Tolerance in Manufacturing
In the manufacturing world, tolerance is used frequently since it defines the accuracy necessary to create a functioning product. Tolerance is the range of variation permitted in maintaining a specified dimension in machining a piece.
Imagine a company that builds engines and needs to ream a hole in the cylinder head where the valves reciprocate. They look around and eventually find a company that manufactures reamers for this specific job.
After ordering reamers, their staff inspects the parts and begins to realize that the size of the holes are not consistent — half of them fit and the other half doesn't. This is due to the reamers not being precise enough for the valve to function properly.
When a product is created, it’s never “perfectly” accurate and having a specific tolerance tells us how close to perfect it needs to be. These dimensional tolerances have a high and low limit, denoted as, + and –, respectively. For example, if the diameter of an object is 45mm and the tolerance is written as +.003 and -.003, the diameter is acceptable between 44.997mm and 45.003mm.
Tolerance is important because if the reamer cuts the hole oversize or undersize, the clearance between the valve guide and the valve increases which causes the valve to seat improperly and can generate an engine misfire due to the lack of compression. Additionally, this can also cause increased buildup of hydrocarbon deposits in the combustion chamber, triggering an increase in compression and unforeseen detonation.
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Basic Types of Tolerance
Upper and Lower dimensions are placed on top of each other. The dimensions show the largest and smallest values allowed. This means that any dimension in between these values will be accepted.
The tolerance variation only occurs in one direction. For example, in this case it's going up by +0.005.
Bilateral Equal occurs when variation from the target dimension is shown going both positive and negative directions equally.
Bilateral Unequal is the same as Bilateral Equal, but both directions are going unequally.