Renewable Tool Coating via Continuous in-situ Deposition

W. Gregory Sawyer ²
Tony L. Schmitz ²
John C. Ziegert ²

Mechanical and Aerospace Engineering

University of Florida
Gainesville, Fl., 32611

Introduction

Tool wear is a primary consideration in machining operations due to the associated decrease in machined surface quality, reduction in part geometric accuracy, and additional process time/cost for tool replacement. In many production operations, the tool life is the most significant factor affecting the manufacturing cost of the part. Fundamentally, tool wear is caused by the high localized stresses and temperatures at the tool-chip interface and friction at both the rake face (contact between chip and tool) and relief or flank face (tool/workpiece interference). 

Recently machine tools with high speed, high power spindles and increased feed rates and accelerations have been introduced, resulting in dramatic increases in cutting speeds and material removal rates. Combined with the push toward dry or minimum quantity lubrication (MQL) machining practices, these changes have placed great demands on tool material/coating technology.

To keep pace with these demands, many new tool materials and coatings have been developed. Cutting fluids are also often applied to reduce friction and high cutting temperatures (which accelerate diffusive wear), as well as for chip evacuation. However, as noted, due to potential environmental contamination, the trend is to move away from substantial use of cutting fluids.

Despite these recent advancements in tool material and coating technology, tool wear remains a significant problem and often serves to limit the available productivity, particularly in hard-to-machine materials such as hardened steel, titanium, and various nickel superalloys. 

Solid Lubrication with Boric Acid

We propose a novel way to improve tool life by reducing friction in the tool chip contact area through the use of a continuously renewed solid lubricant. Boric acid has a lamellar structure, and has been shown to be an effective solid lubricant. Boric acid is inert and biocompatible (it is used in eye wash, for example) and is compatible with aluminum and steel. 

Gearing et al. have used boric acid to dramatically lower the friction coefficient in forming operations where the contact pressures are similar to those in machining. 

In Situ Replenishment of Solid Lubricant Coating

Although boric acid has low wear resistance, we propose to continuously renew the lubricant layer whenever the tool is not in the cut. This concept is shown in the figure below. Here, a solution of boric acid in water is vaporized in a heated nozzle and the jet is directed at the tool. During the time the teeth are out of the cut, the boric acid deposits on the rake faces of the teeth. When the tooth re-enters the cut, the boric acid layer will serve to lower the friction and wear on the tool surface, and will also reduce the incidence of ‘built up edge’, or BUE, where layers of workpiece material weld to the tooth surface.

The ability of the boric acid lubricant to survive the extreme conditions of the cut zone will depend on the amount of lubricant deposited on the tooth relative to the tribological severity during chip formation.

An approximate model of the deposition vs. wear rate of the boric acid coating shows that in an interrupted cut typical of milling, increasing the ratio of deposition time to time in the cut will increase the lubrication capacity of the film.

Closure

For typical milling operations, continuous renewal of the solid lubricant coating is expected to result in significant reduction in the frictional forces between the tool and chip, and to dramatically lower the wear rate of the tool, contributing to improved tool life, better surface finish, and lower manufacturing costs. Additionally, these improvements are achieved without the use of potentially harmful cutting fluids.

Personnel

Dr. W. Gregory Sawyer (wgsawyer@ufl.edu) is an Assistant Professor in the Department of Mechanical and Aerospace Engineering. His primary interests are in tribology. Particularly solid-lubrication, and sliding contacts in extreme environments where the use of fluid lubrication is not available.

Dr. Tony Schmitz (tschmitz@ufl.edu) is an Assistant Professor in the Department of Mechanical and Aerospace Engineering. His primary interests are in the dynamics of the milling process and manufacturing metrology.

Dr. John Ziegert (johnz@ufl.edu) is a Professor in the Department of Mechanical and Aerospace Engineering. His research background is in precision manufacturing and machining operations, and precision dimensional metrology, and is the President of the American Society for Precision Engineering.