High-Speed Hard Milling Tips for Successful Mold Production

By Jay Ball, Product Manager – Solid Carbide Endmills, Seco Tools LLC

If your mold shop would like to reduce or eliminate roughing of mold cavities pre-heat-treat and go straight to post-heat-treat roughing and finishing, you might want to consider high-speed hard milling. Why? Because, when applied correctly, the process, which involves light depths of cut and accelerated feed rates, can boost your productivity and reduce your setup costs.

With hard milling, depending on your application, you could drill holes and water lines in a block, perform heat treatment and then apply high-speed strategies to rough and finish in one setup. But, to be successful, there are several factors to consider, including the 6 mentioned below.

1. Think in Terms of Machinability
Typical hardened mold steels fall into the 48-65 HRC hardness range, but Rockwell numbers are only half of the equation in terms of real-world machinability. D2 tool steel, for example, hardens to about 60-62 HRC, but machines more like 62-65 HRC material due to its 11-13% chromium content. So, when machining multi-constituent alloys, it’s best to apply the recommended machining parameters for hard materials set forth by your cutting tool supplier.  

2. Maintain Constant Chip Load
Maintaining a constant chip load on a tool’s cutting edge is important, otherwise it will wear out prematurely, chip or break. Certain situations, such as machining 3D contours into a mold, make chip load management especially challenging. There are, however, steps you can take to simplify the process, including manually reducing your rpm/feed rate via override controls, or backing the rpm and feed rate down using the combined efforts of your machine’s CAM program and CNC.

3. Minimize Tool Run Out
On average, tool run-out greater than 0.0004" can cut your tool life in half. Therefore, it’s important to do everything you can to minimize it, especially when working with smaller tools. One way to reduce runout is by using high-precision holders such as shrink-fit, hydraulic, and milling chucks in your operations.

4. Reduce Finishing Stock
Be sure to remove as much part stock as you can during the roughing process. If you’re working with cutters that are larger than 1/8" in diameter, it’s best to leave about 1 percent of the cutter diameter for finishing. Smaller tools, however, determining a sufficient amount of stock for finishing may be a case of “feel,” or trial and error. 

5. Beat the Heat
Extreme heat has a significant impact on tool life, especially when processing materials harder than 48 HRC. When working with these materials, it’s better to use an air blast or oil/air mist instead of coolant to avoid thermal shock.

6. Apply the Right Tool
When processing molds with tight tolerances, you want to make sure you have the right cutting tool for the job. Because milling hard materials generates a significant amount of heat, it’s best to use tools with a high thermal-barrier and abrasive resistant coatings such as AlTiN. CBN tools are great for premier finishing applications, while inserted end mills prove highly effective in roughing and some finishing operations.  

While high-speed hard milling is a great alternative to lengthy rough milling processes, careful consideration of your entire machining system is critical to successful application. Understanding these 6 factors is a good place to start, but you should also consult with your cutting tool supplier to gain the best possible results.



About the Author
Jay has been with Seco for more than 10 years. As a key member of the product management team, he is responsible for Seco’s solid carbide end mill products in North America. He works closely with global R&D on new innovations to ensure they meet the necessary market requirements. He also provides technical support for high-speed hard milling and micro milling operations, including CAD file review, tooling selections and programming recommendations.