In milling machines, retention knobs used with tapered toolholders can engage drawbars that secure the toolholder firmly in the spindle. The tapered toolholder has threads designed to accept the retention knob, but improper seating of V-flange toolholders lowers productivity and causes runout and vibration that reduce machining precision, tool life, and spindle life.
Machining exceeding 20,000rpm spindle speed is often used in aerospace, medical, and other industries that process exotic alloys and harder metals such as titanium. At higher speeds, precise and secure seating of tapered toolholders in the spindle (with the help of the retention knob) is even more critical. Failing to pay attention to this detail can be costly.
Strong, well-manufactured retention knobs increase setup rigidity, increasing tool life and speeding cycle times. Beware of retention knobs made of low-quality steel that may be prone to break during machining. Newer CNC milling machines exert significantly more drawbar pressure on retention knobs than in the past. As drawbar tension increases, so much pressure is put on the retention knob that it may be more likely to break if not manufactured to the highest standard.
Retention knob failures may occur due to other reasons, including improper knob configuration, misaligned machines, metal fatigue, over-torquing, and exceeding machine capability. It’s essential to periodically inspect each retention knob for signs of excessive wear, damage, or corrosion, and if found, replace it.
“We deal with a lot of aerospace companies, some with high-speed CNC applications with harder metals or alloys,” says Bob Selleck, manager at SAG Supply, a Long Island, New York-based provider of high-quality industrial tools, machinery, and accessories. “For that, you need a reliable retention knob that will last without breaking or moving. It needs to hold everything tightly in place with precision.”
Although retention knobs often look very similar, they are not interchangeable. Materials, applied coatings, and manufacturing quality can be important differentiators. Another consideration is where the item is manufactured.
“Some [retention knob] imports do not always fit snugly or precisely in the flange and may wobble or move a little bit under high-speed applications,” Selleck adds.
The most reliable retention knobs are those made with superior steel and coated with black oxide to prevent corrosion. In addition, the retention knobs should be subjected to the most rigorous quality control standards, including frequent inspection by the manufacturer.
Selleck points to Ohio-based high-quality retention knob manufacturer T.J. Davies, a company s that uses certified 86L20 and 9310 steel drawn in the United States. He adds that SAG Supply carries T.J. Davies retention knobs for their reliability at higher spindle speeds during CNC machining.
A low alloy nickel, chromium, molybdenum case-hardening steel, 86L20 has high hardenability without temper brittleness along with good external and internal strength and wear-resistance. If CNC operators need a higher quality steel, retention knobs are available made of 9310, a low alloy steel composed mostly of nickel and chromium. This alloy also has high hardenability, core hardness, and fatigue strength, making it ideal for heavy-duty machinery.
“We have found the domestic T.J. Davies retention knobs to be of higher quality and more reliable than imports,” Selleck says. “They do not break; they last longer even at higher speeds with harder metals. We have carried them for more than 10 years, and I have never heard of a problem or complaint.”
Black oxide benefits
Long-term reliability also depends on corrosion prevention so each retention knob is coated with black oxide. The top layer of the ferrous material is treated with a chemical that converts the surface to prevent rust. Black oxide also increases abrasion resistance while providing a decorative finish.
If a retention knob ever needs to be replaced, it also helps to have quick shipping from a domestic supplier rather than dealing with lengthy overseas shipping and logistics. T.J. Davies can ship items the same day with no minimum order and will drop-ship to end users on behalf of distributors.
“When I need retention knobs, I do not want to wait for overseas shipping,” Selleck adds. “When I order domestically, I usually get what I need within two days.”
Because contract machine shops and original equipment manufacturers (OEMs) require CNC precision and productivity at all times, using reliable retention knobs made with superior steel, coated with black oxide, can serve as a competitive manufacturing and production advantage.