spark arrestor and tool rest before the machine goes anywhere near power.

Reset the gaps, every single time

A new wheel is a different diameter to the worn one, so the old rest position is now wrong. Set the tool rest no more than about 3mm from the wheel face and the spark arrestor tongue no more than about 6mm. Anything wider and a workpiece can drag into the gap and jam, which is how most bench grinder injuries happen. Adjust the rest only with the machine stopped and unplugged.

First run and dressing

Plug in, stand to one side of the wheel plane rather than in front of it, and run the grinder for a full minute with no load. If a wheel is going to fail from a hidden flaw, this is when it does it. Listen for vibration or watch for a wobble in the wheel face.

Then dress the new wheel lightly with a star or diamond dresser to true the face. New wheels rarely sit perfectly concentric, and one light truing pass makes a real difference to how smoothly the grinder runs. Recheck the tool rest gap afterwards, since you have just taken diameter off the wheel.