NDT Standards for Bar and Tube Inspection Systems

PH Tool™ is proud to serve many customers in the field of primary metals manufacturing.  This includes iron and steel producers, aluminum mills, and other producers of nonferrous metals.  When metals producers adopted sophisticated, in-line phased array ultrasonic and eddy current technology to inspect their product, they turned to PH Tool to manufacture their complex calibration standards.  

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In-line inspection systems for bar and tube offer a mechanized solution to metals manufacturers seeking to ensure a high level of material quality in their production methods.  The machines have high throughput, flexibility in product shape and size, and also minimize dependency on skilled operators once they have been properly set up and calibrated. Many types of systems exist for inspection of billet, round or square bars, and hollow products like tubing or pipe.  These systems often allow for an entire length of product (several feet) to be inspected quickly and often detect both surface and subsurface flaws. The primary NDT technologies employed in bar and tube inspection systems are Ultrasonic Testing (UT) and Eddy Current Testing (ET).  Many systems are equipped with sophisticated phased array UT (PAUT) scanners and eddy current arrays (ECA) to ensure quick and complete scanning of materials.

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The primary reflector types used for calibration of bar and tube inspection systems are the Flat-bottom Hole (FBH) and EDM Notch. Because billets and bars can have quite a large cross-section, the FBHs often need to be drilled to a depth far beyond what is recommended by drill bit manufacturers. A calibration bar with holes at multiple depths allows for calibration through the cross-sectional thickness. In the past, if customers needed deep FBHs drilled, the standard would have needed to be “stepped” due to machine and tooling limitations. Thanks to the experience and pioneering spirit of PH Tool, our customers can now calibrate on large-diameter standards that are not compromised with invasive steps or access pockets and slots.  To support this effort, we have several vertical CNC mills dedicated to deep FBH drilling.  The largest of these has been modified to have an incredible Z-axis travel of 41 inches. Combined with specially-engineered tooling, PH Tool can now drill FBHs at 20 inches deep into a 21-inch diameter bar.  Bar standards up to 10,000 lbs. are possible on this specialized machinery. 

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This deep FBH drilling is achieved through the use of a larger pilot hole (a.k.a. – “counterbore”) that allows us to slip a tooling extension and precision collet for the smaller drill down to the final required depth. The pilot hole is most often .500” (12.7mm) diameter and ends in a conical transition (normally 118° or 135° included angle). The primary FBH target is placed at the bottom of the pilot hole, terminating at a precise distance from the inspection surface (a.k.a. Metal Travel Distance or “MTD”). Calibration standards can be several feet long, and therefore have many FBHs placed along the bar axis at various MTD. Standards can also include FBHs of multiple diameters to cover more than one quality level. To date, PH Tool has used this deep drilling method for FBHs as small as 1/64” (0.4mm) diameter. EDM notches are often added for detection of surface defects. We routinely drill deep FBHs and machine EDM notches in steel alloys, high-nickel superalloys, titanium, aluminum, and more.  

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 Large Tube EDM3

PH Tool employs a trustworthy process to machine and mechanically measure FBHs. It is good practice to start with a spotting tool or countersink. This provides a shallow, conical starting hole for the initial drill to center itself and not deflect or skid off-center. This is especially important when entering an irregular or angled surface. Next, the spotting tool is removed, and the holes are peck-drilled to within a few thousandths of an inch of the final depth, using a standard, pointed drill bit.  A skilled toolmaker then grinds the bit to 180° (flat) under high magnification in-house on a Swiss-made precision drill grinding machine.  The drill is inspected on a calibrated, NIST-traceable video measurement system to ensure flatness before and after drilling the last few thousandths of the hole.  We use gauge pins to determine the finished diameter of the hole, and then use a calibrated depth gauge with a pointed pin to determine the depth.  The depth we measured using the pointed pin is then compared against that of a flat pin, to ensure there is no concavity at the hole bottom. In addition to the mechanical inspection using gauge pins, the hole bottom is optically inspected using specialized scopes or vision inspection systems. A final look at the flat bottom drill reveals that the drill is still sharp, and has no corner radius, which would reduce the reflective area of the drilled flat-bottom hole. This same, comprehensive process applies to even the smallest flat bottom holes we machine here at PH Tool.

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When it comes to plugging deeper FBHs with long, .500" diameter counterbores, the best option is to use a special brass plug. The brass plugs we use have expandable O-rings and stainless steel hardware, so they are both corrosion-resistant and water-tight. We gently tap the plug into the hole opening, and then expand the O-ring by rotating the screw. With some effort, these plugs can be removed if necessary, for re-inspection at a later date. The most common application for this type of plug is on large, solid bar standards used in automated UT immersion systems.  

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Contact PH Tool Sales today to speak with us about your next in-line bar or tube reference standard!