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Measurements and Tolerances

Measurement science is its own language. These few notes barely scratch the surface. If you are new to this, or will just be peripherally involved, perhaps as a sales support person, it is suggested that you learn decimal equivalents down to the sixteenths, and key metric sizes, such as 5mm, 10mm, 20mm, 50mm, etc. It will make your life a lot easier.

15/16″ is fifteen sixteenths. Not fifteen sixteens.

.010″ is ten thousandths, .003″ is three thousandths, .0005″ is; half a tenth, or five ten thousandths.

In all methods of measurement tolerances nothing is an absolute single number. Tolerances are represented in a range. It is important to both parties involved in a transaction understand what that range involves. If you are not sure, ask. It may vary from company to company, product to prodcut, or even from material to material. In the case of machine tolerances, the spread may be just a few thousandths. Bar length ranges may involve many feet.

The above is not intended to be all-inclusive but a random exposure to the topic. We will try to broaden the scope in future posts.

August 5th, 2020 – Howard Thomas

Bearing Quality Vs. Bearing Fit

Be careful, the two are often confused between end-user and vendor. They are not interchangeable. BEARING QUALITY refers to manufacturing restrictions that are employed, when the mill makes the steel. It is commonly referred to as “Clean Steel Production”. The processing refines the steel, removing non-metallic inclusions and generally improving the quality and core integrity of the steel. This is not something that can be achieved in subsequent processing; it either is bearing quality, or it isn’t.

BEARING FIT (TOLERANCE), is achieved by subsequent processing of a steel bar or shaft. You may accomplish this at any point prior to use of the bar; do-able on-site locally, or as a specification for subsequent processing accomplished at the mill. This, as stated, refers to the tolerance alone; making no statement in regards to the integrity nor the cleanliness of the material. Bearing Tolerance is referred to as a “minus/minus” tolerance, as opposed to a plus or minus tolerance.

Typically, and depending on diameter, the tolerance would be something like minus .001″ to minus .0015″

A NOTE ON EXPRESSING BAR TOLERANCES, it is common to hear bar tolerances specified as; “Plus nothing minus .002”, or whatever the downside tolerance is. To avoid potential problems, it is better to state both plus and minus terms with a specific decimal position. Make sure that both parties know how far out that “plus” side is carried. Is it measured to the third decimal place, or the fourth? Many “Plus Nothing” bars actually may only be measured to the third (thousandths) place which allows the bar to actually be a plus tolerance. Such as plus .0005″. Note that Bearing Fit or Bearing Tolerance insures a minus tolerance.

You may further avoid potential headaches by specifying the actual diameter wanted in terms of the actual decimal, both over and under (plus and minus), such as .2500″/.2495″.

Whether it is Bearing Quality or Bearing Fit, keep in mind that there may be an additional cost to obtain that benefit.

 

-Howard Thomas, Oct 18th 2018