How do I establish / evaluate the life of a thread gauge ?

You can evaluate the life of thread gauges used by you, considering the following factors, which affect the life of thread gauges.

Identify wear parameter i.e. Major diameter & Pitch diameter of Thread Plugs and Minor diameter & Pitch diameter for thread rings. Flank Angle and pitch can also be considered. Most important is the Pitch diameter.

Measure / Calibrate the Pitch diameter periodically for a batch of gauged components.

Analyse the wear pattern of the pitch diameter on reaching the complete specified wear limit as against the number of gauged components.

This analysis should help you to establish the life of a thread gauge and work out a plan to improve the gauge life.

What are the factors contributing for the normal life of thread gauges?

The factors which contribute towards life of thread gauges are :

• Material & Heat treatment of gauges.
• Thread flank surface finish of gauges.
• Material of gauging components.
• Fitment of gauging with its components.
• Hardness of raw material.
• Manufacturing process control of components i.e. Control of Tolerance system on components.
• Lubrication while gauging & after gauging.
• Cleanliness of thread cut components prior to gauging.
• Driving force of gauging.
• Handling and preservation of gauges.

How can I improve the life of a thread gauge?

You can improve the life of a thread gauge by taking the following precautions :

• Use a correct thread gauge i.e. of the correct class of fit and tolerance disposition.
• Clean the component to be gauged of metal chips, sharp edges before gauging.
• Use suitable lubrication while gauging to avoid dry and hard drive while gauging.
• Manufacturing process control of component to give smooth gauging i.e. to manufacture component with free fit and not close fit to thread gauges.
• The thread gauge should enter the threaded portion of the component by using normal hand force.
• Avoid trying to force the gauge in the component.
• Handle thread gauges with care. Thread plug gauges are more prone to damage on the external diameter, if not handled properly.
• Clean the gauge after use and apply oil or a suitable anti-rust formulation.

Is it possible to use thread gauges which are approaching or have exceeded the wear limits?

The use of thread gauges which are approaching or have exceeded the wear limits is not recommended.
This is illustrated by the following example. Let us assume that you want to manufacture bolts and nuts of the same size. The Pitch diameter of the thread plug gauge that you use to check the nuts is at the wear limit. The nuts manufactured and checked with this thread plug gauge will also be on the minimum side or may be undersize.

The thread ring gauge used to check the bolts is worn out and its pitch diameter has reached the wear limit. The bolts manufactured and checked with this thread ring gauge will be at the maximum limit or may be oversize. When you try to mate the bolts and nuts which have been checked with gauges at the wear limits, the probability that they may not mate easily or may not mate at all, is very high.

Can the Maximum metal condition of thread gauges serve the purpose of obtaining maximum life of the gauge?

Yes, but only to a certain extent.

You use a thread gauge to gauge the threads of the components.

Let us assume that you manufacture nuts and bolts of the same size.

You have the corresponding thread ring and thread plug gauges at the maximum metal condition i.e. the effective diameter of the thread plug gauge is at the maximum and the effective diameter of the thread ring gauge is at the minimum permissible level.

Since the thread plug gauge is at the maximum level, the nuts checked by it will also be at the maximum level or in some cases may be oversize. Since the thread ring gauge is at the minimum level (remember: maximum metal condition) the bolts checked by it will be at the minimum level or in some cases may be undersize.

When you try to mate the nuts and bolts which have been checked with thread gauges at the Maximum Metal Condition, the probability that the fitment being loose, is very high. Thus you see, that while you may obtain a longer life for the thread gauge, you may not get the desired fitment between the mating components and the risk of reducing & minimizing the working manufacturing tolerance of nuts and bolts is high.

How do I work out the wear limits of a thread gauge?

The wear limit of a thread gauge is checked by using a wear check gauge. The wear check gauge is used to ascertain that the pitch diameter of the thread gauge has not exceeded the wear limit.

The wear check gauge screwed by hand, without excessive force, may enter into the thread gauge, but not more than one turn of thread. If it can be screwed in more than one turn of thread, the thread gauge no longer meets the specifications.

For direct working of wear limits on pitch diameter. relevant specification guidelines to be followed.

Are all the functional parameters and features certified, when an used external micrometer is calibrated?

No. Certain parameters like flatness of the measuring faces, parallelism between the measuring faces and the measuring force, do not remain the same as they were, when the micrometer was new and not used after being manufactured as per the applicable relevant specification.

The calibration of an used external micrometer certifies only the instrument error of that micrometer.

Can Thread Gauges be repaired?

No. Due to intricate tolerance and complex profile, Thread Gauges can not be repaired.

You can evaluate the life of thread gauges used by you, considering the following factors, which affect the life of thread gauges.

Identify wear parameter i.e. Major diameter & Pitch diameter of Thread Plugs and Minor diameter & Pitch diameter for thread rings. Flank Angle and pitch can also be considered. Most important is the Pitch diameter.

Measure / Calibrate the Pitch diameter periodically for a batch of gauged components.

Analyse the wear pattern of the pitch diameter on reaching the complete specified wear limit as against the number of gauged components.

This analysis should help you to establish the life of a thread gauge and work out a plan to improve the gauge life.

What are the factors contributing for the normal life of thread gauges?

The factors which contribute towards life of thread gauges are :

• Material & Heat treatment of gauges.
• Thread flank surface finish of gauges.
• Material of gauging components.
• Fitment of gauging with its components.
• Hardness of raw material.
• Manufacturing process control of components i.e. Control of Tolerance system on components.
• Lubrication while gauging & after gauging.
• Cleanliness of thread cut components prior to gauging.
• Driving force of gauging.
• Handling and preservation of gauges.

How can I improve the life of a thread gauge?

You can improve the life of a thread gauge by taking the following precautions :

• Use a correct thread gauge i.e. of the correct class of fit and tolerance disposition.
• Clean the component to be gauged of metal chips, sharp edges before gauging.
• Use suitable lubrication while gauging to avoid dry and hard drive while gauging.
• Manufacturing process control of component to give smooth gauging i.e. to manufacture component with free fit and not close fit to thread gauges.
• The thread gauge should enter the threaded portion of the component by using normal hand force.
• Avoid trying to force the gauge in the component.
• Handle thread gauges with care. Thread plug gauges are more prone to damage on the external diameter, if not handled properly.
• Clean the gauge after use and apply oil or a suitable anti-rust formulation. 

Is it possible to use thread gauges which are approaching or have exceeded the wear limits?

The use of thread gauges which are approaching or have exceeded the wear limits is not recommended.
This is illustrated by the following example. Let us assume that you want to manufacture bolts and nuts of the same size.

The Pitch diameter of the thread plug gauge that you use to check the nuts is at the wear limit. The nuts manufactured and checked with this thread plug gauge will also be on the minimum side or may be undersize.

The thread ring gauge used to check the bolts is worn out and its pitch diameter has reached the wear limit.

The bolts manufactured and checked with this thread ring gauge will be at the maximum limit or may be oversize.

When you try to mate the bolts and nuts which have been checked with gauges at the wear limits, the probability that they may not mate easily or may not mate at all, is very high.

How do I work out the wear limits of a thread gauge?

The wear limit of a thread gauge is checked by using a wear check gauge. The wear check gauge is used to ascertain that the pitch diameter of the thread gauge has not exceeded the wear limit.

The wear check gauge screwed by hand, without excessive force, may enter into the thread gauge, but not more than one turn of thread.

If it can be screwed in more than one turn of thread, the thread gauge no longer meets the specifications.

For direct working of wear limits on pitch diameter. relevant specification guidelines to be followed.

Are all the functional parameters and features certified, when an used external micrometer is calibrated?

No. Certain parameters like flatness of the measuring faces, parallelism between the measuring faces and the measuring force, do not remain the same as they were, when the micrometer was new and not used after being manufactured as per the applicable relevant specification.

The calibration of an used external micrometer certifies only the instrument error of that micrometer.

Can Thread Gauges be repaired?

No. Due to intricate tolerance and complex profile, Thread Gauges can not be repaired.

How do I use a NPT basic gauge ?

Click here for photo

Screw the gauge into the threads of the component using hand tight engagement and ensure that the gauge is firmly seated and does not shake. If the gauge is not seated firmly, it means that the taper on the threads is incorrect.

External Taper Component :

The distance between the basic small end face of the ring gauge and the small end face of the component should be within +/- 1 turn. 

Internal Taper Component :

The distance between the basic step at large end of plug gauge and the large end face of the component should be within +/- 1 turn.

For e.g.  if you use a  ½ - 14 NPT Basic plug gauge,  the notch (step) of the plug gauge should not be more than 1.814 mm either out or in from the large end face of the component when screwed with hand-tight engagement. 

Similarly when using a ring gauge, this is measured from the small end face of the ring gauge to the small end face of the component.