Trouble-free and predictable machining processes

Trouble-free and predictable machining processes

In this article we will discuss the relationship between sustainability and productivity in machining production.

Sustainable machining production is a relative concept that is geographically and geo-politically related. Sustainable machining production is divided into several interacting dimensions, with among other things, the following: social sustainability, ecological sustainability, economic sustainability, and moral and ethical sustainability.

With a more pragmatic mindset we can state that a sustainable machining process is defined as a secure, controllable, and predictable process with highest possible productivity and lowest production costs. To realise this, several variables influencing the process and the outcome of it need to be considered, like e.g., the workpiece material, the geometry and dimensions of the workpiece, the used machine tool, CAM systems, fixturing of the workpiece and cutting tools, the applied cutting tools and how they are to be used (cutting conditions and data).

Figure 1 - The NEXT STEP machinability model for sustainable machining. Full insight in a machining process helps the professional machining expert to fully understand and control the process. What happens during machining? Why do certain phenomena appear during machining? What are the variables influencing the process? How do they interact with each other and with the process? How can we work with all them to assure a sustainable process and outcome? Some of the insights that guide the machining expert to make correct selections on machining strategies and how to use the full set of equipment.?

Phenomena that can lead to problems when machining are:

-?????Chip formation and evacuation. If this is not fully under control, too long chips will cause production stoppages and potential damage to workpieces, machine tools and cutting tools, as well as they are hazardous for the operators. Too short chips will cause too fast deterioration of the cutting edges and this will endanger the process efficiency.

-?????Cutting forces acting on the tool. If these go beyond control, there is a risk of broken tools, broken cutting edges and vibrations during the operation. All these phenomena will cause production stoppages and bad quality on the finished workpieces.

-?????Tool deterioration and wear on the cutting edges. This must be of a controllable type i.e., flank wear and crater wear. Tool wear that occurs suddenly, like i.e., breaking or chipping of the cutting edges needs to be avoided.

-?????Heat generation and temperature rise in the cutting tool and the workpiece. A machining process metal generates a lot of heat. And if this head is not sufficiently evacuated by the chips and the process cooling medium, it will concentrate in the cutting tool or in the workpiece surface, jeopardizing the quality of the finished workpiece. The high temperatures in the cutting tool will cause changes in the cutting properties of the cutting material with loss of tool life as a logical consequence. All these factors can make it difficult to finish workpieces with the correct quality in terms of dimensions, shape, surface roughness and surface structure etc.

Figure 2 - There are millions of tooling solutions commercially available on the market. Finding what solution is best in line with expectations on process quality and workpieces quality is a crucial job that needs full insight in the machining process and the variables controlling it, and a professional executed selection procedure.?

To avoid all these potential problems, the machining process and the interaction between the different elements and variables influencing it, need to be carefully considered. A key factor in the full equation is the technological machining method or strategy. We can select traditional machining, high speed machining, high feed machining, high volume machining, or a hybrid strategy (combination of geometrical and technological strategy) like e.g., dynamic machining. Every approach offers advantages as well as constraints. This means that not every method, in each situation offers the same operating security. A well selected cutting edge, in terms of cutting material and geometry, in relation to the selected machining method is of the highest importance in making the cutting process reliable. The cutting material needs to be adapted to the workpiece material with the cutting speed as the linking or interacting variable. The cutting edge geometry has to be well selected to serve the purpose of the operation e.g. roughing or finishing. The interacting or linking variables here are the feed(s) and the depth(s) of cut. Next is the correct selection of the cutting data combination to consider – high cutting speed combined with low feed or high feed with moderate cutting speed or …? Each cutting condition needs to be considered and selected correctly.?The cutting speed needs to be high enough to avoid e.g., built-up edge wear. But, the cutting speed should not be so high that the wear process is mainly governed by thermo-chemical wear patterns. The depth of cut and feed have to be selected so that chip formation and mechanical impacts on the cutting edge (cutting forces) are under control. If process quality (reliability and predictability) is important during the machining process (while maintaining the highest possible productivity), the general preference should be for high depths of cut and feeds coupled with moderate cutting speeds (economical cutting). Cutting speeds can be further increased if circumstances allow and if productivity weighs more than production costs in the total picture. But higher cutting speeds will also involve higher “risk for the unforeseen” during the machining process.

TOOLING EXAMPLES

Figure 3 - Discussed on a company wide level, there are 4 overall production philosophies possible and, when correctly applied in different types of production, they will all lead to a sustainable production: economic machining production (mass production type), productive machining production (mass production type), high quality machining production (production type where one-to-one quality is priority) and quick-response production type (small batch production of diverse workpieces). Off course, all is important but only one can be prioritised in a certain situation. Selection and use of cutting tools need to be in line with the selected production philosophy. Seen as such tool selection is a management decision.?

In Seco we have a long history of looking for more reliable solutions for machining processes, that keep sustainability and productivity high and production costs low.?One example of increasing reliability, controllability and predictability during the machining process, especially in risky machining scenarios where the outcome is crucial (e.g., applications in the aerospace industry), is Seco's Jetstream Tooling (a high pressure directed cooling (HPDC) concept and system). This concept has demonstrated major advantages, such as increased tool life, better chip control, higher overall productivity, lower production costs, lower residual stresses in the workpiece surface, better machining tolerances and improved surface finish to name just a few. And there is also the high feed milling concept based on the classic compensation principles used in machining data selection. In this approach, the basic concept of high feeds to achieve problem-free machining is truly outstanding.

Figure 4 – Sustainable cutting conditions are defined as those that, on a first level, are within what is technical or technological possible in the given combination of machining elements. With other words, cutting conditions that are within the technical boundaries of a trouble-free machining process. On a second level – within the technologic boundaries – it is the cutting conditions that fulfil best the demands made from the economical sustainability thinking, that need to be selected.??

SUMMARY

A sustainable and economical machining process demands maximum security and predictability, while maintaining the highest productivity and lowest production costs. Different approaches to reach these objectives are possible. For general engineering applications, the “increase your feeds” concept offers the best possible results in most situations. Seco has a long history of providing effective tooling innovations (e.g., Seco HFM tools, Seco Jetstream Tooling). When correctly applied, predictability in machining processes can be lifted to a considerably higher level.

SECO’S TECHNICAL EDUCATION PROGRAMME

Seco Tools offers STEP (the Seco Technical Education Programme for applied machining sciences and technology) to make machining expertise and know-how available to all in the machining industry. For more details and to find out how to participate in the courses and classes, contact the local Seco Tools representative.

To comment on the article or for questions, please contact Patrick via [email protected]