Hardened steel parts are widely used in the automotive, gear, bearing, tool, and die industry.
By eliminating the need to finish grind parts, manufacturers reduce capital outlays by as much as 40% and increased production by approximately 30%. As a result, hard turning of machined parts is a production process that holds considerable promise for the future since it is an effective means of increasing productivity.
Many factors are in play to produce the best possible results, tool compensation and geometry, feed rate, depth of cut, and heat transfer are just a few. In this article we will address thermal conductivity and its relation to tool wear and proper finish. There are many informational resources demonstrating numerical simulation of finish hard turning that investigate formulas in depth, here, we will tackle more practical matters.
A good rule of thumb for heat transfer is 80-10-10 or 80% of the generated heat goes into the chip while 10% is absorbed by the turned object and 10% by the tool. Application Engineers and set up specialists expend valuable time and material in fine tuning hard turning systems. For real time in-process results, infrared thermometers determine the temperature of objects by non-contact measurements of the infrared radiation emitted by their surface. A detector converts the incoming infrared radiation into an electrical signal. This results in an accurate temperature value, which is used for further processing. The use of inline infrared thermometry sensors opens up various opportunities to measure and display temperature processes for tool, part, and chip temperatures. This information, combined with information from the CNC, allows real-time optimization of the hard turning process.
In an effort to expedite and simplify set up and advance quality control, Autonetics is incorporating a miniaturized thermal imaging sensor with our hard turning gauging packages, as well as offering stand alone systems for thermal monitoring. In order to gain acceptance as an equivalent of the grinding process, finish hard turning must be able to satisfy the high quality requirements of the workpiece concerning form and size accuracy, surface finish, and surface integrity.
Finish is also tied to temperature. Like grinding, hard turning in some materials can produce a “white layer” or amorphous zone on the surface of the workpiece. This surface defect is invisible to the eye and must be measured using instruments. The white zone usually forms in hard turning because the cutting tool is dull and, hence, delivers too much heat to the workpiece, or tool, rather than the chip. Thermal monitoring is vital to the successful use of hard turning. It’s important to note that developments are ongoing in this area; therefore, the best advice to shops considering hard turning or those already doing hard turning is to periodically ask respected vendors about new developments.