The need for higher feed rates for high productivity machining has driven spindle speeds to levels thought impossible at the start of the modern CNC era twenty years ago.
Turning cutting tools faster has productivity benefits but also introduces new problems in toolholding as the physics of rotating masses become dominant. It’s just not enough to tighten a collet’s drawbar with a box wrench anymore.
Today, balance, form factor and multiple modes of vibration are some of the factors that must be considered if an operation needs to take advantage of the maximum performance offered by today’s machines and cutting tools.
Critical Tool Attributes for Good Holding
Does the cutting tool affect holding ability? More than many machinists and engineers realize. The amount of available shank for gripping is a significant factor.
It’s important not to forget the other factors.
Polished shanks reduce friction, as does the cleanliness of both the shank and tool holder. Oil and coolants reduce gripping power. Cutter shank roundness is often assumed to be close enough to perfect to ignore, but in reality a 25 millionths tolerance is necessary for high-speed performance.
Similarly, ISO h6 tolerance of the tool’s shank diameter is important, which for smaller tools can mean 0-6 micron repeatability. Most shops lack the ability to check low micron diameter tolerances, so a good tool supplier is essential here.
The Importance of Vibration
Vibration is a fact of life in dynamic systems. Whether it’s a tuning fork or an airplane fuselage, with enough energy input, multiple modes of flex will result in unwanted movement of the structure.
For tools spun at high speed, the problem gets worse. Centrifugal forces are set up by the imbalance of the rotating tool holder/tool assembly, which is caused by imperfect mass distribution around the spin axis. Even a set screw offset a few millimeters from center can cause a noticeable vibration at high speeds.
It’s a perfect storm: double the spindle speed and the forces quadruple. Since there’s no practical way to make the tool holder larger in diameter, the key to control is to minimize the offset mass. The easiest way to achieve this without dynamically balancing the assembly is to use intrinsically symmetrical tool holders.
Side Lock? Slow Speed Ahead
Traditional side lock end mill holders are inexpensive, quick to set up and operate, but have several disadvantages. With a TIR of .001”, poor vibration damping and weak gripping power, this traditional toolholding method is restricted to roughing operations at low speeds.
Shrink Fit: More Complex, But Better Performance
Shrink fitting tools would seem like the perfect toolholding technology: full contact with strong clamping forces.
Shrink fit tools are a good choice for moderate to heavy milling and have good speed capability, but the gripping forces are dependent on the tolerance between the tool shank and the holder inside diameter. Heavy wall holders also have superior gripping forces. Tool breakage is a frequent factor in overall shrink fit tooling costs, since broken tool shanks can’t be removed from the holder.
Set up time is necessarily slow with shrink fit systems as holders must be heated and cooled before use. This adds safety considerations due to burn hazards as well as the need to stock a large number of tool holders to maintain productivity.
Runout is five times better than side lock clamping and high-speed capability is excellent, but poor vibration damping characteristics means that the full potential of high-speed milling is limited.
Collet Chucks: Old Idea, Great for High Speed Machining
Most machinists start their training by screwing collet chucks into manual vertical knee mills.
Like a mechanical pencil, collet holding is simple, easy to understand and can generate good wedging action for strong clamping. For more sophisticated high-speed milling, however, there’s another advantage to collet-type clamping: symmetry.
This natural static balance helps reduce the vibration problem at high speeds, making collet chucks ideal for high feed/speed and finish milling. Compared to shrink fit clamping, tool setup is easy, fast and doesn’t require special fixtures or tools. Especially important is the runout characteristic of collet clamping: twice as good as shrink fit and 10 times better than side locking tool holders.
Collets are essentially a series of concentric wedges acting on the tool shank and like any wedge, the higher the applied forces, the greater the force multiplication clamping the tool.
Milling Chucks: All-Around Performance
For applications requiring fast tool changes combined with high grip strength, milling chucks are a good solution.
Milling chucks operate by cam action of multiple rows of needle bearings to apply consistent clamp forces on a collet.
The inclination of the needles is the key to the gripping force of a milling chuck. Like a shallow taper wedge, each element trades off fast action for a strong wedging effect. When combined with multiple elements in a multi-row needle bearing, this gives ample gripping force over a large area of a tool shank.
Larger tool diameters allow more needle bearing elements for even greater clamping power. High retention force combined with a simple twist-to-lock operation makes milling chucks ideal for general purpose operations. Runout, however, is reduced compared to collet chucks, but is still better than double the performance of side lock systems.
Hydraulic Chucks: Low Runout, Fast Tool Changing
Is there a way to combine the quick change capability of side locking systems with the excellent runout and speed of collet clamping?
Hydraulic clamping offers both, with the added benefit of a wider clamping range using straight collets.
The mechanical advantage of hydraulics offers consistent clamping force with little operator-to-operator variation and without special fixtures or tools. Hydraulics are ideal for finish milling, reaming and drilling operations.
Hydraulic clamping combines the best of the other methods with outstanding runout specs and is essential for precise, accurate finish work.
So which clamping method is ideal for your application? It depends on the most critical attributes for your operation.
For precise work, a low TIR may be important. For multiple setups, quick change capability may be essential for high productivity. Tool retention force may be the weak link in a tough job.
Which Is Best?
Again, it is highly task-specific and no single article can cover the full range of problems and solutions associated with advanced toolholding for high-speed milling.
A good tool holder manufacturer has trained personnel and technical expertise that are as important as product price and performance to high throughput shops. They’re a wealth of money saving knowledge and should be consulted for demanding jobs.
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