Kaman’s New ThreadChecker for Threaded Hole Drill Verification

Quality control is the process of ensuring that critical features and properties of parts or devices have been maintained through the manufacturing process. There will often be quality control (QC) checks performed throughout the entire process, but there are two conflicting effects of the process.

On the positive side of QC, any flaws can be corrected quickly and customers are satisfied with the final product. QC trends may also help to identify machine failures which can also be corrected. On the negative side, every CQ inspection at each stage is an additional time consumption, and it requires the extra costs of sensors, vision, and data processing to perform the steps.

Whether we like it or not, QC in some form must be performed, so the sensors detecting features must be properly designed and understood.

One challenging property to inspect is the presence of threads in a drilled hole - notoriously difficult for machine vision and proximity sensors to detect since the difference between a threaded and a non-threaded hole is quite subtle.

Kaman Precision Products, a worldwide leader in the manufacture of sensors using inductive technologies, has recently announced the ThreadChecker line of sensors for detecting the presence of threads inside a drilled hole with speed and accuracy.

 

Kaman Precision Product’s new ThreadChecker sensor. Image used courtesy of Kaman

 

Kaman ThreadChecker Sensors

The Kaman ThreadChecker is a probed sensor with a DIN rail mounted main body. The sensor line is powered with industry-standard 15-30 volts DC, and a signal output consisting of either an analog 0-10 volt signal, or a user-programmable switched discrete DC output of the NPN polarity.

The analog output is suitable for condition monitoring with a PLC, and after the programming process, there will be a significant (5 volt) difference between a threaded and non-threaded condition. Since the discrete output is NPN, it must be used with a sourcing PLC input terminal or module.

All power and signal wires are provided through an insulated, industry-standard cable with the typical brown, blue, black, and white color scheme.

The sensors are physically designed for a range of thread sizes, with the smallest models suitable for M3 (#5) threads, and the largest used for up to an M14 (5/8”) thread diameter.

Inductive Thread Checking

The concept of checking threads is a process very similar to inductive proximity sensors. A magnetic field is generated at the tip of the sensor by the internal electronics. If this magnetic field is introduced to the presence of a nearby electrically conductive surface, the material will generate its own magnetic field by an eddy current. This means the sensor is not useable in plastic materials, with or without threads.

If a hole is drilled without threads, the distance between the sensor tip and the surrounding material will be closer on average (remember, tapping threads removes material, so the average hole diameter is increased). A smaller distance between the sensor and the hole results in a smaller output voltage.

Translated to QC data, this means that a properly tapped hole will have an analog output voltage higher than a pre-drilled, but untapped hole.

 

Both CNC and manual machine processes are used to drill and then tap threads into holes. Image used courtesy of Canva

 

Each sensor model is acceptable for a small range of threads. The difference of just a few mm between hole diameters may be much larger than the height of a thread, so the sensor must be carefully calibrated for each hole diameter.

Summary

Quality control processes are important, but often time-consuming. When many tests must be performed, it is always best to have noncontact sensors to reduce wear and tear on both the part and the sensing system.

Kaman Precision Products’ new line of ThreadChecker sensors is an excellent example of using material properties to quickly test product samples to ensure reliable, efficient operations.