Ball Screw Threads – Rolled vs. Ground

There are three common methods to create a ball screw thread profile – rolling, whirling and precision grinding.  But what’s the best one and what are some key differences between them?  To answer that question one first has to understand the nature of the application.

We recently wrote an article on this subject which appeared in Design World magazine:

Basically, for the less precise applications – pick and place, general automation, etc. – either rolled or whirled screws will usually work.  Per ISO, accuracy grades are classified either as “T” for transport (T5 to T10) or “P” for precision (P0 to P5).  There is one crossover point, at grade 5, which is achievable using any of the three manufacturing methods.  However the accuracy grade 5 here only refers to overall lead error  – which often causes confusion because designers think that means a T5 ball screw is exactly the same as a P5 ball screw.  Nothing could be farther from the truth!  Why?  Because the “T” grades, including T5, are not defined in terms of lead wobble, bandwidth and diameter (and thus preload) variations!  These parameters are never mentioned by rolled screw manufacturers, who focus on just overall (that is, mean) lead error.  The reason is in how the shaft is produced.  For precision ground screws, all machining including thread and journal grinding is normally done while the shaft is supported between centers.  One of the first manufacturing steps for a ground ball screw shaft is to machine centers.  This ensures not only minimal run-out of consecutive thread turns as they are ground into the shaft, but also perfect concentricity of threads and other features such as bearing journals.  And thus also more consistent friction torque.  For rolled shafts, the thread is formed first and only then are any centers added.  A precision ground screw will almost always outperform a rolled screw.  Which is the reason why it’s the component chosen for high end positioning applications such as machine tools and other precision stages. Why the “almost” qualifier?  Well, Steinmeyer is investigating some unique manufacturing methods that we hope will yield a rolled screw that is virtually equivalent to a ground screw.  Check back with us frequently for updates.

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