Difference Between Machining and Grinding

Achieving high dimensional accuracy, close tolerance and surface finish are usually not possible by conventional machining processes like turning, milling, drilling, etc. Grinding can be advantageously utilized for such purposes, especially when desired material removal rate (MRR) is low but accuracy, tolerance and finish are more important.

Conventional machining operations utilize a sharp-edged wedge shaped cutting tool; while grinding uses a circular wheel made of sharp, tiny and random shaped abrasive grits strongly held on a bonding medium. These abrasives remove material from work surface in the form of micro-chips to provide a smooth surface. In general, machining operations are used for stock removal, while grinding is used for finishing. Sometime, conventional machining operations are called metal cutting, while grinding is called abrasive cutting. Important similarities and differences between machining and grinding are discussed below.

Similarities between machining and grinding

  • Both machining and grinding remove material in the form of chips.
  • Both utilize cutter.
  • In both the cases material is removed by shearing (ploughing and rubbing occur in grinding but they do not remove material).
  • Only mechanical energy is utilized for material removal in both the cases (unlike NTM processes where various forms of energy like mechanical, electrical, thermal, chemical, etc. are used to remove material).

Differences between machining and grinding

Machining Grinding
Machining is one bulk material removal process (i.e., high MRR). Thus it is economic and suitable to give proper size as well as to semi-finish it. Grinding has low material removal rate and is preferred for finishing.
Accuracy and tolerance achieved by conventional machining operations are not so good. Achieving tolerance below 50µm is very difficult. It can provide better accuracy and tolerance. In grinding, achievable tolerance can be as low as 2µm.
In machining, cutting tool is usually made of metals or alloys, which is substantially harder than work material. However, ceramic, diamond and cBN tools are also available, usually in the form of inserts or coating. Cutting tool, known as grinding wheel, is made of abrasive materials (such as alumina, silica, etc.) bonded in harder medium (like resin, metal, etc.).
Cutting tool used in machining has specific geometry. Values of the geometrical features may vary from one tool to another, but each tool has pre-defined geometry as per Tool Signature. Grinding wheel may have some specific pre-defined features, but the abrasive grits (which actually participate in material removal) have random geometry and orientation.
Rake angle of a cutting tool may vary from positive to negative. However, the negative rake usually does not go above (–15º) as it may severely degrade machinability. Abrasive grits have haphazard rake angle. A much wider variation of rake angle from (+75º) to (–75º) is noticed among abrasive grits.
While machining operation, each main cutting edge of the tool actively and equally participates in material removal action. Only few (about 1%) abrasive grits actually engage in material removal action (shearing). Some grits just engage in rubbing, scratching, and ploughing. A major percentage of grits do not even touch the work material.
Generated cutting temperature is comparatively less and only a tiny portion (5-8%) goes into the workpiece. Thus thermal damage of the machined surface is usually insignificant. Severe heat is generated and also a substantial amount goes within the workpiece, which causes thermal damage of the machined (ground) surface such as changing hardness.
The maximum cutting speed (rpm) used in conventional machining is limited to around 1500rpm (limited by the capability of machine tool, especially gears and bearings). Cutting velocity (m/min) is also lower. Very high speed of grinding wheel (2000–4000 rpm) can be used. Cutting velocity also becomes very high. Ultra high speed grinding (speed around 20,000 rpm) is also carried out for some specific applications.
Specific energy consumption (power per unit volume of material removed in kW/mm3) is much lower. Specific energy consumption is 5–50 times higher than that of the machining.
There are some surface hardened and inherently hard materials that cannot be machined by conventional methods. Such materials can be finished by grinding.

 

References

  • Book: Machining and Machine Tools by A. B. Chattopadhyay (Wiley). Buy this book
  • Book: Metal Cutting: Theory And Practice by A. Bhattacharya (New Central Book Agency). Buy this book
  • Book: Manufacturing Process for Engineering Materials by S. Kalpakjain and S. Schmid (Pearson Education India). Buy this book
  • Book: DeGarmo’s Materials and Processes in Manufacturing by J. T. Black and R. A. Kohser (Wiley). Buy this book