Thread measurements

How to determine thread

An external thread is determined by the outside diameter, pitches and thread angles.

A slide gauge measures the outside diameter, and a screw pitch gauge measures the pitch and controls the thread angle.

Measuring the internal thread is difficult as a slide gauge also measures the inside diameter, however, the thread angle cannot be determined. Likewise, the screw pitch gauge cannot accurately determine the pitch.

Measuring threads can be difficult, especially the internal thread. Often, tools are used such as the thread plug gauge for the internal measuring and a thread ring gauge or a thread pitch gauge for the external thread measuring.

At the manufacturing of an external thread on lathe, the pitch can be cut incorrectly and thereby result in a too small pitch diameter on the thread. The cutting will continue until the ring gauge can be turned into the thread.

If the thread angle is too big, too small or crooked, the pitch diameter will also be too small, which can be determined using a ring gauge. Thread made with precision tools can be checked for errors by measuring the pitch diameter.

Thread with an oblique thread angle

Measuring thread is usually limited to measuring the pitch diameter, pitch of the thread and the thread angle.
The following measuring instruments can determine the external thread:

Micrometer screw pin
Micrometer screw gauge with thread wires

The following measuring instruments can determine major and minor thread measurements:
Slide gauge (FMS system)
Profile Projector
Non Standard Measuring Tools
Measurements of internal thread length

Measuring with gauges

Micrometer screw gauge with thread pins. Measuring spindle and anvil have precision-tapered bores for accommodating interchangeable anvils. .

Pins with large measuring units measure several pitches.

Pins with smaller units can only measure one pitch but are more accurate.

Measuring the pitch diameter with pins presupposes the angle is correct. If the angle is to large, small or oblique, it will result in a larger pitch diameter.

Measuring with thread wires

When precision measurements are needed, measuring with thread wires should be preferred as it is more accurate, compared to the measuring pins. Thread wires come in sets for fitting on the micrometer screws spindle and anvil.
The three wires are mended at an accurate diameter to touch the thread in proximity of the pitch diameter.

When used, thread wires have to be placed in the same direction as the grooves: at the same angle as the pitch in proportion to the thread axis.

The measurement shown by the micrometer screw, when using thread wires, can be used to calculate the pitch diameter. Most often, a chart is used, as calculations are difficult.

Chart for metric thread:

Pitch P
Pitch diameter D2/d2
Diameters for measuring wire dD
Cable Measurements Z0

Example of how to calculate the measurements of tolerance for thread combination: M20 6H/6g

Figures can be found in the thread tolerance chart.

Nominal pitch diameter D2/d2 for M20	18.376
Fundamental deviation for H	0
Fundamental deviation for g	-42
Tolerance for D2	224
Tolerance for d2	170
Largest pitch diameter, nut D2max.	18.376 + 0.224 = 18.600
Smallest pitch diameter, nut D2min.	18.376 + 0 =18.376
Largest pitch diameter, screw d2max.	18.376 - 0.042 = 18.334
Smallest pitch diameter, screw d2min.	18.376 - 0.042 - 0.170 = 18.164

Measurement of M20 6g screw thread with measuring wire.

Diameter of wire dD (see chart)	1.650
Wire measurements Z0	21.163
Z0 -d2 = 21.163 - 18.376	=2.787
Z0 for pitch diameter d2max. 18.334 + 2.787	=21.121
Z0 for pitch diameter d2min. 18.164 + 2.787	=20.951

FMS System Slide Gauge (Flexible Measuring Systems)

Measuring inside thread of the pitch diameter with a slide gauge.

Profile Projector

A Profile Projector is used for optical inspection of objects and profiles. The object is placed on the focal point and is projected enlarged onto a screen. Using different measurement planes as a background, errors in thread cuts are easily spotted. Measurements down to 0.001 mm. can be read.

Measuring Machine
For precise thread measurements with wire, the measuring wire deformation needs to be taken into account as the pressure can compress it. On a micrometer screw with a measure pressure of 10 Newton, the pressure of the wire is 0.00262 mm. on a M20 thread and 0.00175 mm on a M60 thread.

Combi thread depth gage

The Leitech gage is a combi thread depth instrument - it is a tolerance gage that also measures thread depth. The measuring sleeve is screwed into the thread and you can read the depth on the large scale (X) in mm (14 mm) and on the internal scale in 1/10 mm (2/10 mm).

The result is a depth of 14.2 mm.
For more information go to Leitech’s webpage: www.leitech.com.