Why Tolerance Is Minus On Bars For Machining

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?

And the answer is ...

And the answer is …

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 very 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 assure 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?

Before the era of electric motors, power was transmitted by means of shafting.

Before the era of electric motors, power was transmitted to each machine by means of belts  and pulleys running on cold finished shafting.

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 afixed 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 transimission 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 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…

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