Holier than Thou: Precision Holes by Drilling, Boring, and Reaming

Summary

This video explores the key differences between the ASME Y14.5 and ISO GD&T systems, focusing on the measurement and inspection of cylindrical bores. It explains the two-step process of using a two-point measuring instrument followed by a ‘go’ pin gauge for diameter limits. The ASME system automatically applies the envelope principle, ensuring proper fit, while ISO relies only on size measurements. The video also highlights the modifiers that can make ISO and ASME standards equivalent. Essential for anyone designing precision parts, the video provides valuable insights into tolerances and fitment in engineering design.

Takeaways

• 😀 The ASME Y14.5 GD&T system requires two-step measurement for cylindrical bores: a two-point measurement at various depths and angles, followed by a 'go' pin gauge for the minimum diameter limit.
• 😀 If any measurement is outside of the tolerance limits, the feature is considered out of spec and the part may be scrapped.
• 😀 The envelope principle (Rule 1) ensures that a bore will freely assemble with the mating part by considering both size and straightness issues.
• 😀 The same two-step measurement process applies to outside diameters (ODs), where both two-point measurements and a ring gauge must pass for the part to be in spec.
• 😀 The ISO GD&T system differs from ASME by not including the envelope principle by default, relying solely on two-point measurements at different angles and depths.
• 😀 Under the ISO system, a feature can be in spec by size but still not fit with the mating part due to the lack of straightness control.
• 😀 The envelope modifier can be added to ISO drawings to apply the same interpretation as the ASME system, including the envelope principle.
• 😀 The independency modifier can be added to ASME drawings to exclude the envelope principle, treating the diameter as per the ISO system.
• 😀 The video emphasizes understanding the differences between ASME and ISO systems, as well as when to apply or exclude the envelope principle depending on the requirements.
• 😀 The content concludes with a reminder to subscribe for future videos and mentions additional resources and engineering tools available in the online shop.

Q & A

What is the purpose of the two-step inspection process in the ASME Y14.5 system?

The two-step inspection process ensures that a cylindrical bore or outside diameter is both within specified tolerances and capable of fitting with its mating part. The first step involves measuring the feature at various depths and angles with a two-point instrument, while the second step checks the minimum diameter limit using a 'go' pin gauge, ensuring the feature passes the envelope principle.

What does the envelope principle refer to in the ASME Y14.5 system?

The envelope principle, also known as Rule 1, states that a feature must not only meet the size tolerances but also allow for assembly with the mating part. The 'go' pin gauge ensures that the part can pass through the minimum diameter, guaranteeing proper fit without binding.

How does the envelope principle impact part assembly?

The envelope principle ensures that a bore or outside diameter, when measured at its minimum size, will still fit together with its mating part without interference. Without this principle, parts could be within size tolerances but fail to assemble correctly due to issues like straightness or alignment.

What is the key difference between the ASME and ISO GD&T systems regarding the envelope principle?

The ASME system applies the envelope principle by default, meaning parts must meet both size tolerances and fit with the mating part. The ISO system, however, only requires that two-point measurements fall within tolerance and does not inherently account for straightness or fit, which can lead to parts being in-spec by size but still not fitting properly.

What can be done in the ISO system if the envelope principle is needed?

In the ISO system, the envelope principle can be applied by adding the envelope modifier after the tolerance, ensuring that the feature is evaluated similarly to the ASME system, considering both size and fit.

What happens if a measurement falls outside of the tolerance limits during the inspection process?

If any measurement taken during the inspection process is outside the tolerance limits, the feature is considered out of specification and cannot be used. This ensures that only parts that meet the exact requirements for size and fit are accepted.

Why is the envelope principle particularly important when measuring bores?

The envelope principle is critical when measuring bores because it ensures that the bore can accommodate the mating part, even if it is near its minimum diameter. Without it, a bore could be technically within size limits but still have issues like binding or interference due to straightness or other dimensional deviations.

What is the significance of using a 'go' pin gauge in the inspection process?

The 'go' pin gauge is used to verify that the feature passes the minimum diameter limit, ensuring that the part will fit correctly with its mating component. This step is essential for confirming that the part is not just within size tolerance but also functional for assembly.

What is the main goal of applying the two-point measurement method in GD&T?

The two-point measurement method ensures that the part's dimensions are within the specified tolerances at various depths and angles. This method checks that the part’s size is consistent and within the acceptable range, which is necessary for both functionality and manufacturability.

What is the relationship between straightness and the envelope principle in part fitment?

Straightness plays a crucial role in part fitment because if a feature is near its minimum diameter but has straightness issues, it can cause the mating part to bind or not assemble correctly. The envelope principle ensures that the part’s dimensions are not only within tolerance but also permit proper assembly without binding.