Machining Processes

Machining is the process of removing unwanted material from the workpiece.

Cutting Processes

• Single Point : shaping, planing, turning, boring etc.
• Multipoint : milling, drilling etc.

Abrasive Processes : Grinding, Honing etc.

Chip Formation

1. Continuous Chip

• Occurs at high speed machining of ductile materials
• Desirable as it gives excellent surface finish
• Chip become too long, chip breaking mechanism required.

2. Continuous Chip with BUE

• Built Up edge -  BUE
• The material gets welded onto the tip due to the high compression and diffusion at the nascent surface.
• If it could remain there steadily, it would enhance tool life; but unfortunately it breaks regularly after reaching a critical size. When it breaks, it may at times uproot a part of the tool surface
• It is least desirable as it ruins surface finish and often decreases tool life.

3. Discontinuous Chip

• Occurs in Brittle material
• Desirable as it gives good surface finish
• No need of chip breaking

Continuous Chip	Continuous Chip with BUE	Discontinuous Chip
Ductile Workpiece	Ductile Workpiece	Brittle Workpiece
High Speed	Low Speed	Lower Speed
Larger Rake angle		Smaller Rake angle
Low feed	High feed	High feed
High Tool Life	Low Tool Life	High Tool Life
Good Finish	Poor Finish	Good Finish

Shaping and Planing

• In shaping, the cutting tool is given reciprocal motion and the workpiece is fed at right angle to the cutting motion between successive strikes of tool.
• Whereas in planing, the work is provided with motion and feed is given to the tool.
• The tool reciprocates over the work with forward stroke, cutting velocity V and a quick return stroke at velocity Vr is known quick return mechanism. 

\[m = \frac{forward \quad velocity}{return \quad velocity} = \frac{V}{Vr} = \frac{return \quad time}{cutting \quad time}\] 

• Number of stroke, $N = \frac{w}{f}$
• Time of one stroke, $t = \frac{L(1+m)}{V_{avg}}$ 
• Total time T = N*t
• Material Removal rate, MRR = Vavg *f*d  
• Cutting Power, $P_c = u_c * MRR$

Turning

• Average Cutting Speed, $V_{avg} = \pi D_{avg} N$
• Material Removal Rate, $MRR = V{avg} * d*f$
• Cutting Power, $P_c = u_c * MRR$
• Cutting time, $t = \frac{L}{fN}$

where, N : rpm, L is length of cut, f is feed, d is depth of cut.

Grinding

Grinding is most common form of abrasive machining, used to get desired surface finish, accurate size and shape of product.

Cutting Action of Abrasive grains

Grinding wheel consist of abrasive particles known as grit, bonding material and voids. These grits are characterized by high hot hardness, chemical stability and wear resistance, acts like cutting tool tip.

• High negative rake angle reduce the force per grit.
• The grinding ratio or G ratio is defined as thee volume of work removed divided by the volume of wheel wear.

Horizontal Grinding

Vertical Grinding

(a) Centered grinding                                                 (b) Center-less grinding

Creep feed grinding

In creep-feed grinding, the entire depth of cut is completed in one ot two pass only using very small in-feed rates.

Advantages: 

• Increased accuracy and productivity
• Improved surface finish
• Burr reduction
• Reduced stress and fatigue

Honing

Honing is a finishing process, in which a tool called hone carries out a combined rotary and reciprocating motion while the workpiece does not perform any working motion. 

Honing tool used to improve the surface finish of bored or ground holes.

Honning Tool

Theory of Metal Cutting