Operating Manual

Chapter 2: Fastener Preparation
Guide to Ultrasonic Inspection of Fasteners Page 11
Chapter 2: Fastener Preparation
Prior to measuring a fastener, it must be properly pre-
pared for ultrasonic inspection. The fastener ends must
be machined to be parallel and the end that will be mated
with a transducer must be machined to a controlled,
smooth surface finish. Further, to allow for proper cou-
pling of the transducer and fastener, a suitable couplant
must be applied. Finally, consistent placement of the
transducer on the bolt head or stud end improves the
instrument’s accuracy and repeatability.
NOTE: Most fastener materials are excellent conduc-
tors of ultrasound. However, certain cast irons and
many plastics absorb ultrasound and cannot be mea-
sured with the BoltMike.
2.1 Fastener End-Surface Machining
The ends of bolt heads and threaded sections (bolts or
studs) must be prepared before the fastener is suitable
for ultrasonic inspection. The fastener end that will be
mated with a transducer must be perpendicular to the
fastener’s centerline and machined to a very flat, smooth
surface to allow for proper coupling of the transducer.
The ideal finish for the transducer coupling point is be-
tween 32 to 63 min. CLA (0.8 to 1.6 mm Ra). Inadequate
surface finishes are indicated by poor signal quality on
the A-scan display.
The reflective surface at the opposite end of the fas-
tener must be parallel to the surface that mates with the
transducer. As shown in Figure 2-1, this parallelism al-
lows for identical sound-path distance regardless of the
transducer’s position. The degree to which these two
surfaces are machined parallel determines the upper limit
of an ultrasonic inspection system’s accuracy.
FIGURE 2-1—Fastener ends must be uniform, parallel, and perpendicular to the fastener’s centerline to ensure
acceptable ultrasound transmission.