Precise, secure seating is key to accuracy, longer tool life, and lower machining costs.
Posted by Robert Schoenberger
With high-speed precision machining typically used on exotic alloys and hard metals such as titanium, toolholder quality plays a critical role.
With spindle speeds reaching 30,000rpm or higher, precise and secure seating of a properly balanced toolholder in the spindle is essential. Even minor flaws in toolholder manufacturing can lower machining precision, reduce tool and spindle life, and damage workpieces. For tapered toolholder varieties, two key factors are fit and concentricity.
Without holding or locking mechanisms, self-releasing toolholders must precisely fit within the spindle with only the smallest allowance to maintain accurate location, repeatability, and proper hold.
Concentricity refers to the amount of wobble that can occur when the toolholder is rotating or spinning. Called the whipping effect, it can lead to inconsistent results and out-of-tolerance parts.
Decreased tool life, workpiece damage
“If the toolholder is not concentric or is a little off-center, you will have rubbing, wear, and more friction – which decreases the life expectancy of the tools,” explains Bart Fellin of Fellin Industrial Sales, a company that represents a variety of machine tools and toolholders.
When machining exotic alloys and hard metals, cutting tools must be changed out more frequently as they dull or break, adding tool-replacements costs and adding time and labor to processes.
“The higher-end technical carbide inserts really demand a high-precision toolholder,” Fellin says. “If you end up breaking a tool, it could cause hundreds of dollars’ worth of damage. Not only is the tool expensive, but you have to change it out more often and that takes time. So, when you cost out a job, you may find you are over budget rather than making a profit. It can make or break a deal.”
“You could be spending hundreds of man hours designing a tool and then finding out that it is cutting slightly oversized holes, as an example, because the toolholder could not hold the tool properly,” Fellin says.
Fellin cautions against purchasing toolholders based on price.
“When you have to get a quality part out, and you don’t want it getting rejected, you want to make sure the accuracy is going to be there,” Fellin says.
One way to ensure you are buying a quality toolholder is to look for its certification of spindle tolerance, which should be AT3 or better. Collis Toolholder certifies its products AT3 or better using quality control and metrology testing.
Collis Toolholder also emphasizes concentricity. To prevent the whipping effect, manufacturers often specify the level of unbalance by a G number with units in millimeters per second (mm/sec). Machine-tool spindles and machine-tool parts usually are specified with vibration levels of G2.5 and G6.3. Collis toolholders are balanced to the higher G2.5 standard.
Superior fit and greater balance allows tapered toolholders to run at higher speeds, resulting in more accurate work and better surface finishes.
Fellin says aerospace original equipment manufacturers also are looking for repeatability in each toolholder.
“Being able to know that from the first toolholder they purchase to the fifth to the twentieth, they are going to get the same quality is very important,” Fellin says. “If you are looking for a toolholder, saving a few dollars for a no-name brand is not worth it. It will end up costing you a lot more in the long run.”