Thread Gauges for Blind Holes: How to Specify Depth and Clearance

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Which Thread Gauge Should You Order for a Blind Hole?

This guide addresses straight threaded blind holes. Taper pipe threads and other specialized connections require their own applicable gauging methods.

1. Separate Thread Depth from Drilled Hole Depth

A drawing may specify drilled depth, full-thread depth and an entry feature such as a countersink or counterbore. These dimensions describe different features. Do not use one as a substitute for another when ordering a gauge.

Identify exactly where the required depth begins and ends. Send a section view showing the entry, threaded portion and bottom geometry so the manufacturer can review the inspection requirement.

2. Decide Whether You Need a Limit Check or a Depth Reading

Some applications need a visual check that the required thread depth has been reached. Others require a recorded numerical depth result. State which outcome your inspection plan requires.

A depth notch or groove can provide a visual reference when it is designed for the part and its datum. A dedicated thread-depth measuring arrangement may be appropriate when a numerical result is required. Confirm the proposed method, measurement capability and acceptance criteria before ordering.

3. Define the Reference Surface

Depth needs an agreed reference. Specify whether the measurement starts at the top face, a spotface, a counterbore shoulder or another defined surface.

Vermont Gage’s discussion of depth measurement challenges illustrates how a counterbore can hide a thread-gauge notch and make the indication difficult to interpret. The gauge feature must be designed around the intended reference and visibility.

Ask the supplier to show the reference relationship on the proposed gauge drawing. This gives both parties a clear basis for review.

4. Check Reach, Entry and Bottom Clearance

Provide the entry diameter, counterbore depth, available handle clearance and any nearby obstructions. A gauge that matches the thread designation may still be unsuitable for the physical inspection location.

The review should also consider the gauge’s leading geometry and the available clearance beyond the required thread length. Contact with the hole bottom must not be mistaken for proof that the specified thread depth has been verified.

Do not grind or shorten a gauge on the shop floor to make it reach. Request an approved configuration and the appropriate verification of modified features.

5. Example: A Thread Below a Counterbore

Illustrative example: a component has a recessed internal thread, and the drawing defines minimum full-thread depth from the counterbore shoulder. A gauge with an indication referenced to the top face would need to account for that geometry.

Before manufacture, the buyer and supplier should agree on the reference surface, depth interpretation, gauge reach and readable indication. Ordering only the thread size and “long handle” would leave the main inspection requirement unresolved.

6. Send a Complete Blind-Hole Gauge Enquiry

Include the complete thread designation, applicable standard, drawing revision and required gauge members. Provide the minimum or maximum thread depth as applicable, its tolerance and the reference surface.

Add a section view showing the drilled depth, entry features and available bottom clearance. State whether the inspection occurs before or after coating, and whether you need a pass/fail indication or a numerical result.

Request confirmation of the proposed configuration and the documentation covering any special depth features. A general thread calibration report should not be assumed to verify every added feature.

Buyer Questions

Does a standard GO thread plug gauge measure thread depth?

Not automatically. It needs an agreed depth reference or a separate suitable measurement arrangement to verify a specified depth requirement.

Can the NO-GO member be used to check minimum thread depth?

Do not use it for that purpose unless an approved method specifically requires it. Its thread-limit function is different from verifying the required full-thread depth.

Should I order a longer gauge for every blind hole?

No. Determine the required reach from the part geometry. Excess length does not resolve an undefined datum, unreadable depth indication or insufficient bottom clearance.

Can a part photograph replace a section drawing?

A photograph can explain access and handling, but usually cannot define hidden geometry or depth tolerances. Provide a controlled section drawing or an approved dimensional specification.