A turn of the screw

June 20, 2002
Precision screw forming challenges precision machined and ground screws.

By Wolfgang Becker
Ken Wyman
Danaher Motion
Amherst, N.Y.

Edited by Leland E. Teschler

Precision screw-formed ball screws offer precision comparable to ground screws.

Ball screws are generally considered precise enough for positioning applications only if they are machined or ground. Ground ball screws have tight tolerances which translate into high-accuracy linear positioning. But a new process called precision screw forming (PSF) can produce cold-formed, nonground ball screws with precision superior to that of common rolled ball screws.

PSF and ground ball screws have virtually the same accuracy and performance. Ball-screw accuracy is spelled out in a number of international standards including ANSI and ISO 3408. Accuracy is measured as the maximum lead deviation in microns per 300 mm of travel. Tolerance classes range from 0 to 10; the lower the class number, the tighter the tolerance. Precision or positioning screws are generally Class 5 or lower.

Grinding is generally the best way to achieve class 0, 1, or 2 accuracy. But many applications simply do not require those precision levels. On the other hand, rolled ball screws can only offer accuracies from Class 10 to 7 at best. Until the introduction of PSF technology, there was no alternative to ground screws for accuracy better than Class 7.

Traditional rolled ball screws are made on equipment that uses a fixed forming die and a movable one. At worst, the movable die is positioned for a given diameter screw and remains static while the screw is formed. At best, a servosystem dynamically positions the movable die to maintain correct forming pressure and compensate for slight deviations in screw diameter.

But even in the best case, the straightness of the screw axis and the cylindricality of the ball-screw track diameter can't be guaranteed beyond certain limits. Yet, the ability to maintain close tolerances for these critical parameters is exactly what differentiates the performance of a precision ball screw from that of a nonprecision rolled screw. Without good axial straightness and ball track cylindricality the screw may chatter on reversal, exhibit uneven running torque, or have subpar lead accuracy.

In PSF equipment, both forming dies are movable. A CNC servosystem adjusts either or both dies to correct for axial straightness and cylindricality deviations. With this system, lead error is in the range of 8 to 12 µm per 300 mm of travel, single turn variation of 6 µm or less, and accumulated travel error over a 4-meter length is at most 62 µm.

The PSF process can produce ball screws that in some ways are even better than ground ones. For instance, the surface finish of a PSF screw is smoother than a ground screw, resulting in a smoother running screw that produces less audible noise.

Another advantage of PSF screws is the absence of hard crystalline spots in the steel exposed by the grinding process. In ground screws, these spots can mar surfaces and potentially shorten screw life by up to 20%. PSF ball screws are also less expensive, and generally available with shorter lead times than ground screws.

On the other hand, PSF screws are similar or the same as ground screws in many ways. Both types are made of high-carbon alloy steel and are induction hardened and stress relieved. Consistent drag torque, a concern for machine-tool builders, is the same or better for PSF ball screws as for ground screws. Assembly stiffness and service life expectancy for PSF ball screws are also evenly matched.

PSF technology uses two movable dies under CNC servocontrol to dynamically adjust for screw axis and screw diameter variations. Screw tolerances can match those of ground screws.

Traditional rolled-ball-screw machines use one movable and one fixed die to form the screw. A single movable die system cannot completely compensate for deviations of the screw's axis from a straight line.

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