While Measuring Threads: Internal Thread Inspection

The most important factors when threaded parts must meet engineering specs are accuracy and consistency.

Thread diameter gages inspects the effects of all thread element variations and pitch diameter of the threads.
Functional thread roll gages use matched sets of precision ground thread rolls that seat in the threads during inspection. Each set of rolls is designed to ANSI specifications to measure unlimited diameters for a specific thread form.
Pitch diameter gages use matched sets of precision contact points that seat at the pitch line of the part during inspection. The contact points are interchangeable to allow measuring of different thread pitches

3. Internal Threads Inspection

3.1. Internal diameter measurement with micrometer:

3.2. Pitch diameter measurement.
3.2.1. Internal micrometers with specific contacts

3.2.2. Analog micrometer with V shaped contact probe and cone probe

• One of the two feeler arms is retractable and transmits its movement toward the comparator
• The instrument is valid for internal or external threads by swapping the contact probes

• Internal thread Gauge
• Gage Blocks
• Universal Thread Instrument with spherical probes

3.2.3. Thread gage and measuring instrument
• From angle, the thread pitch and the internal diameter of the thread the gauge to be used is selected and the lengths l1 and l2 to compose to get a nominal average diameter DM0.
• From the comparison between gauge measurement and the workpiece measurement, both made in an universal measuring machine by means of a reference spherical probe you get directly the deviation to the nominal average diameter.

3.2.4. Full Lenght Thread profile Inspection:

Internal micrometers with specific contacts


• Different sets of contacts will allow us to measure not only the pitch diameter, but also external diameter, or even the full length thread profile.

3.3. Complete control in the universal thread instrument


4. Tread Inspection inside Profile projector.

Grinded &Tempered Thread Profile

5. Internal threads Inspection with gauges

5.1 Threaded gauges:

Threated horse-shoe shaped Go / No-go Gauge
Threaded male Go/No-go Gauge


Photo courtesy: GAGEMAKER DETERCO                  Go / No-Go Gauges

Houston, Texas

7.5.2 Smooth gauges for Internal threads nominal


Smooth horse-shoe shaped gauge and male Go/No-Go Gauges

7.6. Gauges with interchangeable rolls
• One of the rollers is connected to a comparator for checking reference measurement deviations


• This allows thread classification depending on the field of tolerances marked
• Provided with two arms probes, one of them retractable and transmits its movement to towards the comparator
• It is valid not only for inside but also for outside threads
• Roller gauge equipped with interchangeable only one fillet roller Cone and V only one fillet enables pitch diameter inspection

Threaded rollers for full lenght thread verification

Sources: MITUTOYO, STARRET, BROWN & SHARPE, TESA, EMUGEand DETERCO flyers and posters

Title: Curso de Metrología Dimensional.
By: Carro de Vicente Portela (Author).
Ed: E.T.S.I.I. de Madrid 1978. page 169 and nexts.

Title: Consejos de metrología de la A.E.C.C. (varios)
By: Comité de Metrología de la A.E.C.C. Madrid. (Authors).
Ed: Asociación Española de Control de Calidad.

Title: Clasificación de instrumentos de metrología dimensional.
By: Ministerio de Industria, Comercio y Turismo. Dirección General de Política Tecnológica.
Ed: Sección Publicaciones Ingenieros Industriales. Madrid, 1992.

Title: Metrología básica
By: Manrique, E. (Author), Casanova, A. (Author).
Ed: Edebé

EGA is registered as 14,302 Engineer at C.O.I.T.I.Madrid

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