Two jobs that are often confused
People often use dressing and truing as if they meant the same thing, but they solve different problems.
Truing corrects the geometry of the wheel. It makes the wheel run concentric with its spindle, flat or round where it should be, and it cuts the correct profile into the rim. A new wheel mounted on a machine is rarely perfectly true, and a wheel that has worn unevenly needs correcting.
Dressing restores the cutting ability of the wheel face. It removes some of the bond so that fresh diamond grains stand out from the surface, and it clears debris from the wheel. A wheel can be perfectly round and still cut poorly if its grains are dull or buried.
In practice the two are often done in the same operation, particularly with conventional abrasive wheels. With diamond wheels they are frequently done separately, because truing can leave the face closed and smooth, so a dressing step is added afterward.
Why diamond wheels need this
A diamond wheel cuts well when grains protrude a small distance above the bond. Over time, several things change that:
- Grain dulling. Flats form on the grains from wear.
- Glazing. The bond covers or surrounds dull grains, so the face turns smooth and rubs.
- Loading. Chips or swarf pack into the spaces between grains. This is common when grinding soft or sticky workpieces.
- Uneven wear. The wheel develops a taper, ridge or out-of-round condition, which shows up as chatter or poor flatness.
A wheel in this condition needs more force to cut, runs hotter and can damage the workpiece, which is a particular risk with cemented carbide and technical ceramics that tolerate thermal shock poorly. These changes follow the same abrasion and adhesion patterns seen in other cutting tools.
Common methods
Dressing sticks
The simplest dressing method is rubbing a stick of relatively soft abrasive, such as aluminum oxide or silicon carbide, against the wheel face while it runs, usually with coolant. The stick wears away the bond and releases debris, exposing grains. The right approach depends on the bond types involved. It is common with resin and metal bonded wheels and is generally gentle, though it consumes some wheel material.
Block and abrasive truing methods
For truing, a stationary block of silicon carbide may be fed against the rotating wheel, or the wheel may be run against a rotating abrasive. These methods are inexpensive but give less precise results than machine-based methods and wear the wheel quickly.
Single-point and rotary diamond tools
On precision grinders, a diamond dresser, often a single-point or a cluster of stones set into a holder, traverses across the wheel to cut a precise shape. Rotary diamond dressers, which are driven discs with diamond on the rim, can form complex profiles and are common in high-volume production grinding, mostly with conventional vitrified wheels.
Electrical and other methods
Metal bonded wheels, which are tough to dress mechanically, are sometimes conditioned using electrical discharge or electrolytic processes that erode the bond selectively. These are specialist methods used in higher-precision settings.
Signs a wheel needs attention
- Shiny, glazed look on the wheel face
- Rising grinding force or power draw
- Burn marks, discoloration or cracking on the workpiece
- Chatter marks or poor flatness
- Noticeable vibration or runout
- Wheel packed with debris
Not every sign has the same cause, so it helps to check coolant supply and feed rates before deciding that the wheel is at fault.
How much to take off
Dressing removes wheel material, so the aim is to do enough and no more. Too light a pass may not open the face. Too heavy a pass wastes wheel and can damage the diamond layer. Makers of wheels generally supply recommended dressing depths and methods, and those recommendations should take precedence over general rules. The thickness of the diamond layer is a finite resource, so every dressing pass is a small cost to wheel life.
Electroplated wheels are a notable exception. They have only a single layer of diamond over a plated base, and are generally not dressed in this way. Once the grains wear, the wheel is replaced or sent for recoating.
Effect on finish and form
Dressing conditions influence the surface of the ground part. A freshly dressed, sharp wheel tends to cut more aggressively and leave a rougher finish at first. A wheel dressed with a fine feed gives a smoother face and a finer result but may cut less freely. Operators adjust the dressing feed to tune between stock removal and finish.
Truing determines the form accuracy that can be held. If a profile wheel is out of true, the profile of the part will be out too, which is why high-precision grinding takes truing so seriously.
Safety and practicalities
- Keep wheel speeds within the rated maximum during dressing as well as grinding.
- Use a proper guard and dressing tool holder.
- Dust from dressing sticks and wheel material can be harmful if inhaled, so extraction or wet dressing is the norm.
- Record dressing intervals. Tracking how many parts a wheel grinds between dressings tells you when something in the process has shifted.
Good dressing and truing habits are one of the least glamorous but most effective ways to keep diamond wheel costs down.