Part dimensions to be gaged should be cleaned and burr free to prevent gaging interference. Grit and part chips which become lodged in thread gages will create scratches and wear on the flanks of threads and on the outside and inside diameters of cylindrical plain gages. Various materials such as aluminum and castings are extremely abrasive and will tend to wear out gages more quickly than other types of materials. Finer pitch and smaller diameter thread gages tend to wear quicker than larger and coarser pitch gages and have less gage tolerance as well. Regarding thread gages, it only takes a small amount of wear to have a significant effect on the pitch diameter. The wear on each flank angle is multiplied by almost 4 times to determine the total impact of wear on the pitch diameter. 50 microinches or 1 micron of wear per thread flank will impact the measured size by .” which can be the total tolerance of many thread gages.

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Selecting higher precision gagemaker tolerances for cylindrical plug and ring gages will consume less product tolerance and will allow the acceptance of slightly more product but with less gage wear life and greater expense. Thread Check’s engineering staff can make recommendations in selecting the correct gagemaker tolerances for a given application. The normal rule of practice requires that 10% of part tolerance be divided between the Go and the No Go gages. Applying this practice results in gage tolerance always being included in the part tolerance by up to 10%.This could result in the possibility that 10% of good parts may fail inspection but that no bad product would ever pass. Assuming that higher precision gagemaker tolerances are better, is not valid, and may create quality issues as these gages tend to wear quicker with the potential of becoming undersized and passing bad parts.

Gages should be turned or pushed slowly and gently into or onto the dimension being checked. Forcing gages will result in faulty gaging and the possibility of damaging both the part and gage. Spinning thread ring gages or thread plug gages onto or into parts will create greater friction and increased wear thus reducing the life of the gage.

Using hard chrome plated thread gages can extend the wear life by more than 100% over standard tool steel gages which can provide dramatic savings on replacement costs. Thread Check supplies all standard size thread plug gages in inches and metric up to 1 ½ or 39mm in diameter in hard chrome at no additional cost. Other options for coatings and alternate material for thread gages and cylindrical gages include tin coat, carbide, alternate steels and even ceramic. There are pros and cons to utilizing various coatings and materials in gage applications which should be discussed with your gage maker. For example, gages made of carbide and ceramic are extremely durable and have excellent wear properties, but can be extremely brittle and break and chip if not handled carefully.

A thin coating of gage lubricant will help reduce friction from gage to part.

Proper training of personnel involved in the use of gages will pay dividends on ensuring the gages are treated with care and last longer. Review the current inventory of gages and look for visual signs of nicks, dents and scratches on gage members and handles. Evidence of this may suggest the gages are not being handled properly.

The effects of thermal expansion should be taken into consideration on both the part and the gage.

The temperature of the part and the gage should be the same. 68° F is the ideal temperature at which both part and gage should be at when inspected because gages are calibrated at 68° F. This effectively eliminates any error due to thermal expansion.

Protecting gages from excessive heat, humidity, moisture and corrosive chemicals will extend the life of your gages. After use, gages should be cleaned and recoated with a thin-film rust preventative or dipped in an easy to peel oil-based waxed coating, and stored properly.

Gages should be periodically inspected and calibrated to assure accuracy. Go member gages tend to wear quicker with normal use. NOGO gages will wear on the ends that receive the greatest usage. Frequency of inspection and calibration should be dependent on such factors as the amount of usage, part and gage material, tolerance, and quality procedures.

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Pin Gauges & Plain Plug Gauges

What is a pin gauge and how are they used?

Pin gauges check holes or features are the correct diameter, in some cases up to 3 decimal places. It has a cylindrical body machined so a very specific diameter. Each pin gauge has a set length of 50mm. A distinction to make is that pin gauges should only be used to accurately measure diameter and not length.

A pin gauge can be used in isolation to gauge or inspect a hole diameter, or in tandem with a pin gauge handle to make a go/no-go gauge. In most situations, the actual diameter will be the smaller sized ‘go’ gauge and the ‘no-go’ gauge will be a size slightly larger than the ‘go’ gauge. Pin gauges are also available in sets for where various sized holes need to be checked.

What different types of pin gauge are available?

As standard, pin gauges are made from a hardened alloy steel (hardened to HRc62-65). These pin gauges can be made to any size with up to 3 decimal places of accuracy from as small as 0.2mm up to 30mm. Hardened steel pin gauges conform to a grade 1 standard, making them ideal for both workshop and inspection-grade applications.

Besides steel pin gauges, ceramic pin gauges are also available. Ceramic pin gauges have the advantage of being extremely wear resistant, corrosion resistant and have superior dimensional stability. ceramic pin gauges are also used where steel gauges cannot be used (such as in food produce, medical and aerospace industries) due to the conductivity of metals. However, ceramic pin gauges can only be made to 2 decimal places of accuracy. In most general applications, alloy steel pin gauges will suffice and the extra cost of ceramic pin gauges would be unnecessary.

Both steel and ceramic pin gauges can be supplied either individually or as sets with pre-determined sizes. Each set will contain a range of sizes and steps (could be either 0.1mm or 0.01mm steps depending on accuracy required). Pin gauge sets can be supplied in either 31, 41, 51 & 101 piece sets which will each cover a different diameter range. Standalone wooden boxes are also available should bespoke sized sets be required.

Furthermore, each pin gauge is supplied with a manufacturer’s inspection certificate which will detail the internal testing completed by the manufacturer. It will contain the actual measurements of that specific gauge and the tolerance with which it falls into. However, this is not equal to a UKAS standard of calibration, which will need to be completed separately. For more information on calibrating your pin gauge, please contact Cutwel today.

What is a plain plug gauge and how are they used?

Aesthetically, plain plug gauges look strikingly similar to thread plug gauges. However, plain plug gauges are designed for gauging the hole size of an unthreaded hole. A plain plug gauge would typically be used to check the fitment of a pin or dowel to a given tolerance. In most applications, a plain plug gauge would be used to verify the size of a hole (or minor diameter of a bore) before it is threaded. A thread plug gauge would then verify the thread size of the same hole.

Up to 50mm diameter, plain plug gauges are double ended, meaning one end is a ‘go’ gauge (the gauge that should fit inside a hole) and the other a ‘no-go’ gauge (the gauge that should not fit inside a hole). Above 50mm, the gauge will consist of two separate bodies; one body containing the ‘go’ gauge and the other the ‘no-go’ gauge. Both the ‘go’ and ‘no-go’ gauge will be made of a hardened steel substrate (HRc60-62) and will typically be to a H7 tolerance.

Custom gauges are available on request. This includes different tolerance classes for very specific or tight-fitting parts (e.g. H8, H9), as well as diameters to 3 decimal places of accuracy (e.g. 19.003mm) and different lengths.

Furthermore, each plain plug gauge is supplied with a manufacturer’s inspection certificate which will detail the internal testing completed by the manufacturer. It will contain the actual measurements of that specific gauge and the tolerance with which it falls into. However, this is not equal to a UKAS standard of calibration, which will need to be completed separately. For more information on calibrating your plain plug gauge, please contact Cutwel today.