Research+development

New developments in the area of cutting systems

March 2016 — Compression springs for the automotive sector are a matter of precision. Additional process steps have often been required in order to reach this level, as common cutting systems present disadvantages when it comes to one of the key parameters: burr formation, operating range, cutting angle, and cutting ellipse.

Over the past few years, different cutting systems have been developed on the basis of the straight cut. The original aim behind this development was to reduce burr formation. An initial approach involved the development of the rotary cut. The fixed cutting ellipse of the rotary cut quickly resulted in a trade-off between a narrow ellipse, which prevents collisions, and a wide ellipse which optimizes the feed speed of a flying rotating cut. The torsion cut was the next to be developed, but this had the disadvantage of a limited operating range. The information below describes patented new developments aimed at avoiding this trade-off and minimizing the central disadvantages. First, there is an overview of the established cutting methods.

1. Comparison of existing cutting systems

a) Straight cut

b) Optimized straight cut

c) Rotary cut

d) Optimized rotary cut

e) Flying rotating cut

f) Torsion cut

2. New developments in cutting systems

a) New torsion cut with predetermined breaking point

aa) Aim and functional description

ab) Comparison of output, cutting energy, and noise emission

ac) Operating range and process limits of DH wire

ad) Application example: stainless steel 1.4310

b) Programmable rotary cut: Multi-E cut

ba) Cutting angle with different ellipse widths and wire materials

bb) Burr height with different ellipse widths and wire materials

1. Comparison of existing cutting systems – overview

a) Straight cut

The straight cut is the standard cutting method for spring coiling. The cutting tool cuts in a vertical motion, as shown in Fig. 1, and the cut is made as soon as the feed rollers stop. The burr forms toward the center of the spring; see Fig. 2.

b) Optimized straight cut

In the case of the optimized straight cut, the vertical cutting motion is slightly staggered and starts earlier, before the feed rollers stop. This shortens the distance that the cutting tool travels before making contact. The shorter distance reduces the total cycle time and, hence, increases the output by around 25%.

c) Rotary cut

In the case of the rotary cut, the cutting tool is moved – unlike the cutting tool of the straight cut – in an elliptical motion; see Fig. 3. Again, the cutting motion only begins once the wire feed cycle has ended. In contrast to the straight cut, the burr forms toward the center of the wire in the case of the rotary cut – see Fig. 4 – and there is less burr formation with this type of cut.

d) Optimized rotary cut

Again, it is possible to maximize the output by shortening the cutting distances. As is the case with the straight cut, the cutting tool needs to be positioned just in front of the contact point with the wire during the wire feed process. This reduces the total cycle time. Changing from the rotary cut to the optimized rotary cut also results in an improvement of around 25% in tests.

e) Flying rotating cut

The elliptical cutting motion of the rotary cut is retained but is carried out at the same time as the ongoing infeed process, as opposed to the conventional/optimized rotary cut. This reduces the total cycle time by around a further 20% and increases the output again to the same extent.

f) Torsion cut

The torsion cut involves two consecutive vertical tool motions in opposite directions; see Fig. 5. The conventional torsion cut makes it possible to minimize burr formation; see Fig. 6. An oil-tempered wire (Si content not less than 1.2%, spring index (C) equal to or less than 3.5) must be used in this case.

Conclusion: Torsion cut minimizes formation of burrs. The operating principle of the torsion cut places particular requirements on the spring geometry and the wire properties, and therefore limits the application possibilities.

2. New developments in cutting systems

a) New torsion cut with predetermined breaking point

aa) Aim and functional description

The aim is to extend the operating range of the torsion cut by creating a predetermined breaking point during spring production. Firstly, the predetermined breaking point is created at the inside of the spring by a servomotor-driven displacement of the mandrel (see Fig. 7, SK2; 154). The spring is then clamped from the top and the predetermined breaking point is created at the outside of the spring (Fig. 7, SK1; 152). The two predetermined breaking points are positioned at the same level. Finally, the spring is cut from below in the same way as the conventional torsion cut (Fig. 7, 172).

ab) Comparison of output, quality, cutting energy, and noise emission

The section below compares the outputs of different cutting systems. It then goes on to compare the conventional torsion cut to the new torsion cut with predetermined breaking point with regard to spring quality, cutting energy, and noise emission.

Comparison of the outputs of different cutting systems:

Predetermined breaking points result in an output reduction of approximately 15% in comparison with the conventional torsion cut. The outputs with rotary cut and straight cut are identical – indicated with * in the graph; see Fig. 8.

Note: This spring cannot be produced with the conventional torsion cut.

Test results regarding quality and process limit 1: Limit of the conventional torsion cut

In the case of the conventional torsion cut, when the spring index C = 3.5 is exceeded, the last coil is plastically deformed by the cutting movement. It does not lie against the adjacent coil and the spring is not dimensionally accurate. In contrast, the predetermined breaking points of the new torsion cut reduce the deformation of the spring during the cutting process. This reduced deformation is entirely elastic and can be neutralized by the spring. This ensures that the spring end is dimensionally accurate; see Fig. 9.

Test results regarding quality and process limit 2: Limit of the torsion cut with predetermined breaking point on patented-drawn DH wire. The springs shown in Fig. 10 do not break due to the torsion cut in the tests; the wire breaks as the subsequent coil is being wound.

Conclusion: The spring index alone does not provide enough information to assess feasibility – the material specifications and wire diameter must also be taken into account.

Test results on cutting energy: Compared to the straight cut, the energy required per cycle was 27% lower for the torsion cut with predetermined breaking point without wire, 47% lower with 5mm wire, and 19% lower with 7 mm wire; see Fig. 11. Of the total energy required per cycle, the cutting energy proportion is between 5-15%; the high measured values in the result with no wire are also due to the use of 7.00mm tools and their higher weight.

Conclusion: The shorter travel distances of the axes with higher weight – cutting with a straight cut or just clamping in the case of the torsion cut with predetermined breaking point – reduce the required cutting energy even without a wire. Creating the predetermined breaking point in the wire reduces the cross-sectional area that must be cut. This explains why the torsion cut with predetermined breaking points reduces the cutting energy.

Test results regarding noise emissions of different cutting systems: (*)In the tests, the noise level was measured 10 cm away from the cut without a cover and the average value over 100ms was calculated during the cutting process. The different outputs were taken into account. The torsion cut with predetermined breaking points generates less noise than the straight cut (-3dB when d=5.0mm, -5dB when d=7.0mm). The measured values (see also Fig. 12) confirm the perception of the noise level at the machine.

Conclusion: The comparison of noise emissions when making a straight cut in wires of different diameters indicates that the noise emissions are also affected by the diameter of the wire being cut. This increase in noise emissions in relation to the wire diameter is also evident in the case of the torsion cut with predetermined breaking points. Directly comparing the straight cut and the torsion cut with predetermined breaking points at the same wire diameter supports the conclusion that reducing the wire cross-section to be cut lowers noise emissions in the same way as it lowers the cutting energy. Test results regarding operating ranges of different torsion cutting systems and wire materials:

Conclusion: Creating the predetermined breaking points significantly increases the operating range of the new torsion cut (in some cases, the maximum possible spring index can be tripled); see Fig. 13. This means that the general advantages of the torsion cut, such as reduced burr formation, can be brought to bear for a wider spring spectrum. The torsion cut with predetermined breaking points can be used for DH wire starting at a wire diameter of 5.0mm; see Fig. 14. The limit may vary depending on the wire characteristics. Different wire materials result in different operating ranges for the torsion cut with predetermined breaking points.

ac) Application example: stainless steel (1.4310)

With the new patented torsion cut with predetermined breaking point, even stainless steel (1.4310) can be processed up to a spring index equal to or less than 10. Different operating ranges are possible depending on the spring geometry and the processed material.

b) Programmable rotary cut: Multi-E cut

Trade-off: When setting up different springs, the aim is to achieve a cutting ellipse that is as narrow as possible in order to prevent the tools colliding, but at the same time as wide as possible in order to achieve maximum feed speeds.

Solution: The newly developed patented Multi-E cut (patent DE102013207028 B3) enables the width of the cutting ellipse to be freely adjusted and the cutting gap to be modified. An additional CNC axis for the swivel movement at the cutting system, shown in Fig. 17, and integration into the control unit make the system easy to operate. The straight cut can also be used if desired.

Advantages of the Multi-E cut: In the case of small spring indices, the coiling fingers are positioned close to each other. The space conditions in the cutting area do not permit use of a standard rotary cut ellipse. The possibility of adjusting the cutting ellipse width extends the operating range of the rotary cut to include spring geometries with small spring indices. Collisions with the tools are prevented. The CNC swivel axis makes it possible to switch between straight and rotary cuts without making mechanical alterations. Thanks to the variable design of the cutting ellipse, the cutting tool can be moved out of the friction zone as soon as the cutting process is complete. This minimizes the frictional wear on the half-coil of the subsequent spring which is already located in the coiling tools. This in turn increases the tool life. The Multi-E cut makes it possible to respond to the material-dependent nature of the cutting angle and burr formation in a targeted manner. These settings can be saved together with the spring geometry. This means that, once a spring geometry has been produced, it can be replicated quickly when needed again.

ba) Cutting angle with different ellipse widths and wire materials

In the case of high-quality springs, the cutting angle and level of burr formation are becoming increasingly crucial factors in the functionality of the component or assembly. Particular applications may require a small cutting angle, for example, or the option to define a specific angle.

Test results regarding the cutting angle with different ellipse widths and wire materials:

Fig. 18 shows that using the straight cut when processing different wire materials results in different cutting angles. Due to system constraints, the kinematics of the straight cut cannot be used to adapt the cutting angles. With the Multi-E cut, the cutting angle grows as the width of the cutting ellipse increases, i.e., the additional CNC axis enables material-dependent adjustments to the cutting angle via the cutting ellipse in line with customer-specific requirements. A further observable trend is the increase in the cutting angle from patented-drawn wire (DH) to stainless steel (1.4310) to oil-tempered wire (VDSiCr).

bb) Burr height with different ellipse widths and wire materials

In addition to the cutting angles, burr height is also a crucial quality characteristic for the compression springs. Short burrs can be removed in subsequent processes

such as shot peening without any problems. If the burr is more pronounced,

additional processes such as recutting or grinding are unavoidable. The burr height was measured along with the cutting angles in the series of tests described above. When using the patented-drawn DH wire, the burr height is virtually independent from the width of the cutting ellipse and the cutting technology. In contrast, in tests with the materials VDSiCr and 1.4310, the burr height increases as the cutting ellipse gets wider.

Conclusion: Material-dependent adjustments to the cutting angle with the Multi-E cutting method help to minimize the burr height.


Summary

The type of cutting used affects the output of the coiling machine as well as the geometry of the cutting surface. Creating predetermined breaking points significantly increases the operating range of the torsion cut. The torsion cut can also be used for patented-drawn wire materials for dynamic loads (DH) and stainless steel (1.4310), depending on the spring geometry, with a wire diameter d = 5.0mm or more. The amount of cutting energy required is reduced in comparison to the straight cut. At the same time, however, output drops by around 15% compared to the conventional torsion cut.

The Multi-E cut makes it possible to make variable adjustments to the width and tilt of the cutting ellipse and the cutting gap. A large operating range is covered between the straight cut and the wide version of the cutting ellipse. A narrow ellipse extends the operating range of the rotary cut to include spring geometries with small spring indices. Depending on the processed wire material, the settings can be adjusted to influence the target variables of outer burr, inner burr, and cutting angle. The input screens, including simulation of the elliptical trajectory, support the machine fitter and the selected settings can be saved together with the parts program.

Uwe-Peter Weigmann (board member technology) and Klaus Wurster (manager research and development), Wafios AG


Wafios AG
wire 2016, hall 10 booth F 22
Silberburgstrasse 5
72764 Reutlingen/Germany
Phone: +49 7121 146-0
info@wafios.de
www.wafios.com

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