6 Questions to Ask BEFORE Running a CBN Hard Milling Test

By Chad Miller, Product Manager-Advanced Materials

Hard milling can be tricky. But if applied properly, you stand to benefit more from this machining practice than if you were using grinding, wire EDM or die-sinking EDM methods. Not only are grinding and EDM machines more costly than milling equipment, they also require a lot more set-up time. When compared to grinding, hard milling can reduce your cutting time by up to 60 percent.

To successfully hard mill a component, the right machine tool, spindle, CNC control and CAD/CAM system are critical to your operation, but even more important is choosing a high-quality cutting tool and having a clear understanding of how to effectively implement it into the machining process. When I’ve seen manufacturers fail at hard milling, it’s usually because they skimped on cutting tools. It’s common; they just don’t realize the right choice early on can help save them money in the long run.

CBN cutting tools are ideal for rough and finish machining of hard steels up to 70 Rockwell as well as abrasive materials, such as grey cast irons. CBN is harder and more abrasive resistant than the carbide and ceramic inserts that are commonly used. CBN costs nearly seven times more than carbide, but users greatly benefit from its adequate balance of strength, toughness, thermal conductivity and chemical inertness.

Here are six questions to ask yourself before making your cutting tool selection and moving forward with a CBN milling test.

1. “What material am I machining?” CBN works well in all types of hard materials, including high tensile steel, high speed steel and white cast iron; soft/abrasive material such as grey cast iron; and difficult to machine materials such as nickel-based and hard facing alloy. However, CBN does not work well in applications involving soft steels or nodular irons.
2. “Will this require rough or finish machining?” Determine whether your hard milling operation will involve rough or finish machining because knowing this helps in selecting the right insert and cutter for the job.
3. “What’s my approach angle?” Another factor that impacts cutter selection is the approach angle; therefore it’s important to decide whether square shoulder or face milling is the best choice.
4. “What is my cutter geometry and setup rigidity? Answer this to determine whether a positive or negative cutter works best in a particular application.
5. “Will I need a wiper insert?” Positioned slightly higher than other inserts, wiper inserts “wipe down” the part to create a superior surface finish. Need a porous finish? Don’t use a wiper insert.
6. “Does every pocket of my test cutter need to be loaded with CBN?” With the high upfront costs of using CBN, you should take steps to ensure you don’t lose a significant amount of money during your milling test. For example, the test cutter does not need to have every pocket loaded with CBN. An effective test can occur with only one or two inserts and an adequately adjusted feedrate. 

Hard milling know-how is vital to its successful implementation and your future productivity and profitability gains. If this is a machining practice you are really considering, but are unsure of where to begin, don’t be afraid to ask for assistance.

An OEM partner that specializes in hard milling tools, such as Seco, can assist you in selecting the best tool for your job, develop a proper cutting strategy for your application and recommend proper feeds and speeds, depths of cut and programming for your cutting material.

About the Author

Chad manages Seco's advanced materials product lines, including all CBN and PCD products. When he's not helping customers implement advanced metalcutting solutions, you can find him training for and running 5K, 10K and 1/2 marathon races and triathlons. Chad can be reached at cmiller@secotools.com.