Anatomy of a CNC Coiler

What to look for in the various machine elements when choosing a compression spring coiler

By Takashi Takumi, Shinko Machinery Co. Ltd.

Advancements in CNC technology have had a big effect on spring machines. Small, low-cost servomotors and high-efficiency controllers have brought the age of multi-axis machines. Fast, easy setup has helped spring companies who lack skilled operators. CNC machine operators can now decide tool positions simply by inputting the numeric data. Spring machine manufacturers have developed various types of CNC equipment, providing springmakers with an abundance of choice. It goes without saying that fast and easy setup is very important, but this should not be the main reason to choose a CNC coiler.

Spring manufacturers often tell us that compression springs are the most difficult to produce, and customer requests are increasingly demanding. Compression springs have simple shapes, but customers’ drawings include many specifications, such as diameter, number of coils, ends, load, free length, pitches, burrs and flaws. For example, some springs must meet more than two specified loads, squareness must be less than 0.5°, and burrs must be minimized. Spring manufacturers must meet their customers’ specifications, along with adding their own input to stand up against the competition and make profits.

Figure 1: Wire feeding structure.

What is the most important feature to consider when selecting a CNC coiling machine? What is more important than fast and easy setup? The answer is simple: high-quality springmaking. High-quality springmaking means making high-quality springs consistently. CNC machines need to have reasonable mechanical structure, and the structure must work better under CNC technology. Compression springs have many particular specifications. However, the basic machine elements are wire feeding, coiling, cutting and machine rigidity for consistent production (repeatable accuracy). This article will explain the general function of these elements, and use a Shinko CG machine (pictured above) as a specific example.

Wire Feeding

Three things are important for wire feeding: First, wire feed accuracy; second, optimum feed roller pressure; and third, machine rigidity to handle high-volume springmaking with a small index and/or oil-tempered wire. To feed wire accurately, movement of the servomotor for wire feeding must be delivered accurately to feed rollers. Gears can have play or backlash, and thus may cause a gap between motor and roller movement. The CG series1, uses accurate reducers for low backlash, along with a minimal number of gears.

Weak pressure can cause wire slippage. Strong pressure can deform wire shape. Only optimum pressure can make high-quality springs. Springmakers must pay attention to the number of feed rollers. If a small number of rollers is equipped, each roller’s pressure may be too strong. A display showing pressure values and repeatability is helpful. It is best to use as many rollers as possible to distribute necessary pressure. For example, the CG60 has four pairs of feed rollers. Scale is used for repeatability, and load cells help operators know pressure values and find trouble areas.

Consistent production comes from machine rigidity. Capacities of servomotors are not everything. Careful attention to machine structure is important. An all-in-one body, box-type frame is recommended. This style provides stability during high-speed machine operation or when cutting thick wire. The CG series machine structure of wire feeding is shown in Figure 1, page 11. Taper roller bearings are used in units to press feed rollers. The bearing is much stronger than normal ball bearings. For example, the load rating of taper roller bearings for a 40mm (1.575") shaft is 61.0kN, while that of normal bearings is only 29.1kN (Figure 2, right).

Coiling

Coiling is making an accurate diameter. There are two methods: one-finger (one-point) coiling and two-finger (two-point) coiling. In one-finger coiling, the edge of the wire guide, arbor and coiling pin support the coiled wire. In two-finger coiling, the edge of the wire guide and two coiling pins support the coiled wire. CNC machines usually adopt a two-finger coiling system because CNC technology works better in controlling two coiling pins than controlling a decided shape of arbor in one-finger coiling.

Figure 3: Best coiling-pin positions for various spring diameters.

Figure 2: Taper roller bearings (right) are stronger than regular ball bearings (left).

Picture 2: The upper and center slides are for right-hand coiling, and the lower and center slides are for left-hand coiling.

Figure 4: One servomotor controls two slides connected by a lever.

Figure 5: One-finger coiling, track of optimum coiling pin position.

Figure 6: Servo cutting allows operators to adjust the horizontal position of the cutter.

Figure 8: Operators input spring specifications, and the computer generates the development data.

Making an accurate diameter in two-finger coiling requires three things: First, the direction of both coiling pins must be exactly toward the center of the coiled spring. Second, the wire must be coiled on the center of the coiling pins. Third, the cutting point must be on the centerline of the coiled spring. Figure 3, below, shows the track of optimum coiling pin positions for different spring diameters. When considering spring-back, as you can understand from the curved track, the coiling pin and upper coiling pin have to move with a different ratio to keep their best positions. If not, then coiled wire departs from the center of the coiling pins (wire can be formed with precision only on the center of the coiling pins), and the cutting point departs from the centerline. If the cutting point is not on the centerline, the wire can be pushed inside. Coiling diameter then becomes inaccurate, and repeatable accuracy is not expected.

Our CG series has three coiling slides to ensure diameter accuracy (Picture 2, below). The upper and center slides are used for right-hand coiling, and the lower and center slides are used for left-hand coiling. When you change the coiling direction, you only have to change the center-slide tool holder. (If there were only two coiling slides, changing coiling direction would become hard work.) The three slides are individually controlled, so they can be kept in their best positions according to diameter. Ball screws and two rails are used for the three slides.

Even in CNC machines, some makers still use a cam slide system. Cam and slide can become misaligned under high-speed operation and make inaccurate diameters. Some machines control two slides with one servomotor using a connected lever, as shown on the right side of Figure 4, page 13. In this method, the machine does not control each slide individually according to the coiling diameter. The coiling pins are not toward the center of the coiled wire, which makes accurate coiling difficult. In another system, the upper and lower slide can move vertically and horizontally. This system can control each slide individually, but at least four servomotors are necessary. Moreover, because slide directions and load direction from the coiled wire are not the same, such machines can have a problem with rigidity.

Some operators prefer one-finger coiling for some springmaking. The track of the optimum coiling pin position in one-finger coiling is shown in Figure 5, below.

If the pin moves horizontally away from the best track, the pin is not toward the center of the coiled spring. It is easy to understand that accurate-diameter springs cannot be made in this case. A center slide is used in the CG series for one-finger coiling. The vertical position of this slide can be adjusted, and the sliding direction can be changed. Even conical spring coiling by one-finger is available.

Cutting Method

Cutting method is always important for springmakers. CNC coilers usually have three types of cutting methods: straight, torsion and rotary.

Straight cutting has been most commonly used since the days of mechanical machines.

Torsion, or twist, cutting involves making a tool mark from one side and breaking the wire from the other side. This is good for burr-free cutting. However, this method can be used only for oil-tempered wire with a small index (usually less than five).

Rotary cutting is the third method, but it still has problems. First, the cutter can hit the edge of the wire guide, coiling pin and push-pitch tool in small-diameter coiling. Second, wire is cut crosswise, so its cross-section area becomes so big that the load to machine and arbor is much greater than with straight cutting. Moreover, the cut edge is especially sharp in thick-wire springs. Sharp edges of springs are easy to break and can be harmful when placed in an assembly. (Imagine that compression spring trying to rotate when installed.) Third, adjustment of the cutter and arbor positions is more difficult than with straight cutting. This is because the cutting line is adjusted in straight cutting, and the cutting point is adjusted in rotary cutting.

A fourth method, servo cutting2, has been developed for the CG series to solve these problems. (See Picture 3, page 13 and Figure 6, above left.) CG operators can adjust the track width of rotary cutting and the horizontal position of the cutter. Therefore, according to operator craftsmanship or spring specifications, the cutting angle can be changed.

For example, if operators need to avoid a cutter clashing with other tools, they can make the track width smaller. If they need to avoid an inside burr, they can change burr direction by using the wide width of the track. Horizontal adjustment of the cutter provides the best clearance between cutter and arbor. Moreover, the straighter the cutter track is, the easier it is to make the arbor’s shape or adjust the arbor (Figure 7, top right).

Operating Software

CNC machines enable operators to decide each tool position by inputting data. What are the important features of multi-axis CNC machine software for high-quality springmaking? For starters, servomotors must work exactly according to the input data. In addition, mechanisms to make full use of electronic performance are very important. Ball screws are a good example. This component is much superior to cams for CNC technology. Moreover, each servomotor must be perfectly synchronized. If these two points are well developed, users can enjoy high-quality springmaking and consistent production.

Of course, functionality is also important. Quick and easy programming or modification of data improves productivity. Figure 8, page 15, shows the CG series operating software. Operators input their spring specifications, such as wire diameter, outside coiling diameter, active coils, total coils and free length. Then, the computer automatically generates the development data. Development data includes two charts: one is the number of coils and outer diameter; the other is the number of coils and pitch. Sequential line graphs for each data set are shown on the left side of the screen. Operators make final and fine adjustments at this point. For example, if they need to make a second coil diameter bigger, they just change the outer diameter data of the specified coil point. Operators can divide the program up to 30 steps, so they can finely adjust the first active coil, pitch stroke and end coil to achieve customers’ specifications of load or squareness. In addition, maximum speed is automatically calculated for the tool movement of each step (in other words, the movement of each motor). Operators do not need to find the best speed through trial and error.

Our software includes a quality-control system. Not only free length but also diameter data can be monitored and controlled during automatic machine running. The CG series can check free length and diameter, display the data, and adjust tool position if necessary. Time-series quality data (CpK3, good and bad quantity, etc.) can be displayed as well as quality-control data spring by spring.

Conclusion

Competition in the spring industry is becoming more global and increasingly challenging. Spring manufacturers have to consider their strategic domain. We at Shinko Machinery hope we can contribute to our users and the industry as a whole through communication about springmaking. Development of valuable equipment must start from knowing how to make high-quality springs.

Takashi Takumi is the managing director of the Shinko Machinery Co. in Osaka, Japan (www.shinko-mach.co.jp/e-top.html). Takumi manages overseas marketing and sales. Readers can contact him by phone at +81-6-6794-6610, fax at +81-6-6794-1025 or e-mail at ttaku@shinko-mach.co.jp.

Notes
1. CG Models: The CG series has five different sizes of products, as listed below:
• CG-12 for 0.016" - 0.047" (0.4mm - 1.2mm) wire.
• CG-20 for 0.035" - 0.091" (0.9mm - 2.3mm) wire.
• CG-30 for 0.055" - 0.126" (1.4mm - 3.2mm) wire.
• CG-40 for 0.079" - 0.158" (2.0mm - 4.0mm) wire.
• CG-60 for 0.118" - 0.236" (3.0mm - 6.0mm) wire.

The maximum wire size is available with a spring index of four (music wire). Push pitch and wedge pitch are available in all models.
2. Servo-cutting is a patent of Shinko Machinery Co. Ltd., (U.S. patent No. 7,055,356 B2).
3. CpK is the capability to process kurtosis. It is an indicator of capability and centering of process variation.