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Partnering September, 2020


Ensuring Aerospace Precision with U.S. Steel Retention Knobs


By Del Williams A


erospace manufacturing re- quires precision and tight tol- erances from CNC milling ma-


chines, even when machining exotic alloys and hard metals, or when ag- gressively removing stock. Consis- tently achieving precisely machined components requires a quality reten- tion knob, particularly in high-speed, high-volume applications. In milling machines, retention


knobs are commonly used with ta- pered tool holders to engage draw- bars that hold the tool holder firmly in the spindle. The tapered tool hold- er comes with threads designed to ac- cept the retention knob, but any im- proper seating of V-flange toolhold- ers costs aerospace manufacturers in lost productivity. It also causes run out and vibra-


tion that reduces the precision of ma- chining while reducing tool and spindle life —a common problem in machining.


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Beyond this, high-speed ma-


chining in excess of 20,000 RPMs is often utilized in aerospace when ma- chining exotic alloys and harder met- als, such as titanium. At higher RPMs, the precise and secure seating of tapered tool holders in the spindle (with the help of the retention knob) is even more critical. So much so, that failing to pay


attention to this single detail can lead to further decreased productivi- ty, less precise machining, reduced tool life, and even damaged work- pieces.


Retaining Knob Precision Strong, well-manufactured re-


tention knobs are vitally important in the aerospace industry. Superior products increase the rigidity of the setup, which lengthens tool life and speeds cycle times. One major problem is poorly de-


37 YEARS OF EXCELLENCE


signed retention knobs from over- seas, often made from low-quality steel that is prone to breaking during machining. Newer CNC milling ma- chines exert significantly more draw- bar pressure on retention knobs than in the past. As drawbar tension continues to


increase in CNC machines, it puts so much pressure on the retention knob that it may be more likely to break if not manufactured to the highest standard. Failure may be due to other rea-


sons, including improper knob config- uration, misaligned machines, metal fatigue, over-torqueing, and exceed- ing the machine’s capability. “We deal with a lot of aerospace


companies, some with high-speed CNC applications with harder metals or alloys,” says Bob Selleck, manag- er, SAG Supply, a Long Island-based provider of industrial tools and acces- sories. “For that, you need a reliable retention knob that will last without breaking or moving. It needs to hold everything tightly in place.” Although retention knobs often


look very similar, they are not inter- changeable. The materials, coatings applied and quality of manufacturing are important differentiators. Anoth- er consideration is where the item is manufactured, given that some over- seas importers have a reputation for low quality. “Some imports do not always fit


snugly or precisely in the flange and may wobble or move a little bit under high-speed applications,” says Selleck.


U.S. Steel and Black Oxide The most reliable retention


knobs for aerospace use are those made with superior U.S. steel and


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coated with black oxide to prevent corrosion. In addition, the retention knobs should be subjected to the most rigorous quality control stan- dards, including frequent inspection by the manufacturer. As an example, Selleck points to


Ohio-based T.J. Davies, a manufac- turer of high-quality retention knobs that utilizes 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 high RPMs during CNC machining.


Long-term reliability also depends on a coating to


prevent corrosion — in this case, black oxide.


A low alloy nickel, chromium,


molybdenum case hardening steel, 86L20 has high hardenability with- out temper brittleness, along with good external and internal strength and wear resistance. However, for even higher quality, 9310 is com- posed mostly of nickel and chromium and is a good choice for use in heavy duty machinery. “We have found that the domes-


tic T.J. Davies retention knobs to be of higher quality and more reliable than imports,” says Selleck. “They do not break, and 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.” Long-term reliability also de-


pends on utilizing a superior coating to prevent corrosion. For this reason, each retention knob is coated with black oxide. In this process, the top layer of the ferrous material is chem- ically treated to prevent rust. Black oxide also increases resistance to abrasion, while providing a decora- tive 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 over- seas shipping and logistics. Because aerospace OEMs re-


quire utmost CNC precision and pro- ductivity at all times, utilizing reli- able retention knobs made with su- perior U.S. steel, coated with black oxide can actually become a competi- tive manufacturing and production


advantage. Contact: T.J. Davies Co., Inc.,


16995 W Park Circle Drive, Chagrin Falls, OH 44023 % 440-248-5510 E-mail: sales@tjdavies.com Web: www.tjdavies.com r


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