Machinists know that when it comes to precision machining, the importance of a toolholder cannot be overstated. The quality of the toolholder plays an even greater role when precision machining at higher speeds. High-speed machining is typically used in medical equipment manufacturing, where machinists often work with exotic alloys and harder metals like titanium.
However, with RPMs reaching 20,000, 30,000, or even higher, the precise and secure seating of a properly balanced toolholder in the spindle becomes even more critical. At these rates of speed, even minor flaws in toolholder manufacturing can lead to less-precise machining, reduced tool and spindle life, and even damaged workpieces. With this in mind, it is important to understand the crucial role of a quality toolholder, which for tapered varieties boils down to two key factors: fit and concentricity.
Without any holding or locking mechanism, self-releasing toolholders must precisely fit within the spindle with only the smallest allowance to maintain accurate location, repeatability, and proper hold. The other factor, concentricity, refers to the amount of wobble that can occur when the toolholder is rotating or spinning. In machining, this is called the whipping effect and can lead to inconsistent results and out-of-tolerance parts.
Decreased Tool Life and Damage to the Workpiece
Using a less-precise or low-quality toolholder for high-speed machining can also decrease tool life or cause damage to the workpiece.
“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 already must be changed out more frequently as they dull or break. The cost of tool replacement, not to mention loss of production time due to frequent changeover, can add up quickly.
“The higher-end technical carbide inserts really demand a high-precision toolholder,” says Fellin. “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,” adds Fellin. “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.”
Improper toolholders can also cause damage to the work-piece, which would then have to be repaired or thrown out.
“You could be spending hundreds of man-hours designing a tool and then finding out that it is cutting slightly oversized holes, for example, because the toolholder could not hold the tool properly,” says Fellin.
Fellin cautions against purchasing toolholders based on price alone. One way to ensure the toolholder is of high quality is to look for its certification, which should be AT3 or better. AT3 refers to the tolerances related to the fit of the toolholder in the spindle.
Toolholders certified to AT3 or better are subjected to a rigorous process of quality control and metrology testing to ensure that the toolholders meet that specification.
To prevent the wobble or whipping effect from occurring, manufacturers often specify the level of unbalance by a G number with units in millimeters per second (mm/sec). This is why machine-tool spindles and machine-tool parts usually are specified with vibration levels of G2.5 and G6.3. To ensure accurate concentricity, high-quality toolholders are also balanced to the higher G2.5 standards. This enables the tool-holders to run at higher RPMs with less fretting, resulting in more accurate work and better surface finishes.
According to Fellin, in the medical device industry, OEMs are looking for repeatability in each toolholder as well.
“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,” says Fellin. “You can always find someone in the industry that will provide a lower price, but there is usually a sacrifice in terms of quality,” says Fellin.
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