Machinists know that toolholders are critical elements in the precision machining process, but the details that differentiate a “standard” toolholder from one designed for more precise, high-speed work can be subtle. Like the parts they are used to create, precision toolholders are machined to exacting tolerances to create the optimal fit within the spindle, concentricity, and balance.  The more precise the work, the more these factors come into play.

If not properly machined, a toolholder that doesn’t fit securely in the spindle or is improperly balanced can vibrate, wobble, or fret, leading to out-of-tolerance parts, oversized tapped holes, and inconsistent, unrepeatable results.

As such, there is value in studying the elements of fit, balance, application of coolant, and other aspects of precision toolholders.  Doing so may be critical to maximizing the speed and accuracy potential of precision. 

AT3 certified tapers — The toolholder is the essential connection between the machine and the cutting tool.  The toolholder fits into and is secured by the spindle, and in turn it secures the cutting tool (such as a drill or end mill) by clamping onto its shank. Toolholder tapers often are conical, including CAT and BT taper specifications. A different kind of taper, HSK, is not tapered at all, but instead includes a variety of flanges for locking the toolholder securely in place.

For free-releasing toolholders with tapered flanges, the quality of the fit between the spindle and the holder is critical for accurate location and proper hold. 

Maintaining firm contact — Unlike toolholders that require a drawbar, free-releasing designs must maintain firm contact over the entire conical surface, so the forces of drilling can be resisted and the tool remains in the spindle when idle.  A holder that is too loose causes the tool to wobble or fret, creating imperfections in the end product and reducing tool life.

“If the toolholders don’t seat in the machine properly, you can’t make a good product,” offered Ron Fulks, tool room manager for National Oil Varco.  The company manufactures reciprocating pumps used in midstream oil and gas transportation.

According to Fulks, he began purchasing precision toolholders for several CNC boring mills after switching from tension compression tapping to a more rigid approach. “The tapping procedure is very important, because if there is runout it will make the tap cut bigger, so your hole won’t gauge,” he said. 

The items he purchased included tapered toolholders for the taps as well as straight ID shank toolholders for end mills for several CNC boring mills. 

To ensure the toolholders would fit optimally, Fulks said he selected products developed by Collis Toolholder, which manufactures and distributes over 4,500 tools and toolholders. 

For its tapered toolholders, Collis exceeds industry “AT” standards for a precise fit in the spindle.  These ANSI/ASME standards (ASME B5.50-2009) indicate the tolerances that must be met in relation to the taper angle. Because most CNC machine spindles are manufactured to an AT2 tolerance, toolholder manufacturers often provide tapered products that are “AT3 or better.” 

Collis products undergo a process of quality control testing to certify and document that they meet the specification. 

For its tapered toolholders, Collis exceeds industry “AT” standards for a precise fit in the spindle.  These ANSI/ASME standards (ASME B5.50-2009) indicate the tolerances that must be met in relation to the taper angle. Because most CNC machine spindles are manufactured to an AT2 tolerance, toolholder manufacturers often provide tapered products that are “AT3 or better.” 

Collis products undergo a process of quality control testing to certify and document that they meet the specification.