Why Tolerance is Minus on Bars for Machining

18. June 2013

The tolerance on cold-drawn steel bars for machining is always specified as plus nothing minus some value. So why are the dimensions on the bars held to the minus rather than plus side? Don’t we want to get more steel per foot for our money? May I have your answer please? The reason for the dimensions being held to the minus side is so that the bars can easily pass through a hole of nominal size.

If the bars were the same nominal size as the hole, they would be difficult to assemble. If the bars were even slighty larger, they would not pass through.

So bars are held to the minus side of each nominal dimension to ensure that they can pass through the nominal size hole, whether it be a bushing, pulley, gear, collet, support bearing or any similar application. The bars must measure less than the nominal hole size to permit assembly.

How did this come to be? Line shafting. The power transmission shafts that ran across the ceilings of shops while being held in bearings were called line shafting. The power was taken from the shafts by belts and pulleys. The shafts were held by bearings affixed to the ceiling joists. The shafting had to fit into these bearings and pulleys. (These shafts were driven usually by a single large motor, steam engine, or water wheel.)

It has been some time since power transmission shafting has been used commercially to drive our lathes and drills commercially.

But we have the legacy of cold finished shafting to thank for the foundational concept of tolerances on bar products being held to the minus side.

Thanks to John Halladay at PMPA technical member Vectron in Elyria, Ohio, for the archival shop photo from the Perry Fay Company.

And if you have a burr problem with some of your production, you can call on Vectron to help you with that, too.

Do you have memories of working with machinery driven by line shafting in your career? We’d love to hear your story.

Originally posted on PMPA Speaking of Precision blog.

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