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What is a profile measurement?

Author: Marina

Feb. 04, 2024

62 0

Tags: Measurement & Analysis Instruments

If there was a college major for the coating of metal substrates, the first class would be Surface Preparation 101. The main theme would be maximizing coating adhesion…and to do this, a number of criteria would be taught including visual cleanliness, removing invisible contamination (salts/chlorides, oils, grit embedment), and how to create the coating’s required surface profile.

Accurate measurement of substrate profile is done multiple times to confirm the correct surface profile reading. This is done because a coating applied on too deep of surface profile will be too thinly applied, potentially exposing the peaks of the substrate, leaving it susceptible to premature coating failure. Alternatively, a coating applied on too shallow of a surface profile will have issues with adherence.

Measuring the profile (or anchor pattern) of a substrate can be done using a variety of tools. Testex, Elcometer, and DeFelsko are industry leaders of surface profile inspection equipment. Technologies for measuring surface profile vary. Some employ the use of digital tools, while others use analog technology paired with visual assessment.

Common surface profile testing methods include: 

1. Replica Tape

Replica Tape (or replica film) consists of a crushable foam on a non-compressible tape. When the replica tape is rubbed onto a profiled metal substrate it creates a reverse image of the surface. This manageable, compressible foam is then measured with a micrometer. The compression in the foam gives the average anchor profile measurement. The Society of Protective Coatings (SSPC) recommends taking a minimum of three replica tape samples per 10m² (100f²) to determine the average profile.

Advantages: Historically proven to be accurate and simple to operate. A physical record of each test sample can be kept for record. The surface being measured doesn’t have to be completely flat.

Disadvantages: The process of using replica tape is time and labor consuming. Like most methods, replica tape requires equipment calibration and attention to reading results accurately. Different tape thicknesses are needed for different profile specifications; using the wrong tape thickness can provide false readings.

The NACE standard for using Replica Tape can be found here: NACE SP0287-2016 (formerly RP0287), “Field Measurement of Surface Profile of Abrasive Blast-Cleaned Steel Surfaces Using a Replica Tape”

2. Pin Gauge

Pin Gauge testing of an anchor profile is one of the quickest ways to test a surface. To maintain accuracy, test kits must be calibrated with a piece of glass to maintain accuracy. Usually a number of tests are done on an area and an average surface profile is calculated. ASTM D4417-14 requires 10 readings of that surface. These 10 results are then averaged. This average is what is used for the surface profile reading.

Advantages: Profile pin gauge testing equipment is frequently compact. Many models offer electronic record keeping and storage. Pin Gauges provide instantaneous results. Units are DC powered. Typically profile gauges have easy to read displays. 

Disadvantages: Unit Accuracy can be affected by positioning. Rapid location transitioning can encourage variance in application technique. Recorded data can be reviewed but not re-measured. Measuring can only be done on a flat surface. Calibrations must be done before every use.

The ASTM standard for using pin or profile gauges can be found here: ASTM D4417-14 Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel

 3. Microscopic Surface Graphing and Analysis of a Sample

Microscopic measurements of a surface profile is done using a microscope under a high magnification, typically 30x or higher. This kind of surface analysis is mainly done by scientists in labs, sometimes after a coatings failure. This method can be done multiple times with a single sample to confirm results.

Many standards created for the analysis of a surface profile after it’s been prepared require an unaided (or unmagnified) view of the surface. This is why microscopic analysis is usually done after a coatings failure or issue.

Advantages: Microscopic review of profiled samples are accurate. Usually an analysis is complete with 3D graphical images depicting the sample’s surface.

Disadvantages: Unfortunately equipment for microscopic surface analysis of surface profiles are non-portable and can be very expensive.

4. Surface Profile Comparator

A Surface Profile Comparator is a two-part tool that uses a specialized magnifying glass, and a wheel that has been blasted to verified profiles. An inspector places the magnifying glass on the surface and under lighted magnification compares the test substrate to the verified profiles.

 Advantages: Surface Profile Comparators are simple, straightforward tools. These kits are portable, and can be taken to most job sites.

 Disadvantages: Surface profile comparator tests are subjective. These tests provide low accuracy results, based on a visual assessment of the surface under magnification. Inspectors can reach different results while analyzing the same surface.

 *Surface Profile Comparator Image from SSPC website found here

 

 

 

 

Practical GD&T: Profile of a Surface – Basic Concepts

The ‘profile of a surface’ (sometimes simply ‘surface profile’) control is used in GD&T (geometric dimensioning & tolerancing) to control the form of any 2D surface in 3D space. It is one of the most powerful GD&T controls and can be used with complex freeform geometries.

GD&T symbol

The symbol which will be encountered for the surface profile GD&T control is shown below:

This is the bare minimum; datums and other modifiers may also occur, but it will always include the symbol shown and the tolerance zone size. Take care not to confuse profile-of-a-surface and profile-of-a-line symbols, especially on a low-quality hard-copy printout; the profile of a surface is a closed profile, while profile of a line is an open profile:

Profile of a surface:

Profile of a line:

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Drawing callout

A basic profile-of-a-surface callout is shown on a drawing below; here we deliberately consider a very simple shape (with the profile only varying in one direction) and a very simple callout, with no modifiers:

An important point in passing is that the surface geometry must be defined numerically somewhere as part of the inspection package, or else inspection is not possible (see the article on profile of a line, where we unpack this in more detail).

This is obviously a very simple example, and a lot can be said about the many variations that are possible.

Firstly, note that this callout has only a tolerance value, and the surface profile symbol. There are no ‘in between’ modifiers, specifying a limited range over which the control applies; no datums specifying a reference frame and constraint on positional tolerance, and no material modifiers. Fully covering these is beyond the scope of this article (please consult the standards referenced below if you need to dig deeper).

As a case example (and it is one of many): one such detail to watch out for is the “all around” modifier; this small symbol (a circle at the junction of the callout arm) changes the meaning of the control to include faces all around the part, as shown:

This is still a very simplistic case; another small step towards more realistic callout would be needed on a 3D drawing to indicate the relationship with the end-faces of the part, not shown in the above simple 2D view. This is one of many points where notation differences between ISO and ASME start to emerge, see below:

Above: indicating an ‘all over’ surface profile with ASME and ISO symbology

ASME and ISO also differ somewhat in how a continuous tolerance zone here would handle sharp transitions at the corners; read the article on profile of a line where this is covered with a graphical example.

The above example geometry (essentially a freeform prism) was deliberately chosen for a simple visualisation; of course, on this part, a ‘profile of a line’ constraint could also be applied, with the same reference line geometry evaluated at each location. However, a line profile could pass if all of the individual line profiles were in tolerance, but positionally were out of alignment with one another – because for a line profile control, each line is evaluated independently, and their position relative to one another is not under control (for some application this might be the desired effect; always consider the needs of your specific application).

Above: each line scan could pass a ‘profile of a line’ check individually, but a ‘profile of a surface’ check would fail, due to the constraint on the overall form.

For the purposes of getting familiar with the basics, the “profile of a surface” control is sometimes described as being to ‘3D’ what “profile of a line” is to ‘2D’. In this sense, it is then compared to the more simple form controls: straightness, flatness, roundness and cylindricity:

If this is helpful for getting familiar with the basics, then it may be a good starting point, but this is an over-simplification and should be treated with caution. All surfaces are inherently 2D and no surface can be 3D, by mathematical definition – and in real life all parts being measured are 3D objects, so ultimately all GD&T callouts operate on 2D surfaces in 3D space. A straightness or roundness constraint can apply across a full surface on a 3D object; it just operates in a different way. And the behaviour of any of these controls can be changed drastically by the way that they are included in drawings or section views, or the way datums or GD&T control modifiers are used. If the ‘2D versus 3D’ table is a useful aide mémoire that’s fine – but don’t let it constrain or over-simplify your understanding of the full spectrum of ways these controls can be applied.

(This idea of ‘2D’ and ‘3D’ controls will be revisited in the comparison of circular runout and total runout, where again it is not the best labeling to use – see the total runout article for more).

Applications

Profile of a surface controls can be applied to just about any geometry imaginable. Often, other controls might be more suitable for simpler geometries where possible, but as the examples below the surface profile control extends the capabilities of GD&T tolerancing across a much wider range of geometries:

 

Standards

It is strongly recommended to refer to the full standards for the ‘profile of a surface’ control, as there are more details than can be included in this short review. The most recent (2018) release of ASME standard Y14.5 contains a number of helpful practice applied examples, explaining different ways to use the control. Similarly for ISO, both ISO 1660 (which specifically focuses the line and surface profiles) and also ISO 1101 (which has a wider scope) give plenty of useful practical examples of different uses of the control with various modifiers and datums.

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Overview of GD&T

For an overview of GD&T including the other symbols, please see our practical guide.

                    

Special case: Sphericity

What is a profile measurement?

Practical GD&T: Profile of a Surface Measurement - Basic Concepts

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