Stainless steel CNC machining service materials
Stainless steels are often referred to as problem materials in the field of machining. Above all, the great toughness of stainless steels and their tendency to stick is causing difficulties for many users. Stainless steels, for example, are increasingly prone to build-up edge with increasing nickel content. Also important for the assessment of machinability is the chromium content. Stainless steels are divided into three main groups: austentia, ferritic / martensitic and duplex steels. The steels of the first two groups are significantly better to cut than the latter (see Figure 1).
The largest share is held by the austenites; here can again be divided into 4 groups: In optimized for machining so-called "prodec" steels, as they are z. B. Sandvik steel under the trade name "Sanmac" and in normally machined materials, such as 1.4401 or 1.4436. In addition, there are two particularly demanding material classes in machining, the precipitation-hardened stainless steels and the so-called super austenites. On the basis of these criteria, the cut data recommendations of the company Sandvik are then constructed, which additionally distinguishes between rolled and cast stainless steel, since the cast structure is generally significantly worse to machine than the structure of forged or rolled steel.
Turning the easiest
Of the three most important machining processes - turning, drilling and milling - turning is the most unproblematic. For turning stainless steels is usually worked with emulsion as a cooling lubricant. It come to roughing indexable inserts with stable but positively executed chipbreaker geometry, such. As the -MM or -MR geometry used. These insert geometries are used in combination with relatively tough coated carbide grades. Especially for demanding turning operations, eg with severe cutting interruptions, the Sandvik grade GC2035 has proven itself here (see picture 2).
Finishing can be carried out with coated fine and ultra-fine tungsten carbide (GC 1025, GC 2015) or cermet (CT 525) with sharp-edged cutting edges. In order to obtain a controlled chip breaking, inserts with -ML (MóISO-M, L óLight cuts) geometry should be preferred.
Drilling stainless steel materials
Drilling stainless steel materials is much more demanding than turning; This is due to the fact that when drilling due to the process in the center of the tool, the cutting speed is always zero. Accordingly, in the case of drills with indexable inserts, such as the Coromant U drill, a tough carbide grade (GC 1020) should be used in the center. In contrast, a wear-resistant grade (GC3040) can be used as the outer cutting edge. This combination of two different cutting materials makes it possible to optimally adjust the cutting edge stress during machining. For solid carbide drills as well, Sandvik has achieved a unique combination of two carbide grades in a single drill with the TwinGrade GC1030.
When milling stainless steels, especially the strong tendency to built-up edge formation is problematic. If it comes increasingly to the formation of built-up edges, the tool life decreases dramatically. The chips stick to the cutting edge and are pulled into the cut when the cutting edge re-enters the workpiece, and the cutting edge is likely to be damaged. In order to minimize build-up edge build-up as far as possible, positive indexable inserts with a rake angle of at least 20 ° are used. In order to still get stable cutting, particularly tough carbide grades are used. In principle, two machining strategies are possible when milling: it can be run at low cutting speeds and emulsion, or it is worked at high speeds and dry. In the former method, built-up cutting is avoided by using the cooling lubricant and the finished surfaces are slightly better than dry machining, but productivity is low due to the low cutting speed required to achieve acceptable tool life (see Figure 4).
When dry milling, the cutting speed is chosen to be much greater, so high that the chips no longer stick to the cutting edge. With the right combination of tool geometry and cutting parameters, most of the heat "goes away" with the chip and the workpiece remains relatively cool. This strategy allows a very high productivity compared to wet processing and is therefore usually much more economical. Sandvik carbide grades GC2030 and GC2040 are particularly suitable for this type of machining.
If the achieved surface qualities are not sufficient for the more economical dry machining, the two methods can also be combined: First, roughing is done with high cutting data without cooling lubricant, but the last cut is made with emulsion and approx. 40% reduced speed. Alternatively, the last cut can also be carried out dry with cermet as cutting material; This is particularly useful when various tools are used for roughing and finishing.