What Are Positive and Negative Rake Angle Cutting Tools?

 

Positive and negative rake angle tools are fundamental types of cutting tools whose geometry plays a crucial role in machining performance. Among the various geometric parameters, the rake angle-the angle between the tool's rake face and a reference plane perpendicular to the cutting surface-is especially important. Based on the direction of this angle, tools are categorized as having either a positive or negative rake. This article provides a systematic overview of their structural characteristics, working principles, and suitable applications.

 

 

I Positive Rake Angle Tools

 

These tools feature a rake face inclined inward toward the workpiece, forming a rake angle between +5° and +15°. Structurally, they have a sharper cutting edge and a smaller contact area between the rake face and the chip. This design facilitates smooth chip flow and reduces cutting resistance.

 

 

II Negative Rake Angle Tools

 

In contrast, negative rake tools have a rake face that tilts outward from the cutting point, creating a rake angle of -5° to -10°. They are characterized by a blunter cutting edge and a thicker, more robust tip, offering greater strength and durability.

 

 

III Advantages and Disadvantages

 

Positive rake tools reduce cutting force by 15–25%, improve chip evacuation, minimize built-up edge formation, and deliver better surface finishes (Ra < 1.6μm). However, their lower tip strength makes them prone to chipping during interrupted cuts or when machining hard materials. They also have poorer heat dissipation, leading to crater wear during high-speed operations and typically shorter tool life-30–40% less than their negative counterparts.

 

Negative rake tools offer more than 50% higher tip strength, improved heat dissipation (cutting temperatures reduced by 15–30°C), and often feature double-sided designs for reversible use. On the downside, they generate 20–30% higher cutting forces, demand greater machine power, and are more prone to vibration, especially in long overhang setups.

 

 

 

IV Application Comparison and Experimental Design

 

To provide practical guidance for tool selection, a comparative study was conducted using carbide tools with +8° and -6° rake angles. Both were TiAlN-coated YG8 inserts with a 0.4 mm tip radius. The workpiece was quenched and tempered 45# steel (Φ50×200 mm, HRC 28–32), machined on a CA6140 lathe. Measurements included surface roughness (Mitutoyo SJ-210), tool wear (OLYMPUS DSX510 microscope), cutting force (Kistler 9257B), and temperature (Fluke Ti400 IR thermometer).

 

Fixed Parameters: Cutting depth (ap = 1 mm), feed rate (f = 0.15 mm/rev)

Variable: Cutting speed (v = 60–180 m/min), repeated three times per condition for reliability.

 

 

V Results and Analysis

 

Tool wear was evaluated using a flank wear threshold of VB = 0.3 mm. Positive rake tools wore slower at high speeds but were prone to built-up edge at low speeds. Negative rake tools performed more consistently at medium-to-low speeds but wore faster at high speeds. Microscopy revealed that TiAlN coatings remained oxidation-resistant even at elevated temperatures.

 

Negative rake tools exhibited up to 50–70% longer tool life. Failure modes differed: positive rake tools failed due to crater wear and edge chipping, while negative rake tools showed more uniform flank wear, indicating better resistance to breakage.

 

In terms of cutting force, negative rake tools required roughly 17% more force on average. A 1° increase in negative rake raised cutting force by about 1%, meaning a shift from +5° to -5° increased force by nearly 10%. Despite higher cutting forces, negative rake tools benefited from improved edge strength and thermal control at high speeds.

 

Temperature readings showed that negative rake tools maintained cutting temperatures 15–25°C lower than positive rake tools, especially evident at speeds over 120 m/min.

 

Surface roughness results favored negative rake tools (Ra: 1.2–1.5μm vs. 1.6–2.0μm). Their machined surfaces were smoother with fewer burrs and vibration marks, thanks to better system rigidity and more stable edge geometry.

 

 

VI Conclusion

 

Carbide negative rake tools demonstrate superior tool life, thermal stability, and surface quality-making them well-suited for high-speed modern machining. Positive rake tools, with their lower cutting forces, are preferable in low-rigidity setups. The best rake angle choice depends on workpiece material, machine condition, and the specific machining stage.