Choosing the Right Cutting Tool for Pre-Hard Materials

There are four common tool types to consider for cutting pre-hard materials: ball-end, toroidal, multi-fluted bull-nose and square-end.

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There are four common tool types to consider for cutting pre-hard materials: ball-end, toroidal, multi-fluted bull-nose and square-end.  

1. Ball-end cutters
The No. 1 go-to tool for 3-D cuts is the ball-end cutter, and the proper way to apply ball-nose cutters is to rough out the part in a constant Z-level roughing routine. Climb milling is the preferred cutting direction. Using this type of tool path helps reduce heat and allows you to cut closer to the required finished shape. 

During roughing or semi-finishing operations, typical radial stepovers (X- or Y-axis movements) are 25 to 40 percent of the cutter’s diameter. Axial depths of cut (Z depth) depend on the hardness of the part. For 30-40 HRc, use a Z depth of 10 percent of the cutter diameter per pass; for 40-50-HRc material, use a Z depth of 5 percent; and for materials greater than 50 HRc, use a Z depth of 4 percent. 

2. Toroidal tools
If you have tight areas like helical bores or ribs, or when the cutter diameter is close to the radius of the part, the optimal choice is a toroidal tool. These cutters are able to cut flat floors with larger stepovers than ball-nose cutters, but are much-better-suited for tight areas. 

When using toroidal tools for profiling applications, the hardness of the material affects the radial stepover. For materials 30-50 HRc, use a stepover of 5 percent of the tool diameter per pass, and if the material is harder than 50 HRc,  use a stepover of 2 percent. In both cases, you can use a Z depth of one time the diameter of the cutter per pass. When facing on flat floors or doing helical bores, the radial stepover is 25 percent of the cutter diameter and 2 percent in Z depth per pass. If going down in a helix, use a 2- to 3-degree helix angle. 

2. Bull-nose cutters
Bull-nose cutters work most effectively for wide-open areas with flat floors or straight walls, and when profiling the outside shape with straight walls or very wide-open areas.  Smaller-diameter tools reduce the amount of surface contact, so operations with less-rigid machines benefit from using such tools. Use the same stepovers and depths of cut as stated above. 

For slotting applications, use a trochoidal tool path and ensure the tool is about 50 percent of the slot width to allow for enough movement and to ensure the engagement angle does not increase and generate too much heat. 

4.  Square-end cutters
The very last tool you want to use for pre-hard materials is the square-end cutter, because all the force and heat are focused on the corner of the tool. For this reason, square-end tools should only be used to pick out the small radius left from a toroidal or bull-nose cutter when sharp corners are required.

To view a video on high-speed hart metal trochoidal machining, visit http://short.moldmakingtechnology.com/robbapr14.

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