Friday, October 26, 2012

Thinking More Positive About Negative Cutters

By Todd Miller, Manager, Rotating Products

At Seco, we are constantly seeking out new ways for our customers to save money and still productively produce quality products. Such commitment has enabled us to continuously add new dimensions to the world of metalcutting where “one size does not fit all.” Consider our brand new take on negative cutter design.
Double Octomill
Negative cutters have been around for years and are quite popular, given they help reduce manufacturing costs by allowing the use of double-sided inserts. However, they prove most effective in steel and cast iron applications. Today, we’ve managed to re-engineer the traditional negative cutter to handle a wider range of applications by using modern, high-positive insert geometries – creating a hybrid negative/positive cutter style. 

This hybrid cutter style retains its strength for roughing operations, generates downward cutting pressure, which pushes the part into the fixture, and provides the economy of multiple cutting edges while reducing cutting forces, maintaining good chip evacuation and providing excellent surface finishes.

There are three common configurations for cutter bodies — positive/positive, negative/negative and positive/negative – all having advantages and disadvantages in different applications, which you can read about here.

By taking a negative cutter body and introducing a high-positive insert geometry, we’ve created a highly effective positive cutting rake angle. The effective cutting rake angle is calculated by adding the radial rake of the cutter body and the cutting rake angle of the insert. With a positive cutting rake angle in a negative cutter, users benefit in multiple ways – freer cutting action as well as reduced power consumption and heat generation, all of which leads to longer tool life in addition to the increase in usable cutting edges.

Square 6
The hybrid combination also takes into account several other design factors in addition to cutter body geometry, including the cutting insert’s geometry, effective cutting rake and edge preparation. 

Cutter body geometry, or the positioning of the cutting edge of the insert, involves lead, axial rake and radial rake angles. The lead angle is the approach the cutting edge takes as it enters the workpiece. It also controls the direction of the axial and radial cutting forces, as well as affects chip thickness, tool pressure and tool life. The axial rake is the insert’s angle along the central axis of the cutter assembly, while the radial rake is the insert’s angle in relation to the periphery of the cutter. In a hybrid negative/positive geometric configuration, the cutter rake angles both remain negative. It’s the introduction of the positive insert geometry that makes the difference.

Insert edge preparation also plays a key role in the new hybrid negative/positive approach. In typical milling cutters for steel and cast iron operations, negative T-land edge protection sufficiently applies, while in tougher applications involving titanium, Inconel or superalloy materials, the inserts are honed and do not include the negative T-land, enabling them to be sharper and cut freer. These tougher workpiece materials are where some of the most significant cost reductions can be found using the new hybrid technology. While in most situations, double negative cutters are not effective for these tougher materials. But now, these materials can be machined successfully using the hybrid style with honed-only inserts.

Additionally, these designs now incorporate integrated wipers of various sizes on each insert. This should be considered when machining different materials. For instance, longer wipers tend to produce more tool pressure and increased heat. Therefore, when working with tougher materials, such as superalloys, a shorter wiper is recommended for roughing to help reduce work hardening and excessive heat. In steel and cast iron roughing applications, a longer wiper can save valuable machining time thanks to its freer cutting action that produces a nice surface finish in just one pass.    

Currently, several Seco customers are finding success using negative cutter bodies with the new hybrid negative/positive configuration. Such cutter bodies include the Double Octomill, a highly versatile and economic face milling cutter with 16 numbered cutting edges, and Square 6 line of shoulder milling cutters. And stay tuned as we look to develop even more ways for our customers to productively and profitably machine quality products.

Watch the Double Octomill in action.
Watch the Square 6 in action.

About the Author
Todd is the manager of rotating products for NAFTA, responsible for solutions and applications involving face, square shoulder and disc milling. Todd and his team of product experts are dedicated to providing a consistent, high-level of support to Seco customers throughout the United States, Canada and Mexico. In his spare time, Todd likes to bowl and cheer on the University of Michigan football team. Contact Todd at

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