The selection of Bench Inspection or Portable Thread Inspection is generally dependent on part configuration. While small parts are easily accommodated with Bench-style Gage Systems, larger parts very often can only be inspected with Portable Gages. Overall part length, configuration, and size all play a part in proper gage selection. Very often, inspection at the point of manufacture will simplify production of high-value parts due to control of thread dimensions as a part of the process.
Selecting a Gage
Bench Inspection or Portable
Careful consideration of the appropriate level of inspection is essential in the design of a comprehensive Thread Inspection System. Very few applications are identical, and there are obvious differences in the requirements of a specific threaded assembly. The load-carrying needs of an aerospace engine support bolt in a tension assembly are far greater than those of a simple screw that fastens a cover plate to an electric wall socket. International thread acceptance standards and specifications recognize this basic engineering fact, and incorporate different levels of thread verification and inspection requirements into these standards. The following table offers suggestions regarding the appropriate level of thread inspection for an individual application:
Table of Dimensional Considerations
|Inspection Level||Suggested Application||Johnson Gage Product SOLUTIONS|
|Functional Size Only||Ability to assemble is the only design requirement||External Thread Inspection Solutions C3, JTR, CX, CH-S, CH-T with Functional Rolls. BX or BXR with Functional Segments. Internal Thread Inspection Solutions GJ or GH with Functional Segments GP or GH with Functional Rolls|
|Functional Size and Pitch Diameter Size||Assembly, dimensional factors, and thread performance are essential design requirements.||External Thread Inspection Solutions C3/C3, JTR2, CX/CX, CH-S/CH-S, CH-T/CH-T with Functional and Pitch Diameter Rolls BC3F, BXR/CX, BX/CX, BH-S/CH-S with Functional Segments and Pitch Diameter Rolls Internal Thread Inspection Solutions GJ/GJ with Functional and Pitch Diameter Segments. GH/GH with Functional Segments and Pitch Diameter Rolls GP/GP or GH/GH with Functional and Pitch Diameter Rolls|
|Functional Size and Pitch Diameter Size with additional inspection of Lead, Angle, Taper, and Roundness.||Failure or design analysis, critical structural or dynamic applications necessary for system safety.||Due to part configuration and special design requirements, please contact Johnson Gage to discuss this critical thread inspection application.|
Fixed Size or Range-Adjustable
Fixed-Size Thread Inspection Systems are often used for High Volume or High Value Parts. These Systems are also used in applications where the same thread size is used for a wide variety of different parts such as fasteners, fittings, or couplings. This simplifies gage utilization as there is no need to adjust the gage to accommodate different thread sizes. Fixed-Size Thread Inspection Systems are lower cost alternative to adjustable designs.
Adjustable Thread Inspection Systems are used for applications where a variety of different thread sizes are commonly produced, and there is no predominant size that dominates production or inspection. These Systems are versatile and permit the inspection of a wide variety of different threads at a reasonable cost. While there is a time factor to consider when interchanging components on an adjustable system, the versatility of the system design clearly offers significant advantages. Common applications include parts qualification for an entire product line, small to medium size manufacturing facilities with insufficient funds to dedicate gaging to a specific application, and post-production thread inspection where there is sufficient time to allow component interchangeability.
Functional Segments vs. Functional Rolls
From a design standpoint, there are two different methods for the Variables inspection of Maximum Material – Functional (Assembly) Size: Full Form Segments with Helical Path Contact, and Full Profile Rolls with Straight Annular Non-Helical Contact.
Both are permitted in various acceptance standards and specifications, and both are commonly used for thread inspection. Johnson Gage manufactures and markets both types of gages, and each has its place in a well-designed Thread Inspection System depending on application.
There are significant technical differences between the Functional Segment and the Functional Roll. Like the GO Ring, the Functional Segment incorporates a full Helical Path while the Functional Roll incorporates only point contact at discrete 120O locations.
There are several advantages to the Functional Segment:
Two Point or 180 O Out-of-Round
The 120O design of the Functional Roll is geometrically unable to determine any Roundness Variation beyond Multi-Lobe or Cloverleaf conditions. As a result product acceptable to the Functional Roll may not assemble. The Functional Segment, with its 180O contact, can easily detect this common thread defect.
Helical Path Deviation or Non-Uniform Lead Error
Without a minimum of 50% peripheral contact, any variation in the Helical Path of the Thread is undetected. The Functional Roll has only point contact around the periphery of the thread form, and is technically unable detect this thread error. Assembly is not guaranteed.
Angle Error and Oversize Minor Diameter
The design of the Functional Roll is based on a straight annular ridge. It has no Helical Profile. When the Roll is engaged in the thread, it creates helical interference and is unable to fully contact the Flank Angle of the thread profile. As a result, the full extent of Angle Variation is undetected by the Functional Roll. Even worse, this interference between the Helical Path of the Thread and the straight annular ridge of the Functional Roll prevents the Roll from fully engaging the thread. An oversize Minor Diameter is undetected, and assembly is compromised. The Segment eliminates these problems.
Additionally, the Segment is identical in design profile to the GO Ring. Compatibility between the Segment and the GO Thread Ring Gage is assured. The difference in profiles of the Roll and the GO Ring may require that any part accepted by the Functional Roll must be further inspected with Ring Gages to assure assembly with mating parts. This simple point has always been acknowledged in the industry, even by manufacturers of the Functional Roll. The cost burden of supporting and maintaining an additional Thread Inspection System in addition to the Functional Roll should always be considered.
Despite its technical limitations, the Functional Roll can be successfully used for thread inspection if accommodations are taken into consideration. In most cases, this will not affect the overall capability of the thread inspection system. When Functional Rolls are used, the following factors should be incorporated in the Quality Plan:
Guard Band the Maximum Material Size
The Functional Roll cannot detect all of the thread form errors, and cannot completely assure assembly. Do not allow the process to approach the Maximum Material Condition.
Consider Sources of Thread Form Error
If the manufacturing process does not include the possibility of Two Point Out-of-Round or Helical Path Deviation, the Functional Roll can be used without concern.
Verify Assembly with GO Thread Rings
If assembly is a concern, supplement Functional Roll Thread Inspection with GO Ring Gages.
Functional Rolls are an attractive and economical alternative to Thread Ring Gages or Functional Segments in many applications. Unlike Rings or Segments, the same set of Rolls can be used for a variety of different threads with identical TPI or MM Pitch. For example, an M10 x 1.5 and M12 x 1.5 would require a single set of Functional Rolls rather than two different Rings or Segments. For large diameter threads, the Functional Roll may be the only cost-effective inspection alternative.