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 …
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 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…
Leave a Comment » | Engineering, Shop Floor | Tagged: Bars Must Measure Less Than Nominal Size To Permit Assembly, Bearings, Bushings, Cold Drawn Steel Bars, Cold Finshed Shafting, Dimensional Tolerances On Steel BArs, Less Than Nominal Size, Line Shafting, Power Transmission Shafting, Pulleys, Shafting, Shafts, Tolerances, Vectron, Why Bar Tolerance IS minus, Why Is Bar Tolerance Negative? | Permalink
Posted by speakingofprecision
October 2, 2012
Here are 8 reasons why you might want to consider stress relieving the steel before machining your parts.
- High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
- Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
- Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
- Long parts. Stresses tend to display and their effects increase longitudinally.
- Assymetric parts– and parts with large differences in section or mass.
- To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
- 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.
- 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.
1 Comment | Engineering, Shop Floor | Tagged: Alloy Steel, Assymetric Parts, Cold Drawing, Cold Drawn Steel Bars, Cold Work, Cold Working Strain, Ductility, Heavy Draft, High Carbon steel, Long Parts, Percent Elongation, Percent Reduction of Area, Reduce Distortion During And After Machining, SBQ, Small Diameter Higher Percentage of Cold Work, Stress Relief, Stress Relieve, Stress Relieve To Decrease Mechanical Properties, Stress Relieve to Increase Mechanical Properties, Tensile Strengtyh, Yield Strength | Permalink
Posted by speakingofprecision