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Manufacturing Processes > Machining processes > Grinding

 

Grinding

 

Process description

The removal of small layer’s material by the action of an abrasive spinning wheel on a rotating or reciprocating workpiece.

Materials

All hard materials. Not suitable for soft or flexible materials.

Grinding

Process variations

  • Surface grinding: workpiece is mounted on a reciprocating or rotating bed and a rotating abrasive wheel (either horizontal or vertical axis of rotation) is fed across the surface.
  • Cylindrical grinding: rotating abrasive wheel is fed along the periphery of a slower rotating cylindrical workpiece. Also includes: thread, form and plunge grinding.
  • Internal grinding: small rotating abrasive wheel is fed into the bore of a cylindrical rotating workpiece.
  • Centreless grinding: workpiece is supported on a work rest blade and ground between two wheels, one of which is a regulating wheel operating at 5 per cent the speed of the other.
  • Tool grinder: precision bench grinding unit for tool dressing.
  • Off-hand grinding: a fixed grinding machine (either bench or pedestal) where the work is manually presented to the grinding wheel.
  • Portable grinding: a hand held unit used for fettling and cutting.
  • CNC machines: movement and control of abrasive wheel and workpiece are performed by a computer program via stepper motors.
  • Extensive ranging of abrasive wheel geometries, abrasive materials (aluminum oxide, emery, corundum, diamond, Cubic Boron Nitride (CBN)), grain size, hardness grading and bond types (resin, vitrified glass, rubber, metal) are available.

Economic considerations

  • Production rates range from 1 to 1000/h.
  • Lead times vary from short to moderate, depending on degree of automation and geometry.
  • Material utilization is poor. Recycling of waste material difficult.
  • Flexibility of grinding is high.
  • Turning can compete with grinding in some situations.
  • Suitable for all quantities.
  • Tooling costs are low to moderate.
  • Equipment costs are moderate to high, depending on degree of automation.
  • Direct labor costs ranging from high to low, depending on degree of automation and part complexity.
  • Finishing costs are very low. Cleaning required.

Typical applications

  • Grinding is used for the generation of basic geometric surfaces and finishing of a wide range of components
  • Parts requiring fine surface roughness and/or close tolerances
  • Bearing surfaces
  • Valve seats
  • Gears
  • Cms

Design aspects

  • Complexity is limited to nature of workpiece surface, i.e. cylindrical or flat, unless profiled wheels and/or special machines are used.
  • Grinding should be used to remove the minimum amount of material.
  • Surface features should be kept simple to avoid frequent dressing of the wheel.
  • Fillets and corner radii should be as liberal as possible.
  • Deep holes and recesses should be avoided.
  • Parts should be mounted securely to avoid deflections as high forces can be generated during the grinding process.
  • May not be suitable for delicate workpieces.
  • For best results use the largest wheel possible for the relevant workpiece.
  • Minimum section =0.5mm . Sizes ranging Ø0.5mm–Ø2m+ for cylindrical grinding. Maximum size for surface grinding is approximately 6m in length. Less than 11m for centreless grinding.

Quality issues

  • Interruptions on the workpiece surface, for example key seats and recesses, may cause vibration and chatter.
  • Unit pressures vary with area of contact. High pressures use hard grade, fine grit abrasive wheels.
  • Surface tensile residual stresses remain in the workpiece due to localized high-temperature gradients. This may be critical in heat sensitive applications or when fatigue strength is important. Low stress grinding can impart beneficial compressive stresses.
  • The final size of the workpiece is determined by the speed of response of the gauging system and the forces built up in machine as a result of cutting loads.
  • Gauging may be contact or non-contact, this will probably be dictated by the part.
  • The properties of the wheel may change in the course of the process. Grinding wheels require occasional dressing to ensure uniform cutting properties.
  • Use of grinding fluid is important for chip removal and cooling of the workpiece.
  • Grinding wheels need careful storage and to be visually inspected for cracks before use.
  • Grinding wheels require balancing before use, to minimize vibration because of the high rotational speeds.
  • Surface roughness is controlled by the wheel grading, wheel condition, feed rate at finish size and cleanliness of the cutting fluid.
  • Surface detail is excellent.
  • Surface roughness values ranging 0.025–6.3 µm Ra are obtainable.
  • Process capability charts showing the achievable dimensional tolerances for surface and cylindrical grinding are provided.

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