What to do during a part drawing revision

Part drawing revisions are something every engineer would have to go through almost regularly. It actually pertains to the very aspect the part was designed to be. Revision also come in a variety of types: it could be something related to non-manufacturability, part function improvement or just a drawing revision to make the specification a little clearer. Whatever it is, it is very important to know what the implications of your revision are and how to follow up(what to do after the revision).

Generally, revisions can be classified into 2 types

  1. Minor revisions
  2. Major revisions

Minor revisions

These are revisions that do not pertain to part specifications.

Let’s say you spelled your name wrong or you drew the profile of a part a little differently (albeit with the same dimensional specifications) or you want to place the dimensions a little further away from each other for clarity. These are minor revisions. They do not affect your part specifications in any way.

How to follow up?

Usually for these revisions, there is no need for any follow up needed. Revise the drawing and that’s it, you are good to go.

Major revisions

Here is when it gets trickier. A major revision is any revision that affects the specification of a part. Such revisions usually need validation/testing before release of the drawing. This is because you might have validated the part for a different specification, and you need to do so with the new specification also before you release the part/product into the market.

How to follow up?

Here is a case by case scenario of follow up actions. If there are any more that you’d like added, please comment below and they will be added asap.

  • Case 1: Dimension change 10±0.2 to 10±0.1

For this case, if you have already validated the part in the range of 10±0.2, there is no need to evaluate the parts in range of 10±0.1 since ±0.1 is a subset of ±0.2. You are only decreasing the variability here. Generally the conditions only get better with such kinds of changes.

  • Case 2: Dimension change 10±0.2 to 10±0.25

For this case, you need to re validate your parts again. Preferably in the maximum range of ±0.25. This is because ±0.25 is not a subset of ±0.2. With ±0.25, there would be parts out of the dimensional specification of your already validated parts.

  • Case 3: Part was dimensioned 10±0.2 but factory made it with 10±0.3. So requesting change of dimensioning to 10±0.3

These requests are usually received from the manufacturing side. However appealing it is to believe the factory completely, it is better to check the dimension of the sample parts that were validated to know if it was indeed made and checked for the 10±0.3 specification. If not, ask for samples again, this time, the ones that can reach as close to the tolerance boundaries as possible (it is hard to get the part on the tolerance boundaries as the dimensions generally lean to the nominal value so you’d have to trust the factory with giving you the bad case parts) and validate the parts again.

  • Case 4: It is hard to make the parts with 10±0.2 specification. So please change to 10±0.3 for improving manufacturability.

Same as Case 3.

  • Case 5: The parts made with 10±0.1 were broken during assembly. But the factory evaluated that the parts with 10.2±0.2 would be ok. So please change the dimensions to 10.2±0.2.

This happens more number of times than you’d imagine. Essentially this too is a specification change and needs to be evaluated. The thing is that factory is only confirming assemblability this time. As an engineer, you’d have to evaluate functionality and extreme conditions test (temperature, humidity, vibration etc whichever is applicable).

For example, the factory might be able to achieve the desired assemblability with 10.2±0.2 and they’d be perfectly fine with it. But as an engineer when you evaluate it, you’d notice that the free play it too much due to increased gap or that the sealing is not perfect or that the hooks/ribs break or create noise during high vibrations due to the free play. Therefore it is important for you to evaluate it too before you give it a go.

That’s all folks. If there are any more that you’d like added, please comment below and they will be added asap.

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