What Does “Free of Defects” Really Mean?
A fundamental issue arises when an engineering drawing says “free of defects,” “free of residue,” “clean of oil, grease, and particulate,” “free of debris,” “no scratches,” or otherwise indicates that no material surface imperfections are acceptable. Drawings also often call out characteristics such as “no draw lines, “no gouges,” and “no longitudinal die lines.”
The trouble with these requests is, what does that actually mean when you are looking at a part’s surface?
If you look closely enough at any surface, you will find flaws. (Ever looked at your skin in an ordinary mirror and then in a 10x magnifying glass mirror?) And this is true in even the most precise metal fabrication.
Differences at a Microscopic Level
From a microscopic point of view, a surface may have lines that are invisible at 10x magnification but look like major valleys when examined at 50x. The surface of a cut-off tube may look uniform at 10x magnification, but at 500x using a scanning electron microscope (SEM), imperfections in the grain alignment will be revealed.
Under extremely high magnification, virtually every material has surface issues. However, when asking for a surface to be “free of defects,” drawings often fail to specify how closely a part’s surface should be examined — or, perhaps more importantly, how closely it will be examined by the customer.
So, how can you overcome issues of material surface imperfections? Apply these four steps!
1. Determine the Magnification Ratio
To make sure that parts are inspected to the proper level of detail, it is critically important to identify the magnification ratio at which the material surface needs to be examined. Magnification ratio is the ratio between the apparent size of an object (or its size in an image) and its true size, indicated as 5x,10x, 20x, 50x, and so on.
By indicating the magnification ratio on your drawing, you and your vendor will have an agreed-upon level at which to measure whether a material’s surface meets your needs.
But, what should you consider when specifying magnification?
When choosing a magnification level, it is also important that it makes sense with your end use. Since there is added cost involved in closer examination, you want to be sure you’re not over-engineering the part and asking for surface finish processing that exceeds your requirements and, therefore, can exceed your budget.
For example, if having a slight groove in a tube or rod will not affect the performance of the part, then examining the part at 100x magnification may be excessive. But if you are producing a ball bearing that will be used within a critical airplane part, a high magnification level may be necessary to ensure functionality and meet safety requirements.
2. Consider the Angle of the Light Source
Keep in mind that the angle of the light source can also have an impact on the appearance of a material’s surface at a microscopic level.
The angle of the light can cause shadows and reflections that may increase or decrease the perceived size of draw lines, cracks, gouges, dents, and other surface finish imperfections. The angle can also change the appearance or hide the presence of discolorations.
Optical microscopes can rely on various light sources, such as ring lights, embedded fiber optic, parallel fluorescent tubes, or even back lighting — all of which will cast different shadows.
3. Identify the Color Temperature of the Light
Another variable is the color temperature of the light, which is a more comprehensive way of looking at a light source, versus just identifying whether a microscope uses LED, fluorescent, or incandescent lighting.
This characteristic of visible light is stated in Kelvins (K). Temperatures over 5,000K called cool colors (bluish white) and lower temperatures (2,700–3,000K) called warm colors (yellowish white through red).
Differences in the color temperature can affect the appearance of a part’s surface. This can make for inconsistencies in:
- How two people looking at the same part under different lighting would see and describe the material surface
- How the surface would look in a photograph or video that is shared with the express purpose of confirming how the part “should look”
4. Consider the Presence of Light
Yet another variable to consider is the presence of light at all — or more specifically, the absence of light as in the case of an SEM.
All microscopy involves viewing an object and/or areas of its surface that cannot be seen with the naked eye. Optical microscopes use the diffraction, reflection, or refraction of visible light beams to create an image.
But an SEM examines a sample by scanning it with a focused beam of electrons, which reflect off the topography of a sample to produce three-dimensional images of the object and/or surface. In other words, to the human eye, an SEM looks at parts in complete and utter darkness!
That means there are no confusing shadow effects.
Is an SEM Right for Your Application?
Because electrons have a much smaller wavelength than visible light, an SEM is able to produce an image at a much higher resolution, with greater detail than that produced by an optical microscope. In addition, an SEM has a larger depth of focus, hence the more detailed 3D imaging.
However, there are major drawbacks to SEM, including that the electron source, lenses, and sample must all be in a vacuum. For everyday applications, an SEM is also extremely expensive, complex, and difficult to use.
In addition, because it is so fundamentally different from ordinary methods of microscopy, an SEM does not provide a basis of comparison with what can be seen by the eye using the more typical optical microscope method. So, while the level of detail produced by an SEM is astonishing, it is not compatible with typical resources in the factory or shop.
That means asking for SEM-level “free of defects” is, at least for the present time, not practical.
Thinking About It Now Helps to Ensure Quality Later
The good news is, you can and should always specify the magnification level under which you want to examine your part’s surface using the more typical microscopy methods.
Common optical microscopes can measure anywhere from 5x to 50x, and there are other, more advanced options. For instance, Metal Cutting Corporation has an optical microscope that is capable of magnification to 200x, and our video inspection equipment can display an even greater level of detail.
Most of all, think about the material surface, the magnification ratio, and how they relate to the requirements of your end use early in the manufacturing process — for instance, as you are doing your drawings or completing an RFQ. That will not only help you avoid problems down the road, but also help to ensure that your parts will pass inspection and meet your needs.
