Hello
On a plane, for a given surface, I have a thickness tolerance of 0/+0.25mm. At the same time, I have a flatness tolerance of 0.1mm (for each side). Although geometric and dimensional tolerances are not the same thing, and here the flatness will constrain the dimensional, I wonder why have a greater thickness tolerance than the flatness tolerance. Thank you in advance for your feedback.
Hello
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Hello
This is because one is included in the other, otherwise they cannot be cumulated. The flatness tolerance is to be considered in relation to the maximum tolerance of the sheet.
For the flatness of a surface, it must be between planes P1 and P2, parallel, spaced 0.1 mm apart.
Both of these tolerances are shape tolerances.
The thickness tolerance indicates the maximum value of the deviation tolerated for the plane P2 from the plane P1 which is for example the reference. Logically, the orientation tolerance with parallelism should be taken into account, but on rolled sheets (before machining) there is never a measurable parallelism defect.
So it is the dimensional that conditions the flatness tolerance in the maximum dimension and not the other way around.
To understand this, you have to look at the GPS rating (but be careful, aspirin is not provided).
Kind regards
Hello @ Ac Cobra 427
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Hello
I understand your answer but why does the flatness have to be done in relation to the maximum thickness? Is this a rule of operation? Normally we apply the tolerances independently of each other, and here the flatness necessarily constrains the thickness tolerance, right?
Hello
Zozo_mp gives you the right definition.
Regardless of the size of your thickness, within a tolerance of 0/+0.25mm, the flatness will always be between two planes spaced 0.1mm apart.
For example, for a dimension of 10mm 0/+0.25, these two planes will be at least between 10 and 10.1mm, and at most between 10.15 and 10.25mm.
For me, these are 2 different things:
- the dimensional tolerance determines the position of an ideal plane;
- The geometric tolerance determines that the actual area must be at a certain deviation from the previous ideal area, but the actual area cannot exceed the dimensional tolerance.
The predominance of one tolerance over the other depends on values.
Attached is a small diagram (I have multiplied the values by 10 to make it easier to read). The purple zone can wander between the 2 lines at 25 without being able to get out.