What size tooling holes do you use?

Design and Layout Design for Assembly
1

I’m curious to know what size tooling holes you typically use, especially when the application is on breakaway assembly rails.

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2

The most common tooling hole size recommended for breakaway assembly rails is 0.125 inches (3.175 mm), which appears to be widely accepted in the industry. Many fabrication specifications I’ve come across suggest using this diameter.
Additionally, I’ve seen panel drawings from contract manufacturers that specify other tooling hole sizes, including 3.96 mm, and 5.5 mm, for breakaway rails.

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3

The standard tooling hole size 0.125”.

4

The only tooling hole sizes I’ve seen used were determined by the requirements of either the pick-and-place/through-hole insertion machines or the ATE/ICT testers: 0.125 in and 0.156 in. These tolerances have worked well for both applications.

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5

In many cases, low-cost prototype shops, particularly those in China, favor smaller tooling holes—typically 2 mm (2.05 mm drill size)—paired with a 5 mm breakaway tab.

For improved alignment, a pin and slot arrangement is often a good option. This involves using one close-fit hole for the primary alignment pin, a slotted hole for a diamond pin to account for tolerance variation, and additional oversized holes if further board stability is needed. This approach ensures reliable alignment while accommodating manufacturing tolerances.

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