Thomas Industrial Library

Positive & Negative Rake Angles

How are positive and negative rake angles defined?

• A positive rake angle tool has the chip moving down the top face of the tool bit.

• A negative rake angle tool has the chip moving up the top face of the tool bit.

See Figure 7–31.

What are the advantages and drawbacks to positive and negative rake angles?

Positive rake angle tool bits, Figure 7–32 (a), are the most efficient shape for removing metal because they produce lower cutting forces and generate less heat than negative rake angle tool bits making the same cut. Use a positive rake angle tool bit when cutting forces must be minimized such as when machining thin sections, slender shafts, or using a lathe with poor rigidity. Negative rake tool bits, Figure 7–32 (b), also have several advantages. Negative rake tool bits:

• Have 90º side and end clearance angles, square ends, and still have adequate clearance. This results in a symmetrical insert which can be flipped over as well as end for end, yielding eight cutting edges on a square insert and two on a diamond-shaped or square one. Conversely, positive rake inserts must have a clearance taper on their sides and ends, Figure 7–32 (a) and so are not symmetrical and can only be turned end for end.

• Provide more tool surface to absorb and distribute heat, so higher cutting speeds are possible.

• Distribute the shock and force of the work hitting the tool onto the top surface of the tool, not its cutting edge as positive rake angle tool bits do. This prolongs tool life.

• Are particularly suited to making interrupted cuts and also work well on cast iron because the tough, abrasive outer scale hits the top surface of the tool, not its cutting edge.

• One drawback to using negative rake angle, tool bits besides the need for a rigid lathe structure, is that the chips leaving a negative rake bit come off as a continuous ribbon and are more likely to create a hazard for the operator than the individual chips made by a positive rake tool.

• Only tool bits with carbide inserts are hard enough to work with a negative rake angle. HSS or alloy bits all must have a positive rake.

• The positive/negative rake design, Figure 7–32 (c), offers the lower cutting force advantage of a positive rake design, and the groove on the outside edge of the insert acts as a chip breaker. This chip breaker is needed because ductile materials produce a continuous, unbroken chip that will move off the tool rapidly creating a hazard to the machine operator. A negative rake tool holder is used with a positive/negative carbide insert.

In small lathes, a negative rake tool may not work well due to high cutting forces and lack of machine rigidity. However, in larger machines, they should be considered where increased cutting speed is important or when cutting cast iron, stainless steel, or titanium.

Lead Angle

How is cutting tool lead angle defined?

• Lead angle is the angle at which the side cutting edge of the cutting tool is positioned with respect to a plane perpendicular to the workpiece, Figure 7–33. Note that when the lead angle equals the side cutting angle, the tool axis is perpendicular to the work axis.

Why is it often advantageous to use tool bits with a positive lead angle ? A positive lead angle puts more of the cutting edge on the work. This has three effects:

• Although the volume of the material removed is about the same as with a neutral lead angle, the chip is thinner and more easily removed from the lathe. See Figure 7–34.

• Cutting force is distributed over a longer length of the tool edge which means that tool life is increased and feed rate may also be increased. See Figure 7–35.

• It increases radial force and reduces longitudinal force on the workpiece which may become a problem with work that is thin or not well supported, Figure 7–35.

How does a positive lead angle protect the cutting tool bit?

A positive lead angle tool subjects the tool bit or insert to less shock when it first contacts the work because the work impacts the tool in the center of its face where stronger rather than it is on its end. See Figure 7–36.