Thread Verification and Chasing Group - Inspection: Internal Threads

Companies Increasingly Use Mechanical Thread Verification
Due to the high cost of nonconforming parts, product companies are increasingly requiring their suppliers to provide 100% inspection of internally threaded parts. Customers want to verify not only thread presence, but thread quality as well. Incompletely threaded holes, or obstructed threads, can cause significant downtime with automated assembly equipment. A supplier that cannot guarantee thread quality can be hit with thousands of dollars in penalties and risks a strained relationship.

Some things are easier said than done, especially in high production. While external threads are accessible to a number of inspection techniques, internal threads are particularly difficult to check. Over the years, three methods of testing internal threads have gained acceptance: vision, eddy current, and mechanical.

Vision
Vision or laser-based optical gauging is the least discerning. Although vision can generally “see” whether a hole is threaded (assuming it is oil-free), it has little ability to determine thread quality. Therefore, while this method is fast, it can only be used for non-critical applications.

Eddy Current
Eddy current has become a popular means for testing. The attraction of this technique is its non-contact means of inspecting threads, which eliminates gage wear. In use over 30 years, eddy current technology has traditionally been used to detect surface flaws in tube and wire stock, but has increasingly been used for high production inspection operations.

Eddy current testing works by inserting a probe into a threaded hole. A coil within the probe, when excited by an alternating current, creates an electromagnetic field that induces eddy currents into any adjacent electrically conductive material. The eddy-current “signature” of the adjacent material is determined by the coil inductance. This signature, which is determined by size, hardness, and base-material chemistry variables, is compared against the signature of a known master. When these signatures match within user-defined limits, the threads pass inspection.

This setup is not affected by cutting fluids and discerns a thread versus no-thread condition. However, thread quality is more difficult to determine. If too much variance with the master is allowed, the system will pass “bad” parts; if too little variance is allowed, the system will fail “good” parts, even for reasons not associated with thread quality such as part hardness and base-material chemistry.

The multiple variable problem can sometimes be addressed by a dual-coil probe where one coil reaches halfway through the threaded hole, the other the rest of the way. The system electronics compare the difference between the two eddy-current signatures, minimizing hardness and chemistry considerations. However, this comparison of geometric elements across the length of the thread is limited by the allowable variance between the coils, as well as the allowable variance with the known master.

Another drawback of the eddy current system is the relative fragility of the probe. If the probe rubs on the side of the hole or collides with a broken tap, the coil can easily be damaged, posting erroneous readings.

Mechanical
A couple of advances in the field of motion control have made high-speed mechanical verification of internal threaded holes a preferred alternative for some manufacturers. The experience of Baltimore, MD-based custom machine builder New Vista Corporation has shown the relative virtues of mechanical thread checking.

New Vista recently built a unit for a leading panel nut manufacturer for high-speed thread checking of panel nuts. The unit is integrated into an existing stamping/forming machine line and takes the place of eddy current units that proved unreliable. Including index, the 6-spindle unit processes 6 nuts every .85 seconds, or 25,400 nuts every hour.

Since the index is .35 seconds, the six mechanical gages must fully probe and exit the part within a half-second. “Ten years ago this wouldn’t have been possible to do reliably,” says Jim Brun, vice president of engineering at New Vista. Two motion control advances have made this very short cycle possible: mid-motion servomotor drive command changes, and the shortening of program logic controller scan times (or time to process signals) from milliseconds to microseconds.

Mechanically, each spindle is designed for high production work. A high response servomotor drives a special coated thread gage probe, essentially a “go” gage. In between is a compression compensating mechanism, which prevents damage to both the drive and nut strip in the event the part is not threaded. Also in the drive is a torque limiter mechanism that permits the gage to stall in incomplete or damaged threads or spin at the entrance of missing threads – either event causes the spindle to fall short of a limit, signaling a reject. (A “probe presence” sensor ensures the gage probe is in place.) The drive in this patent-pending device reverses at full torque to ensure the gage exits the part even if it jams in a damaged thread.

Mechanical verification has a few benefits. For one thing, it simulates how the threaded hole is actually going to be used. If the gage probe passes through the part, then the assembly line screw is sure to thread properly. Also, this type of mechanical system is very robust; if the hole is out of position or has a broken tap in it, the compression compensating mechanism and torque limiter mechanism will protect the unit.

The downside of mechanical testing is the wear on the gage probe. New Vista’s Brun recommends that a manufacturer keep spare gage probes in stock. A probe should be replaced every 1.5 million cycles, he says.