Why Tolerance Is Minus On Bars For Machining

January 29, 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?

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…


8 Reasons to Stress Relieve Cold Drawn Steel Bars

October 2, 2012

Here are  8 reasons why you might want to consider stress relieving the steel before machining your parts.

  1. High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
  2. Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
  3. Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
  4. Long parts. Stresses tend to display  and their effects increase longitudinally.
  5. Assymetric parts– and parts with large differences in section or mass.
  6. To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
  7. To decrease mechanical properties. At higher stress relieving temperatures, hardness, tensile strength and yield strength are reduced while % elongation and 5 reduction of area are increased.
  8. To reduce distortion off the machine. Usually stress relieving is used as a last ditch effort to reduce the distortion  that presents after machining a part with some or many  of the characteristics given above.

There are certain applications where stress relief (of steel) is indicated

Stress relieving is a lower than  the material’s critical point thermal treatment also known as strain drawing, strain tempering, strain annealling, strain relieving, or pre-aging. It is performed to modify the the magnitude and distribution of of residual forces within a cold drawn steel bar, as well as to modify the mechanical properties.

Thanks Seth at Sixthman Blog for the photo.