Ground vs Rolled Ball Screws: Does manufacturing method matter?

There is much debate among manufacturers,and sometimes among users,about the superiority(or lack thereof)of ball screws with ground threads over(or lack thereof)threads formed by rolling.Ground ball screws are traditionally the choice for high-precision applications,while rolled ball screws provide an economical solution for general industrial and automation applications.But over the past few decades,manufacturing technology has changed,and ball screws are no longer the"pepper grinder" device that some users used to experience.Regardless of which side of the"ground versus rolling"debate you most agree with,keep these three things in mind when determining which side to use in your machine or process.

The rolling process forces round screw stock through dies that form the threads by pushing material out of the way

Grinding cuts away material in the screw stock in order to form threads

1.The DIN/ISO and JIS specifications distinguish between two types of ball screw accuracy:"P"(precision)and "T"(transport). 

The lower the number,the higher the accuracy,with accuracy grades from P0 to P5 and transport grades from T5 to T9 (T10 for JIS).The JIS specification indicates accuracy class,with the prefix "C" indicating accuracy and "Ct" indicating transportation.

A common misconception is that accuracy levels specify manufacturing methods,but the two are not intertwined.Rolled screws can be made into P5 or even P3 precision,some ground screws only meet the T precision requirements.The important thing to know is whether the lead error is V300 according to the manufacturer's specifications,piling up on the length of the screw.The P accuracy class does not allow lead error accumulation,while the T accuracy class does.

2.Geometric tolerances are also specified in DIN/ISO and JIS standards.

For grinding ball screws,both thread grinding and journal grinding are done using the same reference center,making it easier to minimize radial runout and keep the thread and end journal concentric.

When making screws by rolling,the end journal is machined and ground after rolling the threads,so it is more difficult to maintain concentricity and runout.However,if the ball screw is manufactured to DIN/ISO or JIS standards,whether it is manufactured by rolling or grinding,it meets not only the lead precision specification,but also the geometric specification. 

3.The rolling and grinding processes produce different surface finishes. 

Rough surface finish is problematic in ball screw assemblies because it leads to higher friction and more wear on the load-carrying balls.The grinding process produces a very smooth surface finish and,in theory,the rolling process as well.But in practice,the rolled screw must also be polished to remove the oxide layer formed during the rolling process.Thus,when considering surface finish,the comparison is not between a ground and a rolled surface;it is actually located between the grinding surface and the polished surface of the rolled screw.The quality of the polishing step,not the rolling process,determines the surface finish quality of the rolled screw.

Threads of a ground ball screw.

Threads of a rolled ball screw.

As with most design standards,the decision about which type of ball screw to use comes down to performance requirements and cost.When ball screw accuracy below P5 is required,a ground screw is necessary because the rolling process cannot produce these accuracy levels.But for P5,and in some cases P3,the precision of both the rolled and ground screws can meet the required specifications.For accuracy grades 7 and 9(10 to JIS),both manufacturing methods can produce ball screws that meet DIN/ISO or JIS standards.

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