What is Tolerance?
One of the questions we receive most frequently is how tolerance will affect the price of a part. To begin answering that question, let’s start with what tolerance is. Part tolerance is an allowable amount of variation of a specified quantity, especially in the dimensions of a machine or part. Generally, tolerance is given in the form of measurement ± tolerance, i.e. 2.0” ± 0.1”. The higher the tolerance, the greater the allowable variation from the desired measurement. Because of this, tolerances are generally inversely proportional to manufacturing cost. To sum up, the tighter the tolerance is, the higher the price and vice versa.
Tolerance and Price
Tight tolerances increase price in three ways. First, the tools required to make a part with tighter tolerances are generally more expensive. Precision machinery and equipment is sometimes required if tighter tolerances are requested. Manufacturers can either purchase expensive precision machinery and include that in their overhead cost or they could outsource to a supplier who owns precision machines. Both increase the price of a part. Second, tighter tolerances add processes in the manufacturing of the part. The extra processes require more machinery, tools, and operations. Finally, tight tolerances additional inspection and testing require to verify that the part is in spec. Such testing sometimes requires inspection of each individual component of a part, with very strict guidelines about rejecting parts that are outside of the required tolerance. Notably, the rejection of pieces usable under more reasonable tolerances causes the cost of manufacturing to skyrocket.
How to Choose a Tolerance
When choosing a tolerance, many people err on the side of being too precise. Choosing a tolerance of ±0.001 vs ±0.01 can increase the price by a factor of two or three. Additionally, tighter tolerances require greater care in fabricating and added inspections to ensure the accuracy throughout the parts. There are certainly times when tighter tolerances are 100% necessary; however, there is no need to drive up the prices of products that don’t need tight tolerances. Most importantly, how the parts fit together, what the parts do, and the manufacturing process and the sensitivity of the features to variation should all be what drives component tolerances.