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CNC Industry Turns to High Torque Retention Knobs to Cut Vibrations
CNC horizontal and vertical machine, tool, and tool holder manufacturers and distributors spend years establishing strong relationships with customers by providing top-quality, precision equipment. Sadly, these relationships can easily be undone by a small, less than $30 part: the retention knob—an overlooked, yet critical component in the machining process.
In fact, the problem is so prevalent in CNC equipment that utilizes drawbars to hold tapered tool holders securely in the spindle that many are adopting a policy of only selling the High Torque retention knob; or at minimum, strongly urging customers to invest a few dollars more for an item they do not even sell.
After all, many suppliers have already discovered a hard truth: when machinists experience vibration, chatter, and fretting after purchasing equipment, they tend to blame the new spindle, tool holder or high-end carbide cutting tool. This, despite the fact that improper seating of the tool holder due to poorly designed traditional retention knobs is more likely the cause.
When this occurs, it reflects poorly on the supplier and can cost them considerable business both short-term and in repeat business.
According to Craig Gilsinger, Applications/Quality Manager for JTEKT Toyoda Americas for more than 20 years, this real-world scenario is the reason the company regularly recommends High Torque retention knobs for use in its vertical and horizontal machining centers.
"If the cutting conditions are poor, the customer immediately assigns blame to the machine," explains Gilsinger. "So, we regularly recommend [the High Torque retention knob] to customers. It not only ensures the equipment will cut better, but also that it won’t get blamed for machining issues when it is not the cause."
More often than not, says Gilsinger, investigations of vibration and chatter issues point to improper seating of the tool holder in the spindle.
That is because when a standard retention knob is tightened, it can produce a bulge in the taper that prevents full contact and correct seating in the spindle. As a result, the tool holder is unable to pull completely into the spindle and, subsequently, cannot make contact with up to 70% of its surface.
"We have seen many situations where retention knobs have been overtightened," explains Gilsinger. "A guy goes to the workbench and uses a wrench and a hammer to tighten the retention knob, and it is certainly not to the proper torque with this method. This ends up distorting the taper and so the tool does not seat in the taper correctly."
This can cause a multitude of problems, which too often get blamed on the suppliers’ equipment: vibration and chatter, poor tolerances, non-repeatability, poor finishes, shortened tool life, excessive spindle wear and tear, run-out, and shallow depths of cuts.
Surprisingly, this known design flaw, with its far-reaching consequences, is often overlooked or even dismissed. So when JTEKT Toyoda Americas learned about the High Torque retention knob offered by JM Performance Products, Inc. (JMPP), the company quickly decided to recommend it with each machining center purchase as well as on its full turnkey systems.
The High Torque retention knob is designed with a longer reach so that it threads deeper into the threaded bore of the tool holder. Consequently, all thread engagement happens in an area of the tool holder where there is a denser cross-section of material to counter deformation.
It also features a precision pilot and is balanced by design, to increase rigidity. What is more, to guard against the High Torque retention knob being over-tightened, which can still produce a bulge, the company provides specifically calculated torque specs based on drawbar pressure.
By combining the High Torque retention knob with the proper torque, spindle contact with the taper is improved to close to 100% every time, which drastically reduces vibration and chatter.
High Torque retention knobs work with all existing tool holders including BT, DIN, ISO, and CAT toolholders from 30 to 60 taper.
For Dan Carlstrom of Carlstrom Associates, a manufacturer’s rep organization that sells tool holders, milling products, and work holding systems, the "light bulb" moment came when he recommended the High Torque retention knob to a customer that was struggling with a boring product he had sold them.
"The customer was having a problem holding size on a component, so they had to take multiple boring passes and then do a final reaming pass to get this hole to size," explains Carlstrom. "When they put the High Torque retention knob on the tool holder, the boring tool was able to cut the hole to size, in tolerance, in one pass.
"They never gave me total cost savings, but needless to say it was significant. It also solved a huge headache for them," adds Carlstrom.
Carlstrom, who emphasizes that his company does not represent or sell JMPP’s High Torque retention knobs, says whenever he runs a test of his product for a customer he does so with the knob installed to eliminate poor seating in the spindle as a variable that could affect performance.
"When I run an end mill test, a tool holder test, or a boring bar test, I will not run it without the High Torque retention knob, period," says Carlstrom. "I know it will make my tools, which I get paid for, run properly."
By recommending or supplying High Torque retention knobs, suppliers can also earn the respect and gratitude of their customers, which can result in increased loyalty and referrals, all of which provide them with a competitive edge.
