The retention knob (sometimes known as a pull stud) is a threaded, bolt-shaped component that allows the drawbar to hold the toolholder securely during the machining process. The drawbar grabs onto the head of the knob when the toolholder is inserted into the spindle.
Not all retention knobs are created equal. To prevent undue damage to the toolholder, we recommend using high torque retention knobs that distribute stress more evenly throughout the toolholder. We’ll explain the advantages of high torque retention knobs in greater detail below.
Problems With Standard Retention Knobs
To reach full engagement, a standard retention knob typically relies on about four threads to be threaded into the toolholder. The balance of torsional stress is not evenly distributed through these threads: the first thread from the flange creates the most stress, while the furthest from the flange has the least. This places the greatest proportion of torsional stress on the narrowest section of the toolholder’s taper, causing the thin walls to expand and distort the shape of the toolholder.
When the taper is off, the toolholder will no longer properly mate with the spindle, resulting in movement within the spindle during the machining process. This causes a host of machining problems, including:
- Increased harmonic vibration
- Shortened tool and spindle life
- Damage to the toolholder
- Poor finishes
These issues lead to wasted time and unnecessary expenses.
What is a High-Torque Retention Knob?
As illustrated in this video by JM Performance Products, high torque retention knobs are designed to eliminate the issue of toolholder expansion by providing superior stress distribution and overall toolholder balance. The high torque knobs have the same head dimensions as standard retention knobs, with the addition of a precision pilot and relief under the flange that gives them greater length.
High-torque retention knobs also have fewer threads than standard knobs, and the threads are cut to start and finish 180 degrees to each other.
When you switch to properly-sized high torque retention knobs, you should expect to see toolholders last longer, perform better, and produce more precise cutting results.
We trust the high torque retention knobs produced by JM Performance Products, a world-recognized leader in manufacturing high-performance milling machine products. Their product catalog includes over 500 styles of retention knobs, including the high-torque design described above, which are made from hot-rolled 8620H fine-grade steel or 9310 steel.