Today, turning of stainless steel represents about 24% of all metal turning, and that proportion is rising. Various stainless steel grades present a challenge to all machining operations. That’s because they have characteristics that yield more friction, heat and chip control problems than most common steels and cast irons.
The good news is that modern toolholders and specific stainless steel inserts are available to make your stainless steel work easier. The new generation of inserts provides higher positive rakes, new coatings and better matches for heavy roughing, medium roughing and finishing operations. With the right tooling, you can effectively apply proven turning principles and techniques to facilitate stainless steel turning and manage the most difficult cuts.
Rules of Thumb:
Here are some rules of thumb based on our own experience in thousands of stainless steel turning applications:
1. Use state-of-the-art inserts. The new ones can improve material removal by as much as 30%.
2. Match the insert to the application. You’ll get better finishes and longer tool life.
3. Learn to visually diagnose and remedy symptoms of premature edge failure.
4. Ensure that your shims are in good shape.
5. Use some proven techniques for the more demanding cuts.
Let’s first look at what makes stainless steel a tougher material to turn than conventional steel. Then, we’ll explore each of the five Rules of Thumb separately.
Turning Stainless Steel
Turning stainless steels is not necessarily difficult, but it can be demanding. Their higher alloy content generally equates to more difficult and costly turning processes. Ingredients added to improve corrosion resistance and tensile strength, especially nickel and molybdenum, often work against machinability. This is because they deform in a plastic manner.
As a result, tools are subjected to more friction, higher cutting forces and higher temperatures, causing a tendency toward smearing and work-hardening of the surface. These characteristics are typical of all grades of austenitic stainless steel.
Obviously, the hardness of workpiece material affects the life of cutting tools. Stainless steel is deformation-hardened when it is cold-drawn. The deformation-hardened layer on incoming mill stock is considerably thicker in austenitic stainless steel than in carbon steel. Even bar stock that has been straightened is cold-drawn to some extent, with surface hardness values of HB300 or more. The inside of the material may be half as hard as the surface, but it’s on the surface where the cutting takes place.
Anticipate the Challenges
Anticipate work-hardened surfaces. Make deeper cuts and set feed rates that ensure entry of the cutting edge past the hardened zone. Another problem is heat. Stainless steel has much lower thermal conductivity, and higher ductility than conventional steel. Therefore, it requires considerable energy to cut a chip, and thus generates more heat. Since stainless is unable to dissipate heat quickly, the workpiece, especially the cutting area, gets very hot.
The chip forming process implies that a fresh metal interface is continually produced and forced, at very high pressure and temperature, along the tool material. Since heat from the cutting zone is not removed with the chips, the higher cutting temperatures increase the tendency for tool wear and plastic deformation, especially when interruptions are encountered.
Excessively hard cutting action in most stainless steel workpieces generates various types of very hard, continuous chips. To break these continuous chips into smaller, less abrasive and more manageable sizes, select an insert with high positive rake and aggressive chipbreaking designs.
Sandvik has recently added more insert choices to their stainless steel “M-line.” These newer inserts were developed specifically for stainless steel, with positive rake angles and strong geometries.
Let’s look at some recent insert advances and how they facilitate turning of stainless steel workpieces.
New developments in state-of-the-art inserts for stainless steel turning are: improved coatings, stronger substrates, higher positive rakes and improved chipbreaking geometries.
It is very important to choose the correct grade/geometry combination, and match it correctly to your application. Fortunately, the carbide substrates and PVD/CVD coatings are already matched to the geometries utilized in the most common applications. New, advanced coating techniques provide better chemical barriers to isolate the workpiece surface. These new coatings help reduce friction and built-up edge (BUE).
Developments in insert geometry and higher positive rakes take productivity one step higher (Fig. 1). The benefits are freer cutting action, as well as longer edge life. A positive rake means more continuous cutting, smoother chip flow, lower cutting forces, lower temperatures and less deformation-hardening of the material. A sharp edge means softer cutting action with lower cutting forces. This, in turn, means less deformation of the workpiece material, decreasing the likelihood of burr formation. A positive sharp cutting edge, combined with a correctly balanced open chipbreaker, is usually the best solution for turning stainless steel.
Selection Made Easier
Tool selection guides, like Sandvik’s CoroKey™, offer users information on which grade and geometry combination will work best in every application – finishing (F), medium machining (M) and roughing (R). Sandvik marks this information directly on the inserts for easy identification. The inserts are also shipped in a box with cutting data for that particular insert printed on the label. The cutting data is at best conservative, and should be taken only as the starting point. We strongly recommend the following: push the inserts to the limit. Think parts per edge, rather than insert cost. In that manner, you’ll maximize productivity all the time.
Whether you’re doing heavy roughing, medium roughing or finishing on stainless steel, select an insert with the correct nose radius. The nose radius is a key factor in insert strength for roughing, and surface texture in finishing.
A nose radius of 3⁄64ths or larger is usually the best choice for stainless steel turning. In rough turning, the maximum metal removal rate is obtained with a combination of high feed and moderate cutting speed. Machine power is sometimes the limiting factor, and in such cases, the cutting speed should be lowered accordingly. Generally, in finishing, setting the feed no higher than one-third of the nose radius will provide good surface texture and accuracy. Often, this smoother, easier turning means you can get by with a lower power machining center.
Tool Wear Doctor
Having covered what’s new in inserts, let’s take a look at tool wear. Some edge-wear patterns are normal, while others are symptomatic of improper application, incorrect machine settings or inappropriate turning techniques. You’ll be further along if you diagnose and correct these first signs of trouble, and apply some innovative cutting techniques to minimize them.
Remember, tool wear in and of itself is not a negative process. Tools will always wear. It’s not a question of if, but when, how much and what type of tool wear will occur. When a tool cutting edge has performed a considerable amount of metal cutting within a reasonable time, wear is very acceptable. It becomes negative when premature breakdown or tool fracture occurs, causing excessive stoppages for edge changes.
To control excessive stoppages, push the insert to the limit for 15 minutes, and at that point index the edge. This practical “15-minute rule of thumb” will ensure you get more parts per edge with fewer indexing stoppages. It makes no economic sense to try to push the edge beyond that time. Productivity should be your primary concern, not saving a few cents on the remaining edge life.
Figure 1 above: Cross-sectional view of Sandvik Coromant’s inserts for stainless steel turning in finishing (MF) and roughing (MR) shows higher positive rakes and improved geometries. The newer M inserts ensure continuous cutting and smoother chip flow, smaller variations in cutting forces, lower temperatures, and less deformation-hardening of the material.