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Measuring Wall Thickness of Gun Drilled Metal

The term gun drilling is applied to the process of drilling a hole along the center axis of a solid rod or bar that has a known outside diameter. Conversely, the hole may be drilled and the outside is machined for consistent wall thickness around the circumference. Occasionally, drifting of the drill will cause wall thickness eccentricity. It is important in the gun drilling process to monitor the position of the drilled hole to ensure that drifting has not taken place.
With ultrasonics it is possible to measure eccentricity of wall thickness by scanning around the circumference of the barrel or by manually placing an ultrasonic transducer at various locations around the barrel. Either method will provide the wall thickness information required to ensure that the gun drilled hole is within concentricity specifications. Measurements with ultrasonic gauges are nondestructive, accurate and dependable.

Equipment: Manual measurements to monitor eccentricity in gun drilling can be done with small ultrasonic thickness gauges. Wall thickness and diameter of the barrel ultimately determine the selection of the instrumentation. In general, the Model 38DL PLUS and 45MG gage with Single Element software are well suited for wall thickness measurements. 

As a rule, size of the contact transducer should increase with the size of the barrel. In some cases when the diameter of the drilled hole is small compared to overall diameter (and the wall is thick), it is necessary to use a focused immersion transducer coupled through a water column. Because of the many variables involved, it is recommended that actual samples be tested in order to select the correct instrument and transducer combination. The most challenging situations involve large diameter parts with very small drilled holes.

Scanning of the barrel may improve speed of measurements and increase the total surface that can be monitored. Scanning for eccentricity is frequently done with immersion transducers. Small diameter barrels can be scanned for eccentricity by using the RBS-1 Recirculating Bubbler System. Large diameter barrels need special fixturing.

Olympus IMS

Products used for this application


The handheld 45MG ultrasonic thickness gage is packed with measurement features and software options. This unique instrument is compatible with the complete range of Olympus dual element and single element transducers, making this gage an all-in-one solution for virtually every thickness gage application.


The 38DL PLUS is an advanced ultrasonic thickness gage. Uses dual element transducer for internal corrosion applications, and has features that include THRU-COAT technology and echo-to-echo. Uses single element transducers for very precise thickness measurements of thin, very thick, or multilayer materials.
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