When it comes to positioning accuracy, the ball screw of choice is a machined or ground screw. It's common knowledge that this process delivers screws with tight tolerances, which translate into precise linear positioning. A cold-formed ball screw, on the other hand, has wider tolerances and is thus considered an economic choice when positioning accuracy is not crucial.
But the days of living in the shadow of ground screws may be over for cold-formed screws. In fact, thanks to a new cold-forming technique developed by German researchers, ball-screw precision (and hence, the selection process) will cease to be a matter of construction. Instead, selection will be based on the specifications set forth in the ISO 3408/3 standard.
A matter of degree
Machined screws, the traditional choice for precision, offer variation tolerances, or lead accuracy, of about 0.012 mm within a 300-mm travel and a cylindricity over the total screw length of less than 0.008 mm. Because of the machining involved, these are usually the most expensive option.
Traditional cold formed, also called commercial grade or transportation- type, ball screws cost less to manufacture. But they offer, at best, a lead accuracy of 0.052 mm. Even so, by using the capabilities of high-end servo and step motor systems, a cold formed screw can position a machine tool accurately. However, lead accuracy is not the only important requirement these screws must meet.
They must also maintain torque on a pre-loaded ball nut to prevent backlash or chatter during reverse motions. This torque is a function of cylindricity, or the consistency of the ball track diameter. Because cylindricity is difficult to control in cold forming, these ball screws have a tendency to chatter.
A revolutionary precision coldforming process promises better control during manufacture, plus it is repeatable, enabling batch ball screw production. The manufacturing costs are comparable to traditional coldformed screws, and tests prove that the process can consistently yield ball screws with the following tolerances.
• Permissible travel variation of 0.008 to 0.013 mm within 300 mm travel. This matches the variation found in machined screws.
• Permissible travel variation within useful travel length, or accumulated error, of 0.062 mm over a 4- m useful travel length.
• Concentricity, or "drunkenness," of 0.008 mm.
• Cylindricity of the ball screw track diameter from 0.003 to 0.008 mm.
• Roundness of the ball track diameter from 0.003 to 0.006 mm.
In with the new
Now, with the new cold-forming technology, machine tool builders and motion-system designers have more options. They can choose from machined, traditional cold-formed, or precision cold-formed screws, making better trade-offs in cost and accuracy according to the needs of the application.
Tolerances of the cold-forming process are well within the specifications required for most machine tools, and therefore, negate the old distinctions between "transportation" screws and ground "positioning" screws.
Furthermore, while transportation screws usually require external linear feedback devices to precisely measure travel distance, positioning screws usually do not, nor do precision coldformed screws.
In terms of travel deviation, transportation screws accumulate lead error, so the farther they travel, the larger the deviation. Precision coldformed screws, on the other hand, maintain tolerances independent of distance traveled as do ground screws. In either case, if you want to program a motion profile, the accuracy of the programmed move will be more predictable.
As for backlash, transportation screws are known to have problems. And depending on the application, say in machine tools, the amount of backlash can be unacceptable. In contrast, on a precision cold-formed screw the pre-loaded ball nut maintains drag torque as well as a ground screw.
What's more, an advantage precision cold-forming has over machined screws is that it produces a smoother surface. During grinding, the cutting tool can expose "corns" along the surface. These corns, which are hard, crystalline molecules within the steel, can mar the surface of bearing balls, and potentially shorten ball screw life by up to 20%. The new coldforming process does not expose corns.
Two motions are better than one
The key to fabricating precision cold-formed screws is a CNC-controlled cold-forming machine with two movable dies. Each die is controlled by a synchronous ac drive with micron accuracy. The CNC commands each drive to position its die, and thus, controls pressure along both sides of the screw. In this way, the machine maintains a consistent center axis along the screw.
The traditional cold-forming process can't hold such tolerances because the machine uses a single movable die that presses against a fixed die. So there's more potential for error as the screw's center axis drifts as the forming tools move.
Guiding the choice
Now that it's possible to produce transportation ball screws with machined- screw accuracy, there needs to be a new way to describe tolerances. Rather than specify the process by labeling a screw as machined or cold-formed, the industry can simply follow ISO guidelines for tolerance classes.
Precision ground screws used for machine tools are usually rated at Tolerance class P3. Ball screws rated for less stringent classes may require more torque to move the load, which may be unacceptable in some applications.
To meet ISO guidelines for obtaining a particular rating, a manufacturer sends a report to ISO indicating an individual screw's lead accuracy and torque variation. Class P3 ball screws must have the qualities listed below, regardless of the manufacturing process. Because precision cold-formed screws can meet these specifications, they can receive P3 classification.
• Permissible travel variation of 0.012 mm within 300 mm travel. • Permissible travel variation within 2π travel (one revolution) of 0.006 mm. • Tolerance on useful travel (on a 3-m screw, for example) of 0.050 mm within 3,000 mm travel. • Permissible travel variation within useful travel (on that same 3-m screw) of 0.034 mm within 3,000 mm travel.
Additional class specifications cover radial runout, which indicates straightness. The two most important runout dimensions are the bearing journal and the nut journal, parameters t6 and t10 in the ISO guidelines. Good runout ensures alignment between the bearing supports and the ball nut.
For machine tool applications, one of the most critical specifications is repeatable drag torque on the preloaded ball nut, parameter t12 in the ISO guidelines. For the nut to maintain drag torque within permissible deviations, the ball track diameter – the component of cylindricity that most affects operation – must be as consistent as possible. Excessive diameter variations create a "tighter" or "looser" fit for the ball nut.
In this way, the tolerance classes can provide a universal standard of measure. Designers who purchase according to these classes will know exact ball screw specifications. And they won't have to worry about different interpretations from various suppliers.
Heinz-Robert Schneider is Technical Director at the Wolfschlugen, Germany office and Jack Grooms is Manufacturing Engineering Manager at the Warner Electric Marengo, Illinois plant.