Article by T.J. Davies Co. Inc.
Contract machine shops and OEMs rely on CNC milling machines to produce very precise components in industries such as medical, aerospace, automotive, motorcycle, and gun manufacturing. However, consistently achieving precisely machined components requires a quality retention knob, particularly in high-speed, high-volume applications.
In milling machines, retention knobs are commonly used with tapered tool holders to engage drawbars that hold the tool holder firmly in the spindle. The tapered tool holder comes with threads designed to accept the retention knob, but any improper seating of V-flange toolholders costs manufacturers in lost productivity. It also causes run-out and vibration that reduces the precision of machining while reducing tool & spindle life. This is an extremely common problem in machining.
Beyond this, high-speed machining in excess of 20,000 RPMs is often utilized in aerospace, medical, and other industries when machining exotic alloys and harder metals like titanium. At the higher RPMs, the precise and secure seating of tapered toolholders 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 productivity, less precise machining, reduced tool life, and even damaged workpieces.
That is why strong, well-manufactured retention knobs are so important. A superior product increases the rigidity of the set-up, which increases tool life and speeds cycle times (the measure of the time it takes to complete each part from start to finish.)
The problem is poorly designed retention knobs from overseas are often made of low-quality steel that may be prone to breaking during machining. Newer CNC milling machines exert significantly more drawbar 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.
Retention knob failures may fail due to other reasons, including improper knob configuration, misaligned machines, metal fatigue, over-torqueing and exceeding the machine's capability. That is why it is essential to periodically inspect each retention knob for signs of excessive wear, damage, or corrosion. If found, the retention knob should be replaced.
“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, NY-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. The materials, coatings applied, and the quality of manufacturing can be important differentiators. Another consideration is where the item is manufactured, given that some overseas imports have a reputation for lower quality.
“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,” adds Selleck.
The most reliable retention knobs are those made with superior U.S. 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.
As an example, Selleck points to Ohio-based T.J. Davies, a manufacturer of high-quality retention knobs that utilizes certified 86L20 and 9310 steel drawn in the United States. He adds that SAG Supply carries TJ Davies retention knobs for their reliability at higher RPMs during CNC machining.
86L20, a low alloy nickel, chromium, molybdenum case hardening steel, has high hardenability without temper brittleness, along with good external and internal strength, and high wear-resistance. However, if a CNC operator desires and even higher quality steel, they can choose a retention knob made of 9310, a low alloy steel composed mostly of nickel and chromium. This allow also has high hardenability, core hardness and fatigue strength, which makes it an excellent steel for use in heavy-duty machinery.
“We have found the domestic TJ Davies retention knobs to be of higher quality and more reliable than imports,” says Selleck. “They do not break; they last longer even at higher speeds with harder metals. We have carried them for over 10 years, and I have never heard of a problem or complaint.”
Long-term reliability also depends 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 treated with chemical that convert the surface to prevent rust. Black oxide also increases abrasion resistance while providing a decorative finish.
“I have never seen one of the black oxide coated retention knobs rust, and we sell a lot of them,” says Selleck.
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. Manufacturers like TJ Davies can ship items 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,” says Selleck. “When I order domestically, I usually get what I need within two days.”
Because contract machine shops and OEMs require utmost CNC precision and productivity at all times, utilizing reliable retention knobs made with superior U.S. steel, coated with black oxide, can actually become a competitive manufacturing and production advantage.
Related Glossary Terms
- alloys
alloys
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- black oxide
black oxide
Black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- fatigue
fatigue
Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.
- fatigue strength
fatigue strength
Maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- hardenability
hardenability
Relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness (50 HRC, for example) or a specific percentage of martensite in the microstructure.
- hardening
hardening
Process of increasing the surface hardness of a part. It is accomplished by heating a piece of steel to a temperature within or above its critical range and then cooling (or quenching) it rapidly. In any heat-treatment operation, the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too quickly, the outside becomes hotter than the inside and the desired uniform structure cannot be obtained. If a piece is irregular in shape, a slow heating rate is essential to prevent warping and cracking. The heavier the section, the longer the heating time must be to achieve uniform results. Even after the correct temperature has been reached, the piece should be held at the temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature. See workhardening.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- quality assurance ( quality control)
quality assurance ( quality control)
Terms denoting a formal program for monitoring product quality. The denotations are the same, but QC typically connotes a more traditional postmachining inspection system, while QA implies a more comprehensive approach, with emphasis on “total quality,” broad quality principles, statistical process control and other statistical methods.
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