Machinability | Concept , Definition , Machinability rating

Concept, definition and criteria of judgement of machinability

The term; ‘Machinability’ has been introduced for gradation of work materials with respect to machining characteristics. But truly speaking, there is no unique or clear meaning of the term machinability. People tried to describe “Machinability” in several ways such as:

• It is generally applied to the machining properties of work material.
• It refers to material (work) response to machining.
• It is the ability of the work material to be machined.
• It indicates how easily and fast a material can be machined.

But it has been agreed, in general, that it is difficult to clearly define and quantify Machinability. For instance, saying ‘material A is more machinable than material B’ may mean that compared to ‘B’:

1. ‘A’ causes lesser tool wear or longer tool life.
2. ‘A’ requires lesser cutting forces and power.
3. ‘A’ provides better surface finish.

Attempts were made to measure or quantify machinability and it was done mostly in terms of:

• Tool life which substantially influences productivity and economy in machining.
• Magnitude of cutting forces which affects power consumption and dimensional accuracy.
• Surface finish which plays role on performance and service life of the product.

Often cutting temperature and chip form are also considered for assessing machinability.

Machinability rating (MR) = 

The machinability index KM is defined by
KM = V60/V60R
where ,

• V60 is the cutting speed for the target material that ensures tool life of 60 min,
• V60R is the same for the reference material. Reference materials are selected for each group of work materials (ferrous and non-ferrous) among the most popular and widely used brands.

If KM Greater than 1, the machinability of the target material is better that this of the reference material, and vice versa. Note that this system can be misleading because the index is different for different machining processes.

Read More : What Is Machinability Index | Ways Of Improving Machinability

Machinability can be tentatively defined as “ability of being machined” and more reasonably as “ease of machining”.

Such ease of machining or machinability characteristics of any tool-work pair is to be judged by:

• Magnitude of the cutting forces.
• Tool wear or tool life.
• Surface finish.
• Magnitude of cutting temperature.
• Chip forms.

Machinability will be considered desirably high when cutting forces, temperature, surface roughness and tool wear are less, tool life is long and chips are ideally uniform and short enabling short chip-tool contact length and less friction.

Role of the properties of the work material on machinability

The work material properties that generally govern machinability in varying extent are:

1.  The basic nature – brittleness or ductility etc.
2. Microstructure.
3. Mechanical strength – fracture or yield.
4. Hardness and hot hardness, hot strength.
5. Work hardenability.
6. Thermal conductivity.
7. Chemical reactivity.
8. Stickiness / self lubricity.

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