Flat-bottom Holes (FBH)
The Flat-bottom Hole - "King of Ultrasonic Testing"
The machining of flat bottom holes constitutes a large part of what we are known for at PH Tool™. These holes appear in many test blocks such as ASTM reference blocks made to E127 and E428, and many other block types as well. In addition, countless application-specific standards employ the use of flat bottom holes at various depths and diameters. They simply make for excellent ultrasonic inspection targets with very predictable and quantifiable dimensions. We take many steps at PH Tool to ensure that you get a drilled flat bottom hole of the highest quality available.
We employ both conventional machining (drilling) and nonconventional (EDM) machining in the hole-making process. The determining factors we consider in choosing a method are machinability of the workpiece, accessibility, hole diameter and depth. Most FBHs are drilled in the range of 0.250” (6.25mm) diameter and under. Many specifications call for them to be drilled in increments of 1/64 inch. PH Tool regularly drills holes in the range of 1/64 inch thru 8/64 inch for most applications. Our modern CNC milling centers allow us to conventionally drill FBHs in workpieces down to 0.008” (0.2mm) in diameter.
Smaller holes are machinable via EDM. With recent investments in CNC-controlled fine-hole EDM machines, our capability for drilling extremely small FBHs has dramatically increased. With EDM technology, we have successfully machined FBHs down to 0.004” (0.10mm) in diameter. We are constantly pushing the envelope on what we thought was possible by machining FBHs in tough materials like high-nickel superalloys, tool steels, titanium, and more. We blow right past the drill manufacturers’ recommendations when it comes to drill diameter/depth aspect ratios.
The image shown below displays a special EDM electrode that has been dressed down from a .040" diameter shank to an ultra-small .003" diameter "business end" at the bottom. A human hair is shown in the image for comparison. Electrodes are dressed right on the same machine that is used to make the holes to ensure excellent concentricity. The electrode is rotated during machining of these holes to ensure even electrode wear, resulting in a perfectly round hole that is right on-size. Multiple dresses of the electrode also virtually eliminate the corner radius at the bottom of the hole, making a reflector that is as sharp and flat as physically possible.
How Do We Do It?
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.
Side-by-side comparison of a perfectly-ground Flat-bottom drill bit next to a pointed drill bit.
Deeper FBHs Using Counterbores
When the bar and tube industry 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. 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 added 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! This 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.
Protecting FBHs During Testing - Plugging Options
For ultrasonic applications, it is good practice to plug completed FBHs to prevent any couplant, or water in case of immersion testing, from entering the hole and causing the surface to degrade. PH Tool offers a number of options for plugging FBHs to keep them water-tight for years.
Silicone or Other Non-metallic Plug
The simplest and fastest way to plug a FBH is to press a small amount of silicone, epoxy, or other water insoluble substance (some older specifications even call for beeswax!) into the hole opening. It is important in this case to ensure there is an air gap at the bottom of the hole. If the plugging material were to get in contact with the hole bottom, it could dampen the signal. We take care to ensure only the opening of the hole is plugged, and that an air interface is left between the plugging material and the reflector at the bottom of the hole. Besides being simple to do, this method also has the advantage of being able to remove the plugging substance should you ever need to re-examine or re-certify the FBH.
Press-fit Metal Plug
Another option for plugging is to use press-fit metal plugs. This is the most common method used on ASTM E127 and E428 test blocks. To start, a metal plug of the same or very similar material as the block is made, typically at .250" diameter. When the hole is done being drilled, a .250" diameter x .064" deep counterbore is drilled at the hole opening. The plug is then press-fit into the counterbore, sitting just slightly proud of the block surface. Next, we take the block to a surface grinder and grind the plug until it is exactly flush with the block surface. The interface between the plug and the block is so intimate after grinding, that it is almost impossible to tell if there is a hole in the block at all. Upon first inspection, you may be tempted to call us and say we forgot to drill the hole in your blocks; however, an ultrasonic check will quickly reveal that the hole is indeed exactly where it should be!
Plugging Deeper FBHs with Counterbores
When it comes to plugging deeper FBHs with long, .500" diameter counterbores as-shown above, 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.