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MegaFORCE Retention Knobs: For High-Torque Machining

Get the most secure hold with MegaFORCE Retention Knobs

little devils putting stress on the pull stud

Earlier this year we released our MegaFORCE High-Torque Retention Knobs, designed specifically for high-speed machining. Since then, we have been excited at some of the great feedback we have gotten on how it out-performs standard pull studs. When developing the MegaFORCE, we wanted to truly examine the issues that cause retention knob failure as speeds and feeds increase. The MegaFORCE has been designed specifically to resolve the issues that lead to imbalance and breakage.  MegaFORCE Retention Knobs provide a more secure hold between spindle and holder, for longer tool life and better overall performance. 

Why Retention Knobs Fail

points of weekness on a retention knbPull studs encounter catastrophic failure as a result of metal fatigue caused by a number of reasons including: poor choice of base material, engineering design, machining process, poor heat treatment, and, sometimes, they have just met or exceeded their service life. Also, the repetitive loading and unloading cycles that the retention knob goes through is a significant source of stress that can cause fatigue and cracking at weak areas of the pull stud. 

The most common failure points for a retention knob is at the top of the first thread, and the underside of the pull stud where the grippers or ball bearings of the drawbar engage and draw the toolholder into the spindle. 

For the same reason we put corner radiuses on end mills, sharp corners are a common area of failure for any mechanical device. The same holds true with your pull studs:  The sharp angles on the head of the retention knob and at the minor diameter of the threads are common locations of catastrophic material failure.

Remember, bigger Radii are stronger than sharp corners. 

ROLLED THREADS VS. CUT THREADS

microscopic level view comparing the geometry of rolled vs cut threads

The first image shows how a cut thread has a higher coefficient of friction due the the cutting process. Image 2 shows how rolled thread has a lower coefficient of friction, which means that it engages deeper into the toolholder bore when subjected to the same torque. The cutting method tears at the thread material, creating small fractures, which become points of weakness and lead to tool failure. Rolled threads have burnished roots and crests that are smooth and absent of fractures common in cut threads.

Rolled threads produce a radiused root and crest of the thread and exhibit between a 40% and 300% increase in tensile strength over a cut thread.  In the cold forming process, the thread rolls are pressed into the component, stressing the material beyond its yield point. This causes the component material to be deformed plastically, and thus, permanently. There are three rollers in the typical thread rolling head that maintain better concentricity by default than single point cutting of the threads.  

Also, unlike thread cutting, the grain structure of the material is displaced not removed. Rolled threads produce grain flows that follow the contour of the threads making for a stronger thread at the pitch diameter which is the highest point of wear. The cold forming process also cold works the material which takes advantage of the nickel work hardening properties of 8620. By comparison, cut threads interrupt the grain flow creating weak points. The Techniks MegaFORCE retention knobs feature rolled threads that improve the strength of the knob by 40%.

Upgrade to MegaFORCE Retention Knobs

Ultimately, the only thing standing between a job well done and catastrophic failure is the retention knob. MegaFORCE Retention Knobs are designed to deliver superior performance and enhanced safety for the critical connection between your machine spindle and the tool holder.  MegaFORCE Retention Knobs have been manufactured to increase the strength and durability of this critical connection.

Overall Length

MegaFORCE retention knobs feature a longer projection, for deeper thread engagement to prevent swelling. While a deeper thread engagement can help prevent taper swelling, applying proper torque to the retention knob always the best way to reduce taper swelling. An over-tightened retention knob may still cause taper swelling regardless of how deep it engages the threads of the tool holder.

Material

MegaFORCE retention knobs are made from 8620H. AISI 8620 is hardenable chromium, molybdenum, nickel low alloy steel often used for carburizing to develop a case-hardened part. This case-hardening will result in good wear characteristics.  8620 has high hardenability, no tempering brittleness, good weldability, little tendency to form a cold crack, good maintainability, and cold strain plasticity.

Blended Radii
With the new MegaFORCE pull studs, stress risers of sharp angles have been eliminated through the blended radii on the neck where the gripper engages under the head of the pull stud.  

Ground Pilot
There is a ground pilot, underneath the flange, which provides greater stability. The pilot means the center line of the tool holder and pull stud are perfectly aligned.

Magnetic Particle Tested
Each MegaFORCE retention knob is magnetic particle tested to ensure material integrity and physical soundness. MegaFORCE retention knobs are tested at 2.5X the pulling forces of the drawbar

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