The One Big Flaw in CNC Milling That’s Costing Tool Life

Tool life is one of the areas almost everyone tries to improve in CNC machining. Even if you have the right flow and processes in place, tool life still impacts the cost directly.  Every time you have to stop to replace tools, you’ve cut into your production output and increased the cycle time. Tooling typically makes up an estimated 3% (upwards of 10%+ for aerospace materials) of the total manufacturing cost of a part.  While better tool life is going to save you on tooling expense, the discernable savings is going to have to come as a by-product of tooling life and tooling performance.

In order to achieve the best results from your CNC tooling, you must first identify and eliminate issues that are costing you money in terms of time, tooling, and production. The fact is, there is an inherent design flaw in V-Flange tooling that costs the industry exorbitant amounts of money. That flaw causes a lack of contact between the spindle and the toolholder, therefore, improving the connection between cutting tools and the machine spindle is critical in CNC milling. Additional factors such as the lack of balance/concentricity of the toolholder, as well as improper seating within the spindle, can slow down machining cycle times – dramatically reducing material removal rates and the time it takes to complete each part from start to finish.

L.H. Thomson. (Macon, GA), a company that specializes in titanium machining for the aerospace industry, progressively addressed these issues head-on. By implementing JM Performance Products, Inc.‘s (JMPP) (Fairport Harbor, OH) patented High Torque retention knobs they were able to overcome a critical design flaw inherent in CNC V-Flange tooling–eliminating the toolholder expansion responsible for costly and ongoing CNC milling and boring issues.

Aerospace Milling/Production Challenge

Since 1981, L.H. Thomson has been a leader in creating aerospace parts and assemblies through CNC precision manufacturing. Their lean manufacturing focuses on monolithic commercial aircraft components utilizing demanding aluminum, titanium, hi-temp alloys, stainless steel, high nickel, copper, plastics and composites. Manufacturing these exotic materials creates a unique set of production machining challenges when considering that high speeds, typically in excess of 20,000 RPMs, are often required.

As L.H. Thomson experienced steady growth in the late 2000s, aerospace production at these unheard-of rates of speed began to result in critical damage to expensive workpieces on their Makino and Fadal machining centers. Notably, during a heavy milling operation a retention knob failed during the operation on a Makino a51 horizontal machining center and the toolholder was ejected out of the machine spindle at 12,000 RPMs which resulted in excessive damage to the spindle, tool holder, cutting tool, work piece, and machine.

The failed retention knob was sent to a metallurgist who analyzed the microstructure failure of the material and pinpointed that the culprit, the retention knob, was improperly heat treated, which caused the failure at the minor diameter of the O.D. thread and the coolant through hole by hydrogen embrittlement.

The bottom line for Thomson was that they needed a retention knob that wouldn’t fail to avoid increased costs per part, and negative impact to their ability to move toward their goal of “lights out” aerospace manufacturing capacity. Imperatively, a retention knob solution had to be found to improve the contact, reduce the spindle loads, and increase the speeds and feeds – all while improving the capacity to remove more cubic inches of metal per minute.

Progressive Retention Knob Search/Test/Solution

Daniel Nietzold (now retired), was Thomson’s R&D Engineering Manager at the time and was specifically tasked by then Thomson President, Loronzo H. “Ronnie” Thomson (now deceased), with the objective to search out the best retention knobs available on the market to produce aerospace parts with exotic materials more efficiently. The critical goal was to overcome the retention knob failure and toolholder ejection problem related to their machining centers’ heavy milling of these materials while operating at high metal removal rates.

In his extensive research of the main standard retention knob manufacturers, Nietzold quickly found that all had deficiencies in their designs as it related to the CAT 40 standard. The fact is that CNC milling using V-Flange tooling is inherently flawed by design, as toolholder expansion occurs at the small end of the holder when a standard retention knob is installed. Extensive testing has proven that a standard retention knob has the potential to expand the small end of the toolholder when it is installed with as little as 20 ft/lbs. of torque–with a CAT 40 toolholder beginning to expand at 15 ft/lbs. Of note, the extent of this expansion is variable, depending on the holder, the retention knob, and the installation practice.

While attending the IMTS tradeshow (Chicago, IL), Nietzold met with John Stoneback, President of JM Performance Products, who demonstrated the patented High Torque retention knobs. The uniquely designed knobs were able to resolve one of the most fundamental and long-standing problems in machining–improper seating of the toolholder in the spindle. Since tight tolerances are essential in high-speed machining, if the toolholder doesn’t fit the spindle precisely, decreased productivity and reduced tool life are inevitable.

The key element of JMPP’s patented design is a knob that is longer on the thread side–reaching deeper into the cross section of the holder’s threaded bore. All thread engagement occurs in a region of the toolholder where the cross-section is thicker to resist deformation. Additionally, a precision pilot also increases rigidity, which increases the rigidity of the toolholder itself at higher RPMs–so the knob can increase the tool life.

According to Stoneback, “Critically, by increasing contact with upwards of 70% more spindle surface, a wide range of CNC milling issues are overcome including:  vibration and chatter, poor tolerances, non-repeatability, poor finishes, shortened tool life, excessive spindle wear and tear, run-out, and shallow depths of cuts.”

After returning from IMTS Nietzold quickly tested the new knob vs. the standard knob. First, he torqued the standard knob, cleaned the spindle, blued up the toolholder, and stuck it back in the spindle. He then removed the toolholder from the spindle to see what the contact patch looked like.  He then cleaned up the toolholder and repeated the process with the JM High Torque retention knob. The results were obvious that the tool holders with the JM High Torque retention knobs were making much more spindle contact than the old retention knobs which led Nietzold to move forward with extensive in-house testing.

Nietzold tested the effectiveness by running a 3” diameter high feed inserted milling cutter in the rough milling operation on a large aerospace titanium component that was already in production with a validated process by switching over to the JM High Torque knob. He realized unexpected results as he immediately saw a 30% spindle load reduction. This allowed for him to increase feed rates and depth of cut, which also was able to bring the spindle load back up to 100% load. The end result was a major cost savings due to the increased metal removal rates, plus an unexpected tool life savings due to the better connection at the spindle.

According to Nietzold, “At this point we were highly invested in our machines and top quality tooling, but we were missing out on the connection at the spindle. Fundamentally, the flaw started at the retention knob, and the High Torque retention knobs solved the problems – we were finally able to solve the CAT 40 riddle!”

Nietzold proceeded to convert Thomson’s entire CNC machining center system to JMPP’s High Torque retention knobs which, at the time, included 40-taper Makino and Fadal machines. By changing their standard retention knobs, they also got rid of the chatter, and ended up with smoother, cleaner finishes.

To make certain that puzzle was completely solved, and to ensure production sustainability, Thomson requested additional, value-added processes to be performed by JM Performance Products which included:

Magnetic Particle Inspection Test

Thomson’s old retention knobs had continually experienced hydrogen embrittlement failure at the minor diameter of the thread and the coolant through hole, which produced cracks that could result in toolholder ejection and damage to the toolholders and machines. Hydrogen-induced cracking (HIC) refers to the internal cracks brought about by material trapped in budding hydrogen atoms. It involves atomic hydrogen, which is the smallest atom that diffuses into a metallic structure. HIC is especially important in iron and iron-based alloys because of their importance as structural materials. It is well established that hydrogen not only causes a reduction in the mechanical properties, but can cause a transition in fracture mode.

To make sure there were no HIC flaws in the new High Torque knobs, Thomson asked JMPP to conduct a Magnetic Particle Inspection test on all purchased knobs, which is very 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, the testing proved out JMPP’s High Torque knobs out at a 100% flawless rate.

JMPP Part # Serialization

A retention knob is not intended to last forever as they are subjected to more repetitive stress than any other parts of a mill.  Therefore, they need to be inspected periodically. JMPP custom laser marks the serial and part numbers for all of the retention knobs to help keep track of their time in service for optimal usage. Routinely tracking the knobs service life ensures against retention knob stress failure, which could result in costly damage and down time.

Typically, knobs should be replaced based on their time in service.  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. JMPP has serialized over 1,000 of their knobs for Thomson over the last seven years.

Machine & High Torque Progression

L.H. Thomson boasts a 65,000-square foot manufacturing facility that features a cell manufacturing setup which reduces direct labor, shortens lead time, and requires fewer inventories per project. From their early adoption/conversion to the High Torque retention knobs, to today, Thomson has expanded their machining center capabilities and has implemented over 1,000 of the knobs.

According to current Thomson President, Brian Thomson, “In essence, we exclusively use the High Torque knobs on every tool on our machines.  They have better concentricity, fit exactly, and the tools perform better across the board. Most importantly, with the heavier milling we do with titanium and stainless steel parts, there are no issues with the retention knobs.”

Currently, Thomson implements the knobs in their high RPM performance CNC 40-taper machining centers including: Ten Makino (Mason, OH) a51nx Horizontals, five Okuma America Corporation (Charlotte, NC) Verticals, and a wide array of Fadal Engineering (Ontario, CA) Verticals.

According to Nietzold, “Fundamentally the knobs work as a system that starts at the connection of the spindle and the knob. The benefits are deep and far reaching as better taper connection results in spindle load reduction, increased speeds and feeds, and the ability to remove more cubic inches of metal per minute.”

In conclusion, Brian Thomson stated, “The High Torque knobs work better and faster on our demanding exotic materials projects–allowing us to be more competitive and increase profit margins dramatically.”

Ultimately, JM Performance Products’ High Torque retention knobs are one of the quickest, simplest, and most cost effective stabilizing investments a CNC milling shop can make. According to JMPP President, John Stoneback, “Conservatively, the High Torque knobs can help job shop operations achieve a 10-20% competitive advantage per hour via longer tool life, increased depth of cut, better feed rates, and more rigid tools–reducing tooling cost by 20-50% or more.”

For more information, visit: www.jmperformanceproducts.com