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Thread Verification Techniques In High And Medium Volume Production
Until relatively recently, manufacturers of auto and truck parts, medical and aerospace components and the like, relied on sampling inspection of threads using hand gages. But in the last dozen years, with an accelerated trend toward automated assembly; and with increased outsourcing of machined parts, it has become more urgent for them to move toward automated 100% thread gaging for many critical components.
There are two basic methods that are employed in automated thread gaging systems: noncontact and contact. We will examine the noncontact systems first.
NONCONTACT GAGING: VISION SYSTEMS:
Vision systems employ the use of cameras, or laser-based optical scanning, to discern whether or not threads are present, and in some cases to measure the threads. A simple vision system can be quite inexpensive, very fast, and within its limitations, quite reliable. Parts can often be inspected on the move, which can make fixturing unnecessary. So vision is usually the first method to be considered.
Vision systems are not generally used for internal thread verification for the obvious reason of inaccessibility. So vision works best with clean external threads. The simpler external thread systems can’t measure pitch diameters of external threads accurately, but they can supply an indication of thread length. The more capable (and more expensive) vision systems can provide a diametrical measurement also, but are generally not regarded as foolproof as “GO” (contact) gaging.
The most commonly seen thread vision systems are for parts and components like bolts or screws or threaded-end rods, where the parts are passing by in rapid succession, and the objective is to pick out the ones with missing threads.
NONCONTACT GAGING: EDDY CURRENT:
Eddy current thread gaging has been popular for 30-odd years. It was an outgrowth of techniques that were (and are) employed to detect surface flaws in tube and wire stock, but then adapted also for thread inspection.
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.
Two other drawbacks of the eddy current system are the relative fragility of the probe and the time required to take a reading. 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. The probes can be expensive to replace. Also, the probe should be stationary for at least 0.5 seconds before taking a reading, which can be a relatively long time in high volume applications.
Eddy current installations are most commonly found where there are large numbers of internal threads to be found in a single part that all need to be gaged simultaneously; and where the primary objective is to make a determination as to thread presence, and not so much to be concerned with thread pitch diameter or thread depth (length).
ROTARY CONTACT GAGING: POWERED “GO” AND “NO GO” TECHNIQUES:
These methods are akin to hand “GO” or “NO GO” gaging, only the automated stations are much faster, more consistent, and, of course, require little or no labor. Rotary contact gaging is the method employed in most of New Vista’s gaging stations. Rotary contact gaging, like eddy current gaging, stretches back 30 years and more, the early ones using stall motors, either air or electric, or simple slip clutches, designed to prevent damage should an obstruction or other resistance be encountered. Rotary contact installations were at one time quite expensive, but costs have come down a lot in recent years, and today a multiple-spindle rotary contact station often sells for less than does an eddy current type.
Cycles are about the same as for eddy current: for reasonably short, small diameter threads, about 0.6 seconds (in and back out) complete. When faster cycles (or higher outputs) are required, as in oil filter work, or in nut-sorting, the parts are dealt with in multiples, up to 6 parts at a time.
The appeal of rotary contact gaging is of course that it is truly functional. The method works with either internal or external threads, and if the thread gage members are selected properly, successful assembly of the gaged part (with its mating part) is assured. In addition, rotary contact gaging can be directly correlated with hand gaging, which is familiar to everyone, and where the gages and techniques are widely accepted and very consistent across wide swathes of industries and manufacturing plants.
Stall motors are still used as drivers in some instances where cycles can be long and speed can be slow. But for fast-cycle work it is necessary to use high speed, low inertia torque devices, such as New Vista’s various patented mechanisms.
The old-style mechanisms suffered from difficulties with thread entry and/or with the gages sticking in the holes; but technological advances in recent years have made these problems things of the past.
Compliant toolholders are a necessity for use with most rotary contact systems. These devices ensure that the thread gage member can freely enter the hole (or external thread) without binding, even in circumstances where there is considerable misalignment between the driver and the threaded feature.
For additional information on rotary contact gaging, you can read the New Vista publication How New Vista Thread Verification Units Work. It is available from New Vista’s website: www.newvistacorp.com.
New Vista Corporation