Incorrect speeds and feeds are a surefire way to make terrible noises, break tools and scrap parts. Our initial attempts to machine a hardened ring gear made noises that would make any machinist shudder. Through experimentation, we optimized reliable recipes that result in an excellent surface finish and provide good tool life.

These ProvenCut recipes were performed on a Tormach 1100, a 499-kg (1,100-lb.), 1.1-kW (1.5-hp) CNC mill. It has similar capabilities as a Bridgeport. Tormach machines target the "prosumer" market and often are found in garage shops and R&D departments.

Although cutting tools designed specifically for hard milling are available, we chose a standard four-flute carbide endmill that many machinists already have on hand. Our testing focused on avoiding cutting tool vibration and ensuring that excessive heat was not created. We pushed the tool and machine to achieve an aggressive but sustainable roughing recipe and concentrated on the surface finish of the workpiece for the finishing recipe. Machining 8620 alloy steel hardened to 55 HRC can leave an outstanding surface finish with luster even when cut on a small CNC machine.

Recipe 502 was a Fusion 360 adaptive roughing strategy run at 12.2 m/min. (40 sfm) and 0.033 mm/tooth (0.0013 ipt) with a 15.875 mm (0.625") axial DOC (a_p) and 0.508 mm (0.02") radial DOC (a_e). Recipe 505 was a 2D contour finishing strategy run at 15.2 m/min. (50 sfm) and 0.025 mm/tooth (0.001 ipt) at the same 0.625" axial DOC to remove the 0.02" radial stock left after roughing, imparting an excellent surface finish.

Larger machine tools with more rigidity can handle higher removal rates and higher surface footage. ProvenCut recipe 81 was a great example of using a Haas VF-2 and a seven-flute, 12.7 mm (0.5") carbide endmill at 45.7 m/min. (150 sfm) to machine 60-HRC D-2 tool steel.

ProvenCut's website provides pictures, videos and cutting information for each speed and feed recipe. You can also view a video of the machining operation at cteplus.delivr.com/29n2k