Milling heads don’t get the shaft

END USER: Machine Tool & Gear Inc., (989) 743-3936, www.machinetoolgear.com.
CHALLENGE: Overcome excessive tool wear and poor surface finishes when machining flats on a steel truck shaft.
SOLUTION: A milling tool that exerts all cutting forces downward and cuts freely.
SOLUTION PROVIDER: Horn USA Inc., (888) 818-HORN, www.hornusa.com.

The last thing a tool distributor wants is for a customer to return an expensive product because it can’t produce acceptable parts. PF Markey Inc., headquartered in Saginaw, Mich., faced this possibility after the distributor sold two 90° milling heads that cost about $10,000 each to Machine Tool & Gear Inc., Owosso, Mich. The heads, which attach to the machine tool spindle and accept a cutting tool, were to machine shafts on a Mori Seiki NH 4000 horizontal machining center at the company’s plant in Corunna, Mich.

MT&G purchased the heads when a new truck shaft design included two flat areas 180° apart that must be machined at 90° from other operations, because adding a machine and fixtures to perform the additional process would have been cost-prohibitive, according to MT&G Manufacturing Engineer Jeff Ochodnicky. However, MT&G experienced excessive tool wear and poor surface finishes when cutting with the heads—even after experimenting with numerous machining parameters.

The Mini Mill tool (below) from Horn USA enables Machine Tool & Gear to achieve a 15 to 20 µin. Ra surface finish when milling flats in a steel truck shaft.

That’s when Brett Kischnick, applications engineer for toolmaker Horn USA Inc., Franklin, Tenn., got a call. “Phil Horn, at the distributor, said, ‘I don’t want them to return these heads’ and asked if I could do anything to help.” Kischnick replied, “Let me at it.”

The challenge was milling flats 1.500 ” (38.1mm) long and 0.060 ” (1.524mm) deep on a steel shaft with a cutting tool 0.854 ” (21.692mm) in diameter that hangs more than 10 ” (254mm) from the spindle and rotates at 90° to the spindle via gears. That arrangement created rigidity and chatter issues, Kischnick noted.

He added that the milling tool MT&G was applying is designed primarily for side milling slots and has a staggered flute arrangement that alternates between a helical twist on the flute that pulls, or lifts, a chip, and a helical twist that pushes a chip. “One flute was pushing down and the next one was pulling up and causing vibration in an unstable setup with the 90° head,” Kischnick said, noting MT&G was using a straight-shank toolholder that went into a collet.