TFMi™ Inspection for monitoring "Blind" Holes

It is sometimes necessary to drill holes with very large depth-to-diameter ratios. A common approach is gun drilling, whereby a cutter is used at the end of a long rotating tool. Ensuring the alignment and consistent diameter of long holes represents an inspection challenge. The TFMi™ feature of the Veo3 provides a useful option when the hole is parallel to nearby surfaces.

Using the TFMi™ NDT technique in the Veo3, with the Intermode set to “Keep Max.” shows the reflected signal from the top surface (LL mode) and the bottom surface (LLLL mode) correctly positioned on screen. A simple gate can be used to measure the gap between them.

TFMi™ Inspection

With the instrument set to a single propagation mode TFM scan only one of the surfaces can be imaged at one time.

Setting the instrument to TFMi™ mode allows us to use up to 4 propagation modes at once. Using the “Keep Max” viewing mode we can see the un-altered results from each mode overlayed on top of each other. 

In this inspection we utilised LL and LLLL modes to show the top and bottom reflections from the blind holes. This requires the thickness of the part to be known. For the block used in this study we used a thickness of 50.mm. (See figure 1)

Fig. 1 – Test block and side drilled holes used in the study, the holes can be seen to decrease in size from left to right   Fig. 2 – Example TFMi™ scan set to LL/LLLL keep max mode showing the upper reflections detected by the LL scan and the bottom reflections detected by the LLLL scan, the second image shows the beam paths taken by each mode to produce the image