High-speed, small footprint milling machines have challenged traditional spindle retention knob technology to achieve the design safety required in today’s advanced shops. “As the trajectory of today’s new milling technology trends toward machines producing extremely high speeds/high rpms within a smaller overall footprint, potential safety issues can’t be ignored,” said John Stoneback, JM Performance Products Inc. (JMPP), Fairport Harbor, Ohio.
“The bottom line is, everyone is looking to get more production per square inch and increase efficiency via heavy-duty, multi-axis machines doing precision milling using less space,” said Stoneback. “The daunting obligation and responsibility for both industry and machine builders is to keep machine operation as safe as possible, while achieving the consensus goal of optimizing milling productivity,” he said.
Stoneback noted that the retention knob is the main interface between the machine and the spindle and, when exposed to severe conditions, failure of a standard retention knob can result in a tool breaking loose during a cutting operation. A tool, holder or knob breaking loose from the spindle at such high speeds and rpms produces a projectile that can damage the spindle, tool, holder, workpiece, workholding device and personnel. “Loose tools moving fast could present the potential for a lot of damage to the machine and workpiece. Indirect costs and liability can run the gamut, from damaged facilities or equipment to serious injuries to personnel,” he said.
Recognizing the potential for machine spindle interface failure that these small-footprint/high-rpm machines represent, JM Performance Products, a developer of CNC mill spindle optimization products, created its patented High Torque retention knobs.
“In addition to solving the critical ‘loose tool’ factor and preventing toolholder deformation, the design delivers inherent safety benefits that are vital to addressing the velocity-driven safety dilemma,” said Stoneback. “The fact is that most standard retention knobs are still being designed and manufactured to standards put in place over 40 years ago while the evolution of the tooling and the mills has been progressive. Even though it is a vital component in milling using V-flange tooling, the retention knob has been largely overlooked in this evolution—including safety factors,” he said.
According to JMPP, its testing has proven that standard design retention knobs often expand the toolholder, leading to excessive vibration, chatter and mill harmonics. The company stated that
in addition to affecting finishes, tolerances and tooling life, this vibration and chatter, caused by a lack of concentricity, can be damaging to the spindle and the draw bar of the mill.
“With the advent of today’s very fast, very powerful small-foot-print machines, this damage can directly result in the high-risk, red-flag safety dangers associated with a tool breaking loose during a cutting operation,” said Stoneback. “In essence, it’s a disaster waiting to happen.”
Brother Industries Ltd., Nagoya, Japan, a leading multinational manufacturer of CNC drilling and tapping centers for automotive, aerospace and medical applications, identified the need to modify the standard used to manufacture its retention knobs, including the material tensile strength to make them stronger to meet the machine’s manufacturing demands.
Material strength of knobs is a key factor in retention knob failure. To ensure their durability and strength, JMPP proactively migrated its 30-taper and 40-taper retention knobs from the traditional B8620H material to 9310H material. The 9310H material offers 40 percent higher tensile strength than the 8620H material. In reviewing the cross-sectional strength of the knobs, JMPP also identified a design flaw. To correct this flaw, JMPP is modifying the size of the coolant holes in many of its 30- and 40-taper knobs to increase this cross-sectional strength.
In reality, any industry that depends on high-speed precision milling, whether for roughing exotic materials to complex geometries or for micro parts, is going to face these tooling safety issues. Using Brother’s modified spec for its BT 30 retention knobs, JMPP quickly responded by introducing a knob with a coolant hole and a knob without a coolant hole. These knobs incorporated all of Brother’s dimensional and radius requirements along with JMPP’s High Torque design features and higher tensile strength material.
In addition, JMPP laser marks its parts, providing dating on each knob. The laser marked “date-in-service” feature includes a unique serial number for traceability of how long the knob has been in service.
“This ensures safety of retention knobs, which are a perishable, consumable part,” said Stoneback. “A typical retention knob is good for one to three years on a machine, depending on how long the machine runs per day. The operator can simply enter the unique identifier serial number to show how long a knob has been in service and when a tool change should be made. Inspection of retention knobs during tool changes can reveal signs of diminished draw bar force. With spindle replacements costing $20,000 to $80,000, maintenance is critical,” said Stoneback.
Currently, JMPP has provided more than 10,000 of the modified High Torque retention knobs to customers for demanding high-rpm machines, in addition to the more than 10,000 standard JMPP High Torque knobs already in use.
All JMPP retention knobs are manufactured and material sourced in the U.S. and made from hot rolled 8620H or Grade 9310H fine grain steel. H13 tool steel is also available. Knobs are shot peened to relieve stress, hard turned for superior fit and finish, and balanced by design. They meet all five world standards: ANSI, JMTBA, ISO or DIN and JIS, according to the company.
For more information from JM Performance Products Inc., go to www.jmperformanceproducts.com, or phone 440-357-1234.