It is common knowledge in the CNC manufacturing community that if you use V-Flange tooling, you use retention knobs, which are the critical interface between the spindle and the toolholder and tool. Often considered nothing more than a bolt, the knob is actually a precision tool made to strict specifications and tolerances. While some machine-tool manufacturers have standardized their requirements, others have defined several different knobs as options. The fact is, V-Flange tooling is flawed, and machinists, operators, and owners must be educated about the importance of the retention knob, and the right retention knob, to correct the flaws.
JM Performance Products, Inc., a leading manufacturing innovator of CNC mill spindle optimization products since 2009, recognizes it is of paramount importance to illustrate that retention knobs not uniquely manufactured to JMPP’s patented High Torque© design standards are just standard retention knobs that don’t fully address the gamut of vital and costly V-Flange production issues.
Extensive testing has proven that toolholder expansion is an inherent design flaw of V-Flange tooling. The following is a deep-dive examination into patented High Torque© retention knob essentials for CNC milling, which correct this flaw:
The V-Flange Tooling Design Dilemma
V-Flange tooling should be the most productive and profitable tooling system available. Its precursor, using NMTB holders, employed a 7/24 taper with an extended, internally threaded neck, which accepted the threaded draw bar. This manual change system was replaced by V-Flange to implement automatic tool changes. In order to facilitate this change, the draw bar was modified to include a collet closer (or fingers), and the extended neck was removed from the holder and replaced by the retention knob, which threads directly into the taper.
While these modifications made tool changes occur quickly and automatically, they introduced a variety of issues that were not present with the NMTB system, such as excessive chatter and run-out, resulting in poor finishes and expensive secondary processes. Holding tolerances can be problematic, requiring skilled machinists tweaking speeds and feeds, affecting production in an effort to reduce scrap. Still a good option for heavy milling processes, V-Flange forced the industry to look to HSK, CAPTO, and other precision tooling systems, all significantly more expensive to adopt in a shop.
The modifications caused these problems essentially because steel maintains its elastic properties, even after hardening, and threads create torsional stress. This stress, exerted by the engagement of the threads of the retention knob with the threads of the V-Flange holder at the small end of the holder, often creates a distortion of the holder. Once expanded or distorted, the holder will no longer pull all the way into the spindle, stopping short of full engagement. The small end of the taper makes contact before the large end at the gage line, leaving the holder to move randomly within the spindle, much like a bell-clapper.
This random movement translates to vibration and chatter (and the above resultant issues), and negatively affects tool life, especially expensive carbide that is fragile and susceptible to microfracturing (which yield poor surface finishes). If a microfracture occurs, the whole tip may disappear, and the inserts can be rendered useless. Toolholder expansion caused by standard retention knob installation can reduce the toolholder to spindle contact by 70 percent or more. Carbide tool life is diminished by 50 percent for every 0.0005” distance short of full engagement.
The High Torque Design Inception
JMPP became aware of this tooling system design flaw, and designed a gage to measure and study the expansion. Using this gage, JMPP redesigned the standard retention knob into its patented High Torque retention knob, which uses the lowest available threads in the toolholder. Key to the design is a relief beneath the pilot that forces the threads of the knob deeper into the threaded bore of the toolholder, where a thicker cross-section of material resists the expansion and deformation.
Notably, the design can be used in any toolholder as long as it is made to industry specifications. Additionally, we found that most holders, unless improperly hardened, will return to their original form, so even those holders that were expanded by a standard knob could be kept in service.
Figure 1 compares a standard knob without a pilot to a standard knob with a pilot, and finally to the High Torque design knob. Note that, from the top of the knob head to the flange that rests on the holder face, the knobs are dimensionally alike. The additional length of the High Torque design is beneath the flange, inside the toolholder bore.
Differential Design Features of High Torque Retention knobs
The pilot is that portion beneath the knob flange above the threads that stabilizes the knob in the toolholder. The High Torque design includes a precision ground pilot that ensures the perfect alignment of the knob in the holder during installation. It is important to note that some toolholder manufacturers do not adhere to strict tolerancing of the counter bore at the small end of the holder. In cases where this counter bore is uncontrolled — while the pilot does add some strength and rigidity to the knob and therefore, the tool — it may not facilitate alignment.
Today’s CNC mills are designed to operate with greater draw bar pressure, faster tool changes, higher feed rates, and greater depths of cuts, which place more demand on retention knobs in V-Flange tooling than ever before. In response to meeting machine manufacturers’ future requirements and ever-increasing demands, JMPP has decided to manufacture all 30 and 40 taper retention knobs, including the patented High Torque knobs, from AISI 9310H material, offering 40 percent more tensile strength, instead of the traditional 8620H material.
40 percent added tensile strength is the minimum benefit derived by combination of the steel composition and the effects of heat treating (carburization). JMPP worked closely with metallurgical specialists to define the proper temperatures, processes, depths, and case hardening values to ensure the standards for retention knobs are exceeded. Additionally, proper quenching and cryogenic freezing greatly reduces the possibility of cracks. The processing of JMPP’s knobs is done by highly skilled and certified heat-treat partners.
JMPP reviewed the standards and identified a design flaw in regards to the expected cross-sectional strength of the knob. To correct this, the size of the coolant holes in many of the company’s 30 and 40 taper knobs was modified. The knobs will continue to supply more coolant than demanded, but will be sized to increase the cross-sectional strength of the knob.
Of note, JMPP uses a black oxide, non-acid process with strict temperature control to safeguard against hydrogen embrittlement of the AISI 9310H material. This process ensures proper adhesion of the blackening process to the dense chromium molecules present in this material.
The engineering catch phrase for “approximate radius or a series of radii next to each other” is called Blend Radii. This relates to common machining/engineering knowledge that a sharp edge, including those on retention knobs, creates stress points that are easily subject to failure. Therefore, JMPP eliminates the sharp edges and corners using radii in transitional surfaces during the machining or finishing processes.
Finally, a hard-turning machining operation is applied to each lot of knobs manufactured from 9310H material. In this process, all critical and control surfaces are controlled to 50 percent of allowable tolerances. Tolerances under 0.0008 are 100 percent inspected to ensure conformity. All surface to surface runouts are 0.0004 TIR or less, which ensures better uniformity of gripper to retention knob contact in machine tool spindles.
JMPP precision cuts its threads, with each thread starting and finishing at 180 degrees to the next thread. Each part is held on the major diameter of the threads to ensure precision runout between threads, and all surfaces being hard-turned are held to 0.001 TIR. Keep in mind that most standards require 0.002 maximum TIR to threads. This thread-making process is designed to provide a precision mating between the toolholder threads and the retention knobs threads.
Notably, the High Torque design takes into account the mechanical functionality of each thread, and the fact the load on each thread diminishes with each step down from the first thread. To that end, JMPP has removed any non-mechanically necessary threads, so the mass of the knob is dynamically balanced by design. Simply adding an additional thread or two will not yield a better installation result and do nothing to resolve toolholder expansion. A review of the load percentage on each thread of a retention knob reveals the maximum number of functional threads is six; any additional threads are unnecessary. Also, the process of forming the threads, whether cutting or roll forming, does not affect the distribution of the thread load.
Magnetic Particle Testing
Magnetic particle testing is a non-destructive test that will reveal faults and cracks in the material. Using only high-quality U.S. made steel that has eddy current testing (ECT) certification, ensures the part’s material is defect-free from surface and sub-surface flaws.
JMPP will conduct magnetic particle inspection tests on purchased knobs, which is useful for detecting minute surface and near-surface cracks down to a depth of about 0.100”. Essentially, a magnetic field is induced in the test specimen, which is then “dusted” with iron particles, either dry or in a liquid suspension. The particles will collect along the edges of any micro cracks or other discontinuities in the structure of the material to provide a readily visible indication of the flaw. Notably, JMPP’s testing typically proves out at a 100 percent flawless rate.
Installation Torque Specifications
JMPP provides an axial force calculation, based on draw-bar force and taper size, to define the installation torque that should be used on each knob during installation. This value ensures against over-torqueing the knob (resulting in premature fatigue and failure of the knob), eliminates expansion in toolholders made to spec, and ensures the knob is installed with sufficienttorque so it does not back out of the holder during cutting operations.
Many competitors will specify torque values well in excess of JMPP’s top-range values, for instance: 76 ft. lbs. for a 40 taper knob. Inasmuch as testing has proven that as little as 13 ft. lbs. of torque can cause toolholder expansion and excessive knob stress, JMPP specifies values less than 40 ft. lbs. for most 40 taper knobs. It is important to recognize that any installation torque values that have been published earlier were based on bolted threads. It is also vital to recognize the V-Flange tooling design flaw and the negative effects of over and under torqueing the retention knobs during installation.
As they are subjected to more stress than any other tool used in CNC milling, retention knobs are not intended to last forever; therefore, any knobs showing wear should be replaced. It is always better to replace knobs before a catastrophic failure occurs, so if the knobs are exposed to multiple shifts, heavy cuts, or excessive side-pressure, they should be replaced more frequently.
Typically, knobs should be replaced based on their time in service. JMPP recommends replacement after three years if you’re running a single shift, after two years if you’re running two shifts, and after a year if you’re running “lights out” or three shifts. Also, retention knobs should not be removed from a toolholder that has been in service and re-installed in another holder. This process will flatten and distort the threads, allowing for unintentional over-torqueing of the knobs.
Patented high torque considerations/conclusion
While many competitive retention knobs on the market recognize toolholder expansion (or swelling), they have not taken any significant measures to eliminate the inherent V-Flange tooling condition. This continues to cost shops significantly as less than 75 percent taper contact with the spindle results in poor T.I.R. (runout), poor tool life, poor tolerances, vibration and chatter, poor finishes, and excessive spindle wear and tear.
JMPP’s patented High Torque design was intended to eliminate (or vastly reduce) toolholder expansion caused by the torsional stress exerted on the thinner wall of the small end of the holder by the knob threads. Uniquely, it provides for a relief beneath the precision pilot, which forces the threads to engage deeper in the toolholder bore, where a greater material cross-section resists deformation. The difference is crucial in terms of today’s time, tooling, and production results.