Small diameter pipe: How to choose the best ID detection method

Optical measurement of needle gauge and tube of small parts

One of the fundamental elements of small part procurement is that making informed decisions about specifications has a huge impact on optimizing manufacturability. For example, by considering the end use of the part, you can determine which tolerances are most critical, driving the production and cost of the part.

There are many factors that affect whether a part meets specified dimensions and tolerances. In this blog post, we will cover one factor that may be important in optimizing the manufacturability of parts such as tubules, namely the method for determining whether the inside diameter (ID) meets the requirements.

How is the casing ID measured -- and why is it important

Small diameter clamp sleeves are used in numerous applications in a wide variety of industries, including (but not limited to) :

Medical apparatus and instruments

electronics

Life science

Physics research

Aeronautics and astronautics

chromatography

Veterinary medicine

The car

Chemical processing

For example, tubes of small parts, often called hypodermic tubes, can be precisely cut to length for use in medical devices that require strength, uniformity and corrosion resistance. This includes equipment for minimally invasive surgical and therapeutic applications such as:

The catheter

In the mirror

Laparoscopy

arthroscopy

ophthalmology

A biopsy

Dental surgery

Blood collection

The orthopaedic

Small catheters used in medical devices have different OD as well as different IDS and wall thicknesses.

The secret to measuring and verifying ids for these and other types of small diameter tubes is to agree in advance on the tools and methods to be used. Consistent testing can mean the difference between meeting very precise application specifications and incurring additional work costs.

Let's look at two common methods for checking tubule ID: needle gauges and optical measurements.

Simple and effective needle gauge

Traditionally, the inner diameter of a small diameter pipe or cylinder has been measured using a pin gauge, which is a simple, fail-safe measurement method. Pin gauges, sometimes called attribute or fixed limit gauges, allow you to check the conformance of a part and quickly determine whether its function is compliant.

The pin gauge is a small pin calibrated to insert into the part to determine the compression ID and ensure that the diameter is not too small. The process involves using a series of calibration pins with progressively increasing diameters.

Pins are inserted into the tubules in dimensional order to find the largest pin that fits without stretching the clamp casing. Depending on the softness of the casing material, water or lubricant can be used to prevent the pin from sticking to the surface of the pipe ID.

For most small diameter stuck sleeves with diameters of 0.005" to 0.200", needle gauges produce acceptable correlation results. However, because this is a simple pass-fail detection method, the pin thickness gauge design cannot provide measurements for individual characteristics of part diameter.

Consider pin gauge tolerances

It is also important to remember that a pin can only measure one tolerance limit, not both positive and negative. In addition, needle gauges have their own tolerances, which can lead to stacking.

Pin gauge tolerance classes are established by the National Institute of Standards and Technology (NIST) and use letter names such as ZZ, Z, Y, X, XX, and XXX. The difference between the different classes of gauges is the tolerance allowed during the manufacture of each gauge.

In general, the earlier in the alphabet (the more letters added), the smaller the allowance. For example, for thickness gauges in the size range 0.001" to 0.08250", the tolerance of class Z thickness gauges does not exceed 0.0001" and that of Class X thickness gauges does not exceed 0.00004".

Other limitations of needle gauges for measuring tubules

For very small ids (e.g., less than 0.004 in.), it can be challenging to insert a precision pin gauge into a small diameter tube without bending the tube or damaging the gauge. Pin gauges themselves are prone to wear and need to be monitored for damage.

In addition, results may vary from user to user, and the pass-fail criteria may not always be met. For very long cylinders, such as small diameter clamp sleeves up to 6 ft in length and requiring a consistent ID, pin gauges are impractical.

More complex options for detecting tubules

For more stringent requirements or small parts with ID stuck sleeves that cannot be measured with standard needle gauges - for example, diameters of 0.002" to 0.004", optical measurements provide a more complex option for calculating diameters of cylinders or tubes.

The method can be as simple as visually checking the tube for blockages using a lamp, or measuring points inside the end of the tube. Typically, however, optical measurements use sensors to measure various points around the circumference and inside the tube, and then algorithms run the numbers to determine whether the average ID is within the tolerance range.

Optical measurements are more accurate than needle gauges and can be used to measure circles, arcs and more complex shapes. It is ideal for ensuring fluid flows through long, small-diameter pipes (again, needle thickness gauges may be impractical applications).

However, optical measurement has disadvantages.

Disadvantages of optical measurements and pin gauges

Sensor-based optical measurement systems are more expensive than using pin thickness gauges. In addition, optical elements can only look so far into the tube, not down into the Z-axis, where a needle thickness gauge can and must always pass through.

While an optical measurement can guarantee that the average diameter of the tube is within the specification range, it will not tell you whether the ID is consistent across the tube. Since the system averages what it "sees", problems such as protrusions in tubes may be average only.

However, because the pin thickness gauge needs to pass through the smallest point in the ID, the gauge may detect bumps that cannot be measured by the optical system.

If another part needs to be installed into the tube, the optical method may also be inaccurate and the ID needs to be carefully checked for suitability using pin gauges or other Go/No Go methods.

In addition, optical measurements may be affected by the surface finish and cleanliness of the part under test, as well as the repeatability of the part placement associated with the optical sensor.

Some general guidelines for piping inspection of small parts

In much the same way that you spend time and effort determining part sizes and tolerances, you should carefully weigh your options when deciding how to determine if the ID of the tubules meets your specifications.

As always, it's important to consider the end use of the part. For example, does the pipe need to carry gas or liquid? If so, optical measurements may be the most reliable way to ensure that the pipe diameter is large enough to allow gas or liquid to flow.

However, if the tube length is short (e.g., less than 1") then the pin thickness gauge may be perfectly acceptable. As long as the calibration is correct and there is no wear or damage, the needle gauge will tell you if the ID of the stub is too small. As an added advantage, pin thickness gauges are cheaper than optical measurements.

Some guidelines for using needle thickness gauges

When a pin thickness gauge is used, a calibrated primary thickness gauge should be used, both internally and externally, and that gauge is NIST traceable.

Also, make sure that you and your supplier use the same quality calibration gauges. This way, you won't incur unnecessary additional costs due to incorrect differences in measuring ids for small diameter tubes.

Determining which pin gauge tolerance class is most appropriate will depend on two things:

The tightness of the process

Whether further tightening of tolerance boundaries with a lower grade gauge can still achieve the results you need

If in doubt, your manufacturing partner can help you consult the gauge manufacturer's tolerance chart to determine the pins that best suit your needs.

Specify your tubular ID detection method in advance

It's also important to tell your partner in advance which method you plan to use to inspect finished parts. Why is this so important?

A metal cutting customer has supplied us with tubing with a maximum ID of 0.015" that the supplier says meets the customer's requirements. However, when the customer inspected the finished parts, they were rejected because the 0.015" calibrated pin gauge did not fit the tube.

What are the differences? The customer did not specify that they would use pin gauges to inspect the parts, and since the parts were destined to carry liquids, the sleeve supplier has inspected the sleeve using optical measurements.

In this case, you can clearly see that checking methods must be considered, not just the specified ID and end functionality. While these parts were functionally correct, they were rejected because they did not meet the customer's ID checking requirements.

Most importantly, by keeping the lines of communication open and making sure you and your manufacturing partners are informed, you can work together to achieve your small pipe ID goals and pass inspections with flying colors.

Please contact us at zhang@pride-cnc.com