Flat Bottom & Other Holes

PH Tool offers many different types of machined holes used in NDT today. The most common types are listed below. If you don't find what you're looking for, call us and we'll be happy to discuss how it can be done. We employ both conventional machining (drilling) and non-conventional (EDM) in the hole-making process. The determining factors we consider in choosing a method are machinability of the workpiece, accessibility, hole diameter and depth.


FBH and Straight Beam with sound2Flat-Bottom hole (FBH)

The Flat-bottom Hole (FBH) is machined to have a flat reflecting surface at the hole bottom. The hole bottom is typically parallel to the beam entry surface, but can be machined at other desired angles. Primarily used in Ultrasonic Testing (UT,) with some application in Eddy Current Testing (ET,) In UT applications, FBHs are typically used for straight beam inspections, but they are occasionally used for angle beam inspection as well. FBHs can be machined just about anywhere they are needed for a particular inspection. Available in diameters from .004".

The majority of FBHs made for NDT processes are made by conventional drilling methods using modern CNC machining centers. The tiniest FBHs (under .010" diameter) are made via specialized EDM techniques. To ensure the best testing results, it is critical to machine a hole that is perpendicular to the eventual inspection surface.  Learn more about FBHs on our dedicated FBH Page.


SDH with soundSide-drilled hole (SDH)

The Side-drilled Hole (SDH) is typically a drilled hole, the side wall of which is used as the reflecting surface. Like FBHs, the smallest SDHs (under .010" diameter) are machined via specialized EDM hole drilling techniques. Unlike the FBH, however, the bottom of a SDH serves no purpose, and is therefore most often left conical at the bottom (a vestige of the standard, pointed drill bit used in the drilling process.) The bottom of an EDM'd SDH is typically radiussed (hemispherical) as a result of the EDM process.  SDHs are extremely useful for angle beam ultrasonic inspection due to the fact that they are "omnidirectional" reflectors - i.e. they can be used to calibrate at any angle, simplifying TCG calibrations.

SDHs are often machined into calibration blocks at specified locations from the diameter, e.g.: 1/4T, 1/2T, and 3/4T for setting up a distance amplitude correction (DAC) curve or time-corrected gain (TCG.) For piping applications, they can be drilled either longitudinally or tangentially to the axis of a pipe. SDHs can be machined in plate, bar or other products as well. Primary use is Ultrasonic Testing (UT,) with applications for straight beam and angle beam inspection. Multiple holes can be stacked vertically, arranged in a particular pattern, or staggered incrementally based on application. Available in diameters from .003".


Micro hole with EyeMicro-hole

The Micro-hole is a small-diameter hole used primarily in Liquid Penetrant Testing (PT) applications, where they are also referred to as, "non-linear indications." The hole diameter is normally .004 to .005", and hole depth is .008 to .010". Micro-holes of this size meet the requirements of the U.S. Navy's NSTR-99 Rev. 2 (Qualification Examination Requirements for Nondestructive Test Personnel). Other diameters and depths are available upon request.

Micro-holes can be machined in welded plate, pipe, bolts, bevel canopies, c-canopies, socket welds, lifting hooks, or any specialized test prop. They are machined via EDM techniques, and are therefore most often rounded (hemispherical) at the bottom as a result of the EDM process. Often times, the layout of Micro-holes is randomized at either our, or the customer's, discretion, and the finished test prop is provided with a map of defect locations as a part of the final certification package. Visual detectability of indications prior to performing the test is often not desired in PT props. For this purpose, Micro-holes can be hidden in weld features or rough surface textures, where they will not become visible until after penetrant and developer are applied. Micro-holes are the primary indication type in PH Tool's popular 3-Weld Liquid Penetrant Test Prop Plate. Click here for more information on this standard product offering.     


RBH with ETRound-Bottom hole (RBH)

The Round-bottom hole (RBH) is machined with a hemispherical shape at the hole bottom. Applications include ID pitting of tube or pipe for Eddy Current Inspection, and Liquid Penetrant test props. Larger RBHs are occasionally used in UT applications to evaluate beam spread.  Available in diameters from 0.001". Holes over .010" are conventionally machined with round-nose drill bits or ball mills.  Smaller holes are machined via EDM. Any hole machined on the inside of a tube is machined via EDM.  



Irregular PitIrregularly-shaped Pits

More of an art-form than a machining process! These holes and pits are machined in irregular configurations, not described above. Includes star-shaped, square holes, hex holes, and realistic pits. Custom EDM electrodes are designed and made at the hands of a skilled toolmaker.  Electrode tooling can also be cast from real pits and used to reproduce actual pits very accurately. Irregular Pits are typically used in detailed corrosion mapping applications, where close simulation to real-world conditions is required. If you still haven't seen the hole you need, use your imagination... we'll figure out how to accomplish it.



This shows a cast replica of pits at 48X magnification.


Curved Hole

Yes, we can do that too! Used on curved pipe calibration blocks for special applications. Not required by ASME Code, but available for customers who need a cylindrical reflector (hole) at a constant depth from the scanning surface. Extremely high cost and manufacturing time compared to "tangential holes" detailed in ASME Code. Tangential holes only fall at the correct depth from the scanning surface at a single point. This "tangency point" is typically located at 1/2 the length of the hole. The Curved Hole may simplify inspection by giving a consistent reflection at all points,       


If you have further questions related to reference standards with specialized machined holes, please reach out to us

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irreg pit 2This shows a cast replica of pits at 192X magnification.


Curved Hole with Circle