by Ken Parlee
Which inspection system,
contact or noncontact, best meets your requirements? This question is as timely as ever today due to the growing trend toward miniaturization, especially in the computer, medical device and
electronics industries. The resurgence of selective assembly and match-fitting practices, which depend on more precise measurement data, also makes the question especially timely. By their
nature, smaller parts are often more prone to damage, deformation and/or contamination by even slight contact with a probe. Another factor is that current sampling rates often run higher than
they once did, creating the need for higher inspection speed. Noncontact inspection--optical, video and laser--solves all of these problems. The overall impact of these trends is that current
noncontact systems are handling about 40 percent of the inspection workload, and that number is on the rise.
The stakes in choosing the right inspection system for your
particular operation include measurement throughput, repeatability, accuracy, cost, process control and traceability, operational simplification, part quality and ease of documentation. The
stakes rise proportionally to assembly volume and degree of added value to the product throughout the process.
Most manufacturing quality assurance departments require
measurement uncertainty to be quantified separately from the manufacturing process as well as from the measuring process itself. To reduce this uncertainty, you need instruments with speed,
precision and excellent statistical software to facilitate tight, accurate gage reproducibility and reliability (gage R&R) studies.
The most common part-inspection methods
today include contact and noncontact instruments, such as coordinate measuring machines, microscopes and machine vision systems. When referring to noncontact inspection, we include microscopes,
profile projectors, vision measuring machines and laser micrometers. All these instruments measure a dimension without contacting the work piece. Let's look at each of these alternatives
Coordinate measuring machines
(CMMs) play an important role in a number of industries as functional, fast, accurate, reliable and economic measuring and
inspection instruments. With their sophisticated mechanical probes, they provide a practical solution for measuring complex shapes in a large number of tasks.
Essentially, they work by touch. For that reason, they are not as suitable for products that shouldn't be contacted for fear of contamination, damage,
deformation or part size. If you have high sampling rates, dozens of features, features laid out in patterns or multiple axes, contact methods are slow because the
probe must traverse the part point-to-point.
Microscopes and profile projectors are optical tools that magnify a work piece's surface features and enable the user to measure dimensions on a linear scale. Some
profile projectors can also be linked to a central process control system. They are simple and inexpensive but slow. They are mostly limited to two axes (X-Y)
inspection, and most profile projectors rely on human vision and judgment and are almost always operated manually.
Standard 3-D vision systems offer all of the automation features and
measurement capabilities of contact-type mechanical CMMs, but they do so without actually making contact with the part. Instead, images are generated by
high-resolution video in combination with microscope optics, with edge readings on a gray scale of 0-255 for more accurate resolution and finite-edge detection. A video
system discerns edges and line straightness much more precisely than one that depends on the human eye. That's because it relies on the video camera to capture
the shape and measure on a pixel level. Vision systems will do the job faster than a microscope or profile projector due to their larger field of vision and
program/automation features. Computer numerically controlled (CNC) video systems and vision systems can measure parts up to 32 x 32 x 6 inches and weighing up to 22 pounds.
Economy-size vision systems
represent a new trend in 3-D CNC noncontact systems. Typically, they're smaller and correspondingly lower priced than a
standard CNC vision system but are functionally comparable. In essence, they're small vision systems for near the price of a high-end profile projector. Both
noncontact vision systems (full-size and economy) are equal in their connectivity to the plant quality assurance and computer database. A typical economy-size unit is a
benchtop configuration with a measuring range of 8 in. X-axis, 8 in. Y-axis and 4 in. Z-axis. They're programmable and work by CCD (video) imaging, but are for
smaller parts and smaller workloads. Video imaging grayscale users are assured of very powerful (in terms of accuracy and repeatability) edge identification.
Laser-scan micrometers use laser energy to pick up edges. They play an
important role for those inspecting single dimensions on highly repetitive work, such as diameters, intervals, roundness, heights, widths and linearity. An application
example is measuring diameters generated by automatic screw machines.
On a price comparison, microscopes, profile projectors and laser-scan micrometers
are less costly than programmable CNC and vision systems.
Now that we've covered five options in noncontact inspection, the obvious
question is whether noncontact or contact inspection is right for you. Let's compare the capabilities and limitations of the two.
Contact vs. noncontact
Noncontact systems are definitely faster than mechanical systems, particularly for
high sampling rates. This is especially true if you're dealing with dozens of features, features laid out in patterns, or multiple axes. That's because you can measure more
points, see patterns at once and measure in three axes in a single setup. Contact mechanical devices, by contrast, must traverse the part point-to-point, which can slow things down.
Noncontact systems, as their name denotes, measure with absolutely no contact or probing. They are therefore ideal for measuring soft, deformable, sensitive work
pieces. This is also pertinent when the sterility of surgical instruments or the safe handling of hazardous, unwieldy, hot parts pose a concern.
The principal trade-offs of noncontact systems are increased size and weight
limitations and higher capital cost. Therefore, it's important to judge whether the economies in inspection throughput and cost justify the larger investment.
Another limitation of a noncontact system is that it will only clearly discern visible features on the exterior of a part. To measure an internal feature, such as an
undercut on a bore, one would have to resort to a contact, mechanical system.
Noncontact equipment also needs a cleaner operating environment than contact
types. In addition, most noncontact systems require that the operators be more computer literate than they need to be to use contact, mechanical systems.
Judging your needs
If your sampling requirements are medium to high, CNC vision systems will do the
job faster and more cost-effectively than microscopes. If you want to set Six Sigma failure rates as a quality assurance benchmark, advanced 3-D CNC machine vision
systems may be your only choice. The key is to judge whether the economies in inspection throughput and cost justify the larger investment over present and
anticipated volume. A microscope starts at $7,000, whereas vision systems cost about $45,000, with economy versions starting at $30,000.
When considering machine vision systems, be sure they provide connectivity to the plant quality assurance and computer data acquisition system. Even smaller units
should be programmable and work by CCD video imaging. Be sure that the system you select also provides grayscale edge-detection accuracy.
Noncontact equipment will need a cleaner operating environment than contact types even when all sensitive parts are enclosed for protection from dust and the usual contaminants.
Additionally, most noncontact systems require that operators be more computer literate than do contact mechanical systems. In reality, operators who handle
calipers or micrometers can be trained to use vision systems in one or two days. Icon-based operation permits quick and intuitive learning.
Now you're ready to pull the pieces together and decide which system--contact or
noncontact--will work for you. Based on your specific work pieces, customer specifications and inspection objectives, answer these questions:
What is the sampling rate or required inspection volume for each work piece?
The higher the required throughput, the more sense noncontact makes.
What is each work piece's size and weight? The smaller and lighter, the more likely a noncontact system will benefit you. Conversely, if the work piece is too big
to hold in your hand, you may be better off with a contact system. Parts larger than 32 x 32 x 8 inches and heavier than 22 pounds will dictate an alternative to noncontact.
Is the part so sensitive that readings may be affected by contact, contamination or deformation? If so, a noncontact device is obviously your answer.
What are the specified tolerances on the part print or customer specification?
The rule of thumb you can apply here is that if tolerances are less than +/-0.001 to 0.002 in., a noncontact system is best suited.
How many points, axes and geometric features have to be measured? The more
points and axes and the nearer to a pattern you need to inspect, the more you should consider noncontact. It will be faster.
Are the features clearly visible? If not, a mechanical gage is more appropriate.
Cutting to the chase: noncontact
Which generic type of noncontact system is right for you? Look at these factors about your operation: part size and weight, required inspection throughput, number
of axes to measure, print specifications on magnification, and tolerances/required resolution. Let's look at each separately:
Part size. The bigger and heavier the part, the more you'll need a vision system,
with its larger field-of-vision. Vision systems are designed to handle heavier parts than are microscopes or profile projectors. A good rule of thumb: If you can hold
the part in your hand, a microscope or profile projector will handle it fine. If not, either a standard vision or economy vision system will be needed.
Required inspection throughput. For small-volume work, a microscope or profile
projector will suffice. But as throughput requirements increase, so will the justification for a vision system. Throughput, in fact, is the main force driving the
trend toward noncontact vision inspection.
Number of axes. For single-axis work, all five alternatives are worth considering. However, for high-volume single-axis work, a laser micrometer will be a logical first
choice. For two axes, consider the microscope or profile projector. But for 3-D work, vision systems (which can measure in all three axes with a single setup) are the best choice.
. Some prints or customer specifications call for measurement at a particular magnification. If so, this will usually swing the choice
toward a profile projector or microscope that delivers the required magnification.
Tolerances and resolution. Higher-resolution requirements on multi-axis parts usually favor vision systems because the image and part edge are developed by
video and measured on a gray scale, which is more precise than the human eye.
Now that we've discussed hardware, let's turn to software. The computer system should be assisted by an open architecture program and standard user displays. A
Windows NT operating platform, for example, makes the system easy to operate or connect to a CAD program to insert additional algorithms. Make sure your system
comes standard with versatile processing tools for flexible measurement and intelligent, powerful algorithms. This greatly reduces the total measurement time,
especially when the part to be measured is complex. Some manufacturers offer an open software architecture that enables extremely versatile measurement capabilities
through the use of a common language, such as Visual BASIC.
If you are benchmarking your failure rate, select a machine that offers
seamless-performance high-level statistical programs. You can get it with today's advanced CNC vision systems.
Once you have decided on your noncontact system requirements, it's time to compare vendors. For example, ask yourself:
Does the supplier have a track record for making significant investments in innovation? Is the supplier a technological leader?
Does the standard package provide programmable four-quadrant lighting? Higher
light intensity provides ideal conditions for high-accuracy edge detection. You'll find wide variations among vendors on what constitutes standard lighting.
What accuracy standards are you using in your accuracy claims? U1 or U3
formula? What's included as standard features? Optional? Does the standard package include software, optics, videocam or image processing?
Compare warranties and after-purchase support. Are installation and training included in the quoted price?
Are hardware and software upgrades offered, and at what cost?
Does the software offer SPC data collection or work in conjunction with an SPC software?
How user-friendly is the equipment and documentation?
Does the standard package offer CAD connectivity? How easy is it to import a CAD file?
Does the system offer statistical data collection? Does it have powerful
algorithms? Does it meet your current and future needs?
How do you rate the supplier's credentials? Do they manufacture a comprehensive line of inspection instruments to meet all of your present and future
needs? Will they be around next year? Are they willing to handle your unique technical problems?
Inspection for the future
Process control simplification and quality are important in any competitive business
today. The trend toward smaller, lighter, simpler products is forcing more manufacturers than ever to deal with parts too fragile to measure with conventional
contact, mechanical equipment. In such cases, inspection requires systems that accommodate the parts, yet provide the necessary accuracy and cycle time.
Noncontact inspection may be key to unlock your throughput, cost and quality challenges.
About the author
Ken Parlee is QuickVision product manager for Mitutoyo America Corp. He can be reached by telephone at (626) 961-9661, ext. 4203, or by e-mail at email@example.com .