3 key decisions for successful turning insert selection

Let’s face it, selecting the optimum turning insert for each application can be a daunting task. With hundreds of thousands of different inserts available from dozens of suppliers, how can you make the process more manageable?

Experienced operators know that the workpiece print dictates the shape of the insert for a given operation. Size of the insert will be determined by the amount of material that needs to be removed, the machine and the set-up stability. Once you’ve whittled down your options based on those criteria, there is no magic bullet or ChatBot to get you to the perfect insert choice. There are, however, three baseline decisions that can help spell success for any insert purchase.

Decision #1: Positive or Negative Rake?

Negative rake inserts are most often double sided, which provides good economics. They’re also easy to index, strong and reliable. For this reason, we generally default to double-sided negative rake inserts as a first choice. Positive rake inserts provide much lower cutting forces—a big advantage on smaller workpieces, unstable set-ups, and more difficult to machine alloys.

Here are some general rules to follow when making this decision:

• Choose negative rake inserts on external machining over 1-inch workpiece diameter. For internal machining, the ID needs to be 1½ inches or larger , in most cases, for a negative rake insert to be the best choice. This is due to the inclination angle required to prevent the side flank of the insert from heeling / rubbing on the ID. Hold a negative rake boring bar in your hand and look at it from the end view. You might be surprised to see how “negative” the insert has to sit in the pocket to achieve the clearance needed inside a bore.
• For sticky materials, internal machining, 35° Diamond Inserts, unstable workpieces, thin walls or long slender shafts: Positive rake inserts should be the first choice. In these cases, better performance outweighs the cost efficiency of a double-sided insert.
• Always remember, the goal is to achieve a stable and repeatable machining process. You don’t want to be chasing part quality.

Decision #2: Geometry or Grade, which is more important?

There is a long standing rule here: The right geometry insert in the wrong grade will always outperform the right grade in the wrong geometry. Memorize that rule and be sure to tell everyone on your team that is involved in insert selection.

The physics of metalcutting are all about energy and geometry. The energy comes from the spindle rotation. That energy converts to heat, which is why choosing the correct cutting speed (SFM) is important. Then it’s up to the geometry of the cutting tool to take that energy (heat) from the spindle rotation to reach the yield strength of the workpiece material and to separate the chip. When you’re successful in this separation process, then the grade of carbide determines tool life or the ability for the insert to resist breakage. Spend the necessary time to find the correct insert geometry (often oversimplified as “insert chipbreaker”), then look at the grade choice.

Decision #3: CVD-Coated or PVD-Coated Grade?

There is no perfect answer here, because the variables are far and wide. There are, however, guidelines that can be quite helpful.

CVD-coated inserts are best exploited at high cutting speeds (SFM), and continuous heat (for example, external turning in steel or cast Iron machining). CVD Coatings are usually highly specialized for one or two application areas. If a shop is frequently cutting the same range of workpiece materials for those types of applications, it might make sense to conduct trials and optimize with a handful of high-performance CVD-coated grades.

PVD-coated inserts are very much at home in stickier materials, running at more moderate cutting speeds; and applications where having a “sharper” edge improves the performance of the operation. Small diameter boring is an area where a PVD grade will likely be the better performer. Most milling applications on smaller machining centers will benefit from PVD-coated inserts. PVD coatings almost always have a wider range of materials for which they can perform well. For shops that don’t know what material they’ll be cutting from one day to the next, PVD-coated inserts are generally the best choice.

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