What is the difference between allowance and tolerance?

Question

I have to have a pin fit in a hole, and there needs to be a gap so the pin can turn freely. Can I make the pin smaller by changing the tolerance? Oh, and my parts often have "elephant foot", where they get a little too big sitting on the hot plate.

Answer 1

There are four terms that many people find confusing:

• Tolerance is the amount of random deviation or variation permitted for a given dimension.
• Allowance is a planned difference between a nominal or reference value and an exact value.
• Clearance is the intentional space between two parts.
• Interference is the intentional overlap between two parts.

Use tolerance when specifying the amount of error permitted in making a part. Use allowance when specifying a gap between two mating parts.

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Two other related terms that are important when specifying dimensions for manufacturing:

• Accuracy is the maximum dimensional variation between parts.
• Precision is the size of the steps your machine is capable of.

A machine cannot reliably produce parts with a smaller tolerance than its accuracy. If elephant foot expands your parts by 0.1 mm, then your accuracy is only 0.1 mm. You will not be able to repeatably print parts with 0.05 mm tolerance. (You might get lucky and one out of ten might accidentally fit the dimensions required, but that's not a good manufacturing plan.)

Precision is often confused with accuracy. People think that if their machine is advertised as having 0.05 mm step size (its precision is 0.05 mm) that they can print parts with 0.05 mm tolerance. But if your printer has a precision of 0.05mm and an accuracy of 0.1 mm, you cannot count on it to repeatedly position itself to within 0.05 mm of the desired dimension.

To measure your machine's accuracy, you'll need to print some pins and holes and carefully measure the differences between what you defined and what you printed. The difference between the largest and smallest measurements you take is the accuracy. And be sure to check the accuracy in your X, Y, and Z dimensions; your printer might have a difference between them that would impact the roundness of the parts.

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In the case of the sizes needed to fit a pin in a hole so that it can pivot freely, you need to define an allowance in order to create the clearance you desire.

What is the minimum gap between parts you are looking for, and what is the maximum you can accept? That's the clearance.

Let's say you want a clearance of at least 0.2 mm between the pin and hole, but no more than 1.0 mm. And let's say you measured your printer's accuracy to be ± 0.2 mm. If you print a 5 mm pin, your pin would be anywhere between 5.0 mm ± 0.2 mm, so the hole must therefore be 5.6 mm ± 0.2 mm. The minimum clearance of 0.2 mm would be an minimum sized hole (5.4 mm) and a maximum sized pin (5.2 mm); the maximum clearance of 1.0 mm would be a maximum sized hole (5.8 mm) and a minimum sized pin (4.8 mm).

Note that a clearance of 1.0 mm is really loose, and is just too sloppy for your application. You might think to tighten the tolerances to 0.05 mm in order to reduce the clearance. But since your printer's accuracy can't produce a part that meets your specified tolerances, you would need to find a different way to manufacture or "finish" the parts.

The traditional way to solve the problem of inaccuracy in production is to create the part larger than the maximum material condition, then use a subtractive method to finish it to the desired dimensions.

Say we need the pin and hole to have 0.2 mm clearance, but we've already established that our machine only has 0.2 mm of accuracy. How do we print the parts to fit? We print the hole undersized and the pin oversized by the amount of accuracy in our machine, plus we include an allowance to ensure we always have some material to remove in the finishing step.

Let's establish the hole's final dimension to be 5.0 mm ± 0.05, so we print the hole to 4.7 mm ± 0.2 mm (resulting in a hole that's between 4.9 mm and 4.5 mm). After printing we run a 5 mm drill bit through it to finish it to 5.0 mm ± 0.05 mm.

Then we do a similar operation with the pin. Print it to 5.1 mm ± 0.2 mm (giving us a pin between 5.3 mm and 4.9 mm), then chuck it in a lathe or drill and carefully sand or file it until it becomes 4.8 mm ± 0.05. Now we finally have achieved our clearance of 0.2 mm ± 0.1 mm, which is a good enough fit for our purpose.

These types of secondary finishing steps have been used by craftspeople for hundreds of years.