Dressing/conditioning diamond and CBN Borazon grinding wheels, resin- and metal-bond only, is one of the most overlooked processes. And most dressing is done insufficiently.
The simplest and basic dressing process is to use a white aluminum oxide dressing stick. Maximum exposure is necessary to optimize the wheel performance. Because of the many grades of carbides and the combination of wheel specifications, the grinding process does not always allow the wheel to stay open and exposed. The same with CBN Borazon wheels. There are so many steel alloys harder than 54 HRC and so many wheel bonds (resin and metal bonds only), particle sizes and hardnesses that it is sometimes necessary to even dress a CBN wheel.
Hand dressing with a DIT Type A dressing stick (aluminum oxide) is the simplest and most basic process.
1st: Use the proper grit size and abrasive dressing stick
For wheels with a mesh size of 270 and coarser, use a Type A 220-grit stick. For wheels with a mesh size of 320 and finer, use a Type AF 320-grit stick.
Generally, an aluminum-oxide stick is used for dressing polyimide, resin, and some hot-pressed vitrified-bond wheels. Silicon-carbide sticks are used on metal, some vitrified-metal, and hot-pressed vitrified bonded wheels. Black silicon carbide is usually used.
Use an SC 80-grit stick for wheels with a mesh size under 270, and a SCF 320-grit is for mesh sizes 320 and finer. A black silicon-carbide stick is harder than aluminum oxide and still some operators prefer aluminum oxide even on tougher bond systems. Since there are so many different grinding applications, one has to try what is the most effective stick. When dressing cold-pressed vitrified-bond wheels, extreme care is to be used to dress very lightly and at a maximum 1” length of stick. Cold-pressed vitrified bonds have a built-in porosity.
2nd: Soak the dressing stick in water
Use only a stick which is thoroughly soaked Why? A soaked, wet stick does not grind off as easily as a dry one. Also because it is wet, you have greater control of the removal of the bond. In addition, you will not get abrasive dust all over yourself and the machine.
3rd: Dress at non-grinding speeds
Before pushing the stick into the wheel, turn off the power. As soon as the power is turned off, shove the stick slowly straight into the wheel until it comes to a stop. Why? As the wheel slows down, it acts softer and therefore it will
erode easier and is more controllable. Or, if possible, reduce the rpm so the cutting speed is 1,000 sfm.
4th: Do not move the stick sideways
In tooth type steel saw blades there are gullets, some larger and some smaller. A grinding wheel is similar. Larger mesh sizes have more clearance - small mesh sizes have smaller chip clearances. But by moving the stick sideways, the bond on the side of the particle will be eroded. This will weaken the particle bond so only plunge straight for optimal results.
5th: New wheels always need dressing
They usually need some truing also. More so with CBN Borazon, as they must grind more concentrically to consistently make the correct chips. Even if a wheel is dressed open at the factory when they are mounted and trued
they then will need dressing.
6th: Other dressing procedures
A. A speed control which changes speed of the grinding wheel to a lower rpm will make the wheel act softer and this will open up the chip clearance.
B. An air jet abrasive stream will optimize automatically the exposure of the particle.
C. An automatic dressing device which pushes a dressing stick into the wheel.
D. A motorized device that trues and dresses at the same time.
E. A diamond cup wheel or roll, which trues and dresses at the same time the parts are being ground (continuous dress).
F. A computer-aided grinding system monitor, which senses the energy level of the grinding process and changes the infeed rate and rpm automatically.
Dressing of coated (plated and/or brazed) diamond wheels is a marginally effective way of cleaning out scale and swarf. Use a 220-grit stick for 500 mesh and coarser or a wire brush.
Dressing of coated (plated) CBN Borazon wheels. This is not effective in dressing out a steel loaded wheel. Usually a loaded coated wheel cannot be opened up! Even abrasive blasting could destroy the bond. Try a wire brush under power.
Note that one of the biggest misconceptions is that dressing reduces the life of the wheel. A loaded wheel grinds with greater grinding pressure and therefore gets a lower grinding ratio.
Related Glossary Terms
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- aluminum oxide
Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.
- chip clearance
In milling, the groove or space provided in the cutter body that allows chips to be formed by the inserts.
Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.
- cubic boron nitride ( CBN)
cubic boron nitride ( CBN)
Crystal manufactured from boron nitride under high pressure and temperature. Used to cut hard-to-machine ferrous and nickel-base materials up to 70 HRC. Second hardest material after diamond. See superabrasive tools.
- cutting speed
Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).
Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grinding ratio
Ratio of work material removed to grinding-wheel material lost.
- grinding wheel
Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.
- grit size
Specified size of the abrasive particles in grinding wheels and other abrasive tools. Determines metal-removal capability and quality of finish.
- sawing machine ( saw)
sawing machine ( saw)
Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).
Metal fines and grinding wheel particles generated during grinding.
Using a diamond or other dressing tool to ensure that a grinding wheel is round and concentric and will not vibrate at required speeds. Weights also are used to balance the wheel. Also performed to impart a contour to the wheel’s face. See dressing.