What Is Machinability Index | Ways Of Improving Machinability

What Is Machinability Index | Ways Of Improving Machinability 

Definition Of 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.

Machinability is a term indicating how the work material responds to the cutting process. In the most general case good machinability means that material is cut with good surface finish, long tool life, low force and power requirements, and low cost.

What Is Machinability Index ?
A closer definition of machinability requires that some quantitative judgement be made. Several possibilities are available, but in practice so called machinability index is often quoted. 

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.

Example: Machinability rating

The reference material for steels, AISI 1112 steel has an index of 1. 

For a tool life of 60 min, the AISI 1045 steel should be machined at 0.36 m/s. 

Hence, the machinability index for this steel is,

 KM = 0.36/0.5 = 0.72. 

This index is smaller than 1, therefore, AISI 1045 steel has a worse workability than AISI 1112
Machinability Rating
Machinability Rating 
Machinability of different materials

Leaded steels: lead acts as a solid lubricant in cutting to improve considerably machinability.
Resulphurized steels: sulphur forms inclusions that act as stress raisers in the chip formation zone thus increasing machinability.
Difficult-to-cut steels: a group of steels of low machinability, such as stainless steels, high manganese steels, precipitation-hardening steels.
Other metals

Aluminum: easy-to-cut material except for some cast aluminum alloys with silicon content that may be abrasive.
Cast iron: gray cast iron is generally easy-to-cut material, but some modifications and alloys are abrasive or very hard and may cause various problems in cutting.
Cooper-based alloys: easy to machine metals. Bronzes are more difficult to machine than brass.
Possible Ways Of Improving Machinability Of Work Materials
The machinability of the work materials can be more or less improved, without sacrificing productivity, by the following ways :

• Favourable change in composition, microstructure and mechanical properties by mixing suitable type and amount of additive(s) in the work material and appropriate heat treatment

• Proper selection and use of cutting tool material and geometry depending upon the work material and the significant machinability criteria under consideration
• Proper selection and appropriate method of application of cutting fluid depending upon the tool – work materials, desired levels of productivity i.e., VC and so and also on the primary objectives of the machining work undertaken
• Proper selection and application of special techniques like dynamic machining, hot machining, cryogenic machining etc, if feasible, economically viable and eco-friendly.

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