{"id":7318,"date":"2020-07-31T17:37:20","date_gmt":"2020-07-31T14:37:20","guid":{"rendered":"https:\/\/fractory.com\/?p=7318"},"modified":"2024-01-26T15:26:04","modified_gmt":"2024-01-26T13:26:04","slug":"engineering-tolerances","status":"publish","type":"post","link":"https:\/\/fractory.com\/engineering-tolerances\/","title":{"rendered":"Engineering Tolerances"},"content":{"rendered":"

In mechanical engineering, tolerances set the allowable deviation from assigned dimensions. The use of tolerances helps to ensure that the final product is readily usable, especially if it is a part of a larger assembly.<\/p>\n

Not setting a tolerance in a critical area may render the part unusable according to the design intent, as each fabrication method comes with a certain level of inaccuracy.<\/p>\n

However, pinpointing a suitable tolerance makes sure that the manufacturing company knows to tackle a few specific points in the production process with more attention. This can be the difference between perfectly mating parts and scrap metal.<\/p>\n

What Is Tolerance in Engineering?<\/h2>\n

Engineering tolerance is the permissible variation in measurements deriving from the base measurement.<\/p>\n

Tolerances can apply to many different units. For example, the working conditions may have tolerances for temperature (\u00b0 C), humidity (g\/m3<\/sup>), etc. In mechanical engineering, we are mainly talking about tolerances that apply to linear, angular and other physical dimensions.<\/p>\n

But regardless of the unit, a tolerance states an acceptable measurement range from the base point (nominal value).<\/p>\n

Let’s say you are designing a sieve to separate 3.5 mm pebbles from 2.5 mm pebbles. You want the smaller pebbles to fall through the holes while keeping the larger ones on the sift.<\/p>\n

The larger pieces of rocks vary in size between 3.3 mm and 3.7 mm. The smaller ones are in the range of 2.3…2.7 mm.<\/p>\n

To ensure that only the smaller ones, all of them, will actually fall through the holes while keeping the larger ones on the sift, you can set the nominal value for the hole diameter as 2.8 mm. At the same time, manufacturing accuracy will mean that you may end up with some holes at 2.6 mm.<\/p>\n

Adding a lower limit of -0 mm and an upper limit of +0.3 mm guarantees that all the holes will be between 2.8 and 3.1 mm in diameter.<\/p>\n

Dimension Tolerances<\/h2>\n

As machines can not perform to perfection, the final dimensions of a product will definitely vary from the stated measurements. For example, a 15 mm hole on a drawing may end up 15.1 mm for laser cut parts<\/a>.<\/p>\n

So let’s see what you can do to make sure that the deviations are in the direction you would prefer them in. For this example, we are going to use linear dimensions.<\/p>\n

Nominal Value<\/h3>\n

\"Nominal<\/p>\n

The nominal value is the basic dimension you usually give on a drawing. Without specifying the allowed tolerances, manufacturers will try to stay close to the value but there will be some sort of deviation as machine capabilities, setup, machinist competence, etc. all play a role.<\/p>\n

Lower Deviation<\/h3>\n

\"Lower<\/p>\n

Adding a lower deviation tells the manufacturer how much smaller a certain measurement can be. This is noted using the “-” sign.<\/p>\n

When making the part on the drawing, a measurement between 99.5 and 100 mm is acceptable. Anything under or above is not within the set limits.<\/p>\n

Upper Deviation<\/h3>\n

\"Upper<\/p>\n

Upper deviation is the exact opposite of lower deviation. Adding it shows how much larger a measurement can be compared to the nominal value.<\/p>\n

So the final measurement can be anywhere between 100 and 100.5 mm according to the tolerance limits on the drawing.<\/p>\n

Bilateral deviation<\/h3>\n

\"Bilaterail<\/p>\n

A third way to give a tolerance range is by using bilateral deviations.<\/p>\n

The drawing states that 99.75 is the minimum acceptable dimension and 100.25 mm is the maximum. Thus, the total<\/strong> “room for error” is still the same – 0.5 mm – but it can go either way from the nominal value by 0.25 mm.<\/p>\n

A founded question here might be – is there any difference between a nominal value of 99.5 mm and an upper limit of +0.5 mm and a nominal value of 100 mm and a lower limit of -0.5 mm?<\/p>\n

Now, if the manufacturer has made a box full of parts that fit into the range of 99.5 to 100 mm, they can send the parts out in both cases. So at this stage, there is essentially no difference.<\/p>\n

However, the production partner will take the nominal value as the main reference point to strive for during the manufacturing phase. Thus, the 99.5 +0.5 mm box will likely contain more parts with a measurement of 99.6 mm and the 100 -0.5 mm box will come back with a larger portion of parts having a measurement of 99.9 mm.<\/p>\n

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