Considerations for toolholders in micro machining applications
“The trend of miniaturization in consumer and industrial products today require microscopic features, says Stephen Veldhuis, director of the McMaster Manufacturing Research Institute and Micro Manufacturing Laboratory, and associate professor of the University’s Mechanical Engineering department.
Veldhuis spoke with Shop Metalworking Technology in 2013, but his statement reflects the ongoing demand for small parts with tolerances of less than 100 micron and sub-micron ranges. And small parts require application-specific stable machines, cutting tools and toolholders.
“Many factors need to be considered when purchasing holders for micro machining,” says Mike Roy, sales manager, Ontario, Tooling and Workholding for Schunk Intec Corp., Mississauga, ON. “Run-out accuracy, vibration dampening, balance quality and flexibility. All are dependent on the specific application and process.”
An important consideration is repeatability, which has a significant effect on process stability and precision machining. “To achieve exact results, the toolholder should have a concentricity and repeat accuracy of less than 0.003 mm and a balance grade of G2.5 at 25,000 rpm,” says Roy.
There are different types of toolholder interfaces, but HSK is considered the best type for micro machining. One of the main reasons, according to industry suppliers, is because the HSK toolholder has excellent run-out conditions compared to other types of toolholders due to the dual contact interface design.
Indeed, toolholder design is an important consideration when selecting toolholders for micro machining. “Design influences vibration, run out, repeatability and lifespan,” says Thomas Mittmann, whose company, Mittmann Inc., represents WTO toolholders in Canada. “At WTO, bearing geometry and bearing quality are important for high rigidity, and a labyrinth seal prevents coolant from entering the toolholder.”
For Schunk, its polygonal clamping technology is key to the success of its toolholders for micro machining applications. Polygonal clamping technology offers advantages compared to collet-style holders, says Roy. “We use the elasticity of the steel to clamp the cutting tool. The toolholder is ground out of round (polygon shape) and we apply pressure at the three points of the polygon to make it round. Once it is round, we insert the cutting tool and then release the pressure. The elasticity of the steel springs back to its original state to clamp the cutting tool.” And since the clamping system has no moving parts, it makes the holders unsusceptible to mechanical damage and eliminates maintenance and wear over time, adds Roy.
Here’s a snapshot of some toolholders for micro machining applications.
BIG Kaiser: Mega Micro Chuck
Designed for micro drills and micro end mills, the toolholder features a slim nut with a 10 mm (.394 in.) diameter and taper design to prevent interference and provide access into tight locations. The notch-free design prevents vibration and noise and offers what the company describes as “superior” balance and conentricity. The line of toolholders accepts tool shanks down to 0.45 mm (0.0177 in.). The new Mega Baby chuck holds shanks as small as 0.25 mm (0.0098 in.). The clamping range has been expanded to 8 mm capacity and the company now offers a sealed nut for directed coolant delivery for 6S and 8S body sizes.
Haimer: Mini Shrink Chuck HSK-E25
The shrink-fit version of the company’s HSK-E25 series of toolholders provides runout of less than 0.003 mm. The company says the slim and short design is suitable for micro machining applications. It has no disturbing edges so difficult-to-access jobs are now penetrable. To improve ease of use, the company offers the Power Clamp Nano, a shrinkfit machine that supports solid carbide and HSS tools in diameters from 3 to 16 mm.
Schunk: Tribos-RM
Tribos-RM is a precision toolholder for micro machining with an ISO interface. The one-piece, rotationally symmetric design ensures long life of the machine spindles and the complete transmission of power, claims the company. Compared with heat shrink clamping, the Tribos-RM features steady tool clamping that Schunk says will not distort the toolholder. It offers a run-out accuracy of less than 0.003 mm, suitable for small tools for micro machining. It’s designed for high spindle speeds of up to 60,000 rpm. Available in sizes ISO 10 D 1-6 mm. The balancing grade is G 2.5 at 25,000 rpm.
WTO: Thread Whirling Unit
The Thread Whirling Unit with Quick Change System from WTO enables users to change the cutting tool holder from the backside of the tool. The unit can stay in the machine and doesn’t need to be realigned. The system is designed with up to 12 blades to increase productivity and reduce vibrations. The quick tool change system allows for changing of the blades outside of the machine. And because of the open interface of the thread whirling unit, insert blades from several cutting tool suppliers are available. It has a nominal diameter of 12 mm and blade repeatability of a maximum +/-0.005 mm.
