Tapped Hole Position Tolerance: How Tight Is Enough?

Tapped hole position tolerance is one of the callouts machinists push back on most.

How Tight Should the Position Tolerance Be on a Tapped Hole?

Size it from the screw and the clearance hole in the mating part, not from a habit number. For an M5 screw in a 5.5 mm clearance hole that works out to ⌀0.25 mm (about .010 in), and machinists in the source thread put .005 in as the floor.

Where the question came from

A designer posted a drawing for critique on r/Machinists with a .003 in position tolerance on tapped holes. The top reply called that "really tight" and explained why: the pitch diameter of the thread is what locates the screw, and checking position that tight on a thread takes special gauges. The advice was to open it to .010 in or more. Other replies said around .014 in, or ".005 or larger." One also suggested reordering the datums to C|A|B instead of A|B|C.

Sizing tapped hole position tolerance from the fastener

For a fixed-fastener case (the screw threads into this part and passes through a clearance hole in the other), the common Y14.5 approach is T = (H − F) / 2, where H is the clearance hole diameter and F is the screw's major diameter. The full standard is on the ASME Y14.5 page.

Worked example for an M5 screw in a 5.5 mm clearance hole: (5.5 − 5.0) / 2 = 0.25, so ⌀0.25 mm at each part. Use the actual hole and screw sizes on your drawing, since a looser clearance hole gives you more room and a fitted one gives you less.

A few extras from the thread are worth keeping. Use basic dimensions to locate the holes, and let the position callout carry the tolerance. If one of the mating holes is a slot, it frees up one direction and you can loosen accordingly.

Why tight thread position is hard to inspect

A smooth hole can be checked with a pin or a probe. A thread can't, at least not directly. Measuring where the pitch diameter actually sits takes special gauges, as the thread pointed out, and tight callouts mean more time per hole. Our post on parts that pass the CMM but still don't assemble covers how that goes wrong. For a short run, a first article inspection is the cheapest way to catch it.

At DigiForge

Our capabilities page lists ISO 2768-m as standard for CNC, with tighter work down to ±0.01 mm on features that need it. If your drawing has a tight position callout on threaded holes, or you need an inspection report for them, ask us when you quote.