Rake angle

• is a parameter used in various cutting and machining processes, describing the angle of the cutting face relative to the work. There are two rake angles, namely the back rake angle and side rake angle, both of which help to guide chip flow. There are three types of rake angles: positive, negative, and zero.

Positive rake angles

• Make the tool more sharp and pointed. This reduces the strength of the tool, as the small included angle in the tip may cause it to chip away.

• Reduce cutting forces and power requirements.

Negative

• Make the tool more blunt, increasing the strength of the cutting edge.

• Increase the cutting forces.• Can increase friction, resulting in higher

temperatures.• Can improve surface finish.

Zero

• Is the easiest to manufacture, but has a larger crater wear when compared to positive rake angle as the chip slides over the rake face

• Negative angles result in a “scraping” action. Positive angles result in a cutting action

• Land:The plane surface immediately following the cutting edge• Clearance angle: The angle between the back of the bur tooth and tooth

being cut. Eliminates rubbing friction of clearance face Provides stop to prevent bur edge from digging into

the tooth• Carbide burs: – slightly negative rake angle90 degree edge angle Low

clearance angle

Modifications in bur design:

– Reduced use of crosscuts:- • Crosscuts produce unduly rough surface when used

with high speed

• Extended heads on fissure burs:- – Light pressure needed for high speeds permits this

modification– Roundening of the sharp tip angles-

– lower stresses- – enhance strength of tooth- – facilitate adaptation of restorative materials

FACTORS INFLUENCING THE CUTTINGEFFICIENCY OF BURS

1. Rake angle:• Positive the rake angle - more the cutting efficiency• Positive rake angle>radial rake angle>negative rake

angleBut negative rake angle is used because of following

reasons- – The size of bur tooth and tooth angle is more thus

increasing its bulk and increased resistance to fracture- – The cut chips moves directly away from the blade

• Positive rake angle-the chips are larger and tend to clog the chip space