The Trend Toward Precision Toolholders


Precision Tooling Now Replacing Collet Chucks

For many years, collet chucks have been popular because of their low pricing and have been the established toolholders for many machining applications. Unfortunately, the same scenario happens in many companies: Because of worry over loose tools, many machine operators are tightening the coolant tube nut of the collet mountings with oversized extensions. They also have to repeatedly clean the dirty components of the multi-part toolholders and be aware of how fast the tools wear in the collet mountings.

Because of this, experts agree that precision toolholders offer enormous potential concerning quality, performance, process reliability, and lower costs. Investments in modern toolholders generally pay for themselves in a very short time and ensure future competitiveness in metal-cutting manufacturing.

Two strong arguments for precision toolholders: Quality demands of customers and cost calculation

On one hand, tolerance allowances of modern workpieces become better, on the other hand, cost is increasing which makes it necessary to maximize profit in all areas of productivity. At the very least, when the operator is thinking about automated manufacturing, a higher efficiency or the use of high-quality tools, the toolholding equipment has to be critically analyzed. "Compared to conventional universal clamping solutions, sophisticated and proven precision toolholders have enormous benefits", says Michael Kraft, Product Manager of SCHUNK Germany. "They reduce tool and machine costs, the number of tool changes and adjustments, as well as the costs for maintenance of the toolholder. Moreover, they provide higher efficiency and improved machining quality."

Collet chucks are restricted in efficiency

A collet chuck consists of three components: A base body with a cone, a collet chuck and a coolant tube nut. All three in combination ensure clamping of the tools. At the same time, the chuck is composed of three pieces, each having additive tolerances, which affects run-out quality. As long as the mountings are merely used for simple tasks, this weakness is not essential. With increasing spindle speed and metal removal and increasing demands on precision, the collet mounting has become a critical factor. An uneven cut affects precision, as well as causing higher levels of wear at the cutting edge of the tool. Collet chucks are also very sensitive to dirt and process reliability is adversely affected.

Shorter production times per piece require a safe hold within seconds

Many operators know that tools could fall out during machining because they are drawn out of the collet mounting. With oversized hook wrenches, operators try to minimize this risk by over tightening. However, the collet chucks wear out very quickly if this method is applied. The run-out accuracy of the mountings is reduced and the quality of the workpiece surface and tools are damaged. If the tools have smaller diameters, there is the risk of the tool breaking because it was exposed to high forces and deformations in the collet chuck. Such a mistake is very expensive because even small tools are costly. In contrast to collet chucks, the clamping operation of modern precision mountings is simple. Hydraulic expansion toolholders should be always equipped with a stop for the clamping pressure. This ensures that the toolholders always generate the same optimum clamping force. One turn with a hexagon socket key to the end stop sets the machine up and prevents the chance of over-clamping.

Length adjustment is maintained during clamping operation

When the collet chuck is closed, an unintentional axial movement occurs, which causes a misalignment in the tool, whereby it loses its exact position of the pre-adjustment. This misalignment may be up to 0.6mm.This does not happen with hydraulic expansion toolholders or elastomer expansion toolholders. The expansion sleeve carries out an absolutely radial expansion. The tool is neither pressed out of the toolholder nor is it drawn into it.

Symptom of fatigue

The sliding surfaces of the collet chuck between the collet and the base body are heavily loaded since they are moving towards each other, while a high surface pressure also exerts force. This causes high wear, which can occur after 1,000 clamping cycles and causes a significant decrease in the run-out accuracy. Expansion sleeves in turn are basically wear-free and no loss of the clamping accuracy can be measured before 10.000 clamping cycles.

Advantage of the damped expansion toolholders

In case of rough machining, not only a tight and safe hold of the tool is important, but also are the dampening characteristics of the toolholder. When the tool edges are immerging, vibrations of the tool are generated, which can spread out over the whole machine tool. If these vibrations occur in the right frequency, the frequency of the machine, the vibration can become very strong and damage the tool. Expansion toolholders offer secure clamping and excellent dampening characteristics. The oscillation amplitude considerably reduces vibration and the surface becomes smoother instead of rough and bumpy.

By avoiding high oscillation amplitude, the load of the tool's cutting edge is considerably lowered. Thus the service life is increased and the tool's cost is reduced. The noise is also lowered and the machine's spindle is protected from damage.

Design of the expansion toolholder

Expansion toolholders have many advantages, such as the length adjustment, vibration dampening, and low symptoms of fatigue.

Hydraulic expansion toolholders generate pressure by expanding the inner sleeve and by compressing oil, and elastomer expansion toolholders, compress the volume-stable elastomer mechanically.

Hydraulic expansion toolholders have their strength in true-running accuracy, which especially distinguishes them for the applications such as high-precision drilling and milling. Elastomer expansion toolholders dispose of high transmissible torques in connection with strong dampening characteristics, which make them excellent for rough machining and high material removal rates.

Summary

The economic point of view

By using expansion toolholders, higher feed rates are achieved and in contrast to collet chucks, they are almost maintenance-free. Expansion toolholders are very attractive in price since a considerable reduction in price has taken place in the last few years.

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