Analog Scanning vs. Touch Probing

In the manufacturing world, inspection professionals are tasked with identifying the most optimal measurement solution to match their needs and budget. Gathering and analyzing large amounts of dimensional data in the shortest amount of time is the most effective technique for maintaining control of manufacturing processes. Analysis of this information can indicate out-of-tolerance conditions and suggest ways of modifying the process to improve work-piece quality. However, accurately collecting enough dimensional information for effective process control has been a challenge, particularly for contoured shapes.

One of the most common solutions, the Coordinate Measuring Machine (CMM), is the creme de la creme when it comes to efficient data gathering of even the most complex of geometries.

However, getting a CMM is not just as simple as going online and selecting "buy now". Many different configurations exist that can either make your machine a gold mine or sink the project like the Titanic. When considering a CMM, none of these configuration options matter more than selecting the type of probing necessary. Special care must be taken to select a configuration that accommodates the measurement applications and is neither overkill nor inadequate.

Today, we are going to break down this complex world and help you select the option that is best for you.

For purposes of simplicity, our primary focus will be on the two main categories of CMM probing that have been the mainstay for decades, Touch Probing and Analog Scanning.

The more traditional option available are touch-trigger probes, which take discrete points on a part. The newer, more advanced option, analog scanning probes, will run along the surface of a part, taking thousands of individual points at a time.

Touch Probing

While there are advanced sensor solutions available for specialty applications such as laser stripe scanners capable of taking tens of thousands, or even millions, of points on a surface. Chromatic white light sensors (CWS) which offer high accuracy, non-contact inspection. Blue light scanning capable of measuring large throughput of shiny, reflective components, etc. Measurement equipment manufacturers are constantly changing with the landscape of manufacturing.

Today, there are coordinate measuring machines (CMMs) that automatically measure 3-D forms and use that information in conjunction with computer-aided design (CAD) systems to provide insights into processes involved in the work-piece manufacturing operation. They are called scanning CMMs. Scanning is simply a way of automatically collecting a large number of data points to accurately define the shape of an object.

When faced with the decision of whether to scan rather than touch probe a part, metrology professionals inherently lean toward analog scanning when the need is to capture large amounts of high-accuracy 3D coordinate data. It is also especially useful because it is more likely that all critical features are captured.

As well, using analog scanning improves repeatability of the results while decreasing measurement uncertainty. However, in reality, the decision to scan a part is more dependent on the application and therefore should not be generalized. There are far too many variables to consider when it comes to  identifying the most appropriate probe configuration answer.

To begin with, evaluating inspection techniques is to look at the overall manufacturing process. For example, is the material such that there may be situations where there may be a thicker area prone to shrinkage as the material cools and settles? If the answer is yes, and it is a critical feature with high tolerances, it should be scanned. If the feature is not critical and has low tolerances, touch probing is likely sufficient.

Also, when evaluating the process, the component’s use should be taken into consideration. A plastic connector for automotive parts, as an example, requires a tight seal to eliminate the possibility of water ingress. Though the connector has simplistic features that could easily be touch probed, the edge where it forms a seal will likely have tighter tolerances that require analog scanning.

A skilled metrology professional will review the manufacturing process from start to finish to determine the best approach.

Faster Data Collection:

Conventional CMMs equipped with touch-trigger probes use the stitch scanning method to record streams of points from a part surface. In stitch scanning, the CMM lifts the probe head from the surface of the part, moves it slightly forward and lowers it until contact is made again for every data point that is collected. This single-point procedure is slow and not suitable for efficient measurement of complex shapes.

Analog probes for continuous scanning--designed to send an uninterrupted stream of data back to the system’s computer--are a solution to this problem because they eliminate the time-wasting auxiliary movements required by point-to-point measuring probes.

The mechanics of a CMM and its control system, drives and filtering procedures must be designed to take full advantage of the benefits offered by continuous scanning. For example, the mechanical system must provide rigidity for high repeatability. The control system is also critical, as it links the mechanical system, the scanning probe, the drives and the data collection system for computer data analysis. The system must quickly identify surface form changes so that the contour path is precisely followed. The speed and accuracy with which the control system reacts, even to the smallest contour changes, determines the CMM's throughput. Fast, parallel data transfer must be accomplished to ensure that measurement analysis is not delayed. Many factors can determine the actual measuring speed of a scanning CMM, including acceleration, maximum velocity, probing speed, probing method and the computation power of the CMM software.

Because forms measurement requires the collection of so much data, there are special scanning functions that aren't found in other CMM software packages. One of these is the filtering function, which allows scanning software to distinguish between subtle changes in the surface form and variability in the surface finish, such as the rough area in a turbine blade.

As with any measurement system, a scanning system's measurement results should help identify possible part problems easily, find root causes and take corrective actions on the process. The result of the comparison between "as-built" and "as-designed" parts can be displayed in various graphic forms that enable a faster understanding and help detect and troubleshoot manufacturing problems. Variations are displayed as color-coded plots, with nominal tolerance bands laid over the scanned curves.

In some systems interactive programming and editing take advantage of CAD graphics. Advanced scanning software packages allow a CAD representation of the 3-D part geometry to be imported into the measurement program, automatically extracting the nominal values and the correct vectors from the mathematical definition of the surface provided by the CAD model. The "patches" and the profiles to be scanned on the part surface are selected by clicking on the screen representation of the part. The system then automatically scans each patch and profile and reports on the data collected.

The most advanced scanning software includes a number of application-specific packages that simplify and optimize the measurement and analysis of diverse complex part geometries. These include dedicated modules for the inspection of airfoils and turbine blades, screw compressors, scroll compressors, spur and bevel gears, cams, and many others.

Much boils down to this question: What are the Tolerances?

As alluded to previously, the tolerance required on a given datum will play into determining the ideal inspection method.

Touch probing delivers high accuracy, but it has lower repeatability when compared with analog scanning probes.

Analog scanning is considered the most accurate method of inspecting a part and can hold tolerances of as high as ± 0.0005 inch or better. In the case of certain medical components, scanning is the ideal option. This is because the profile needs to be inspected to a high level of accuracy.

On the other end of the spectrum, certain parts lend themselves to discrete point probing. A stamped component with loose tolerances, where form is not an issue and the profile of hole locations is required, is best suited for point probing. Even though the part may contain multiple features, scanning is overkill and a waste of money.

Investment:

A significant variable when examining different probing options is the available budget. There is a reason this factor is the last consideration, rather than the starting point. An organization should first research the best technology for its application. It should then look at the benefits, as well as the associated cost savings, and calculate the predicted return on investment. Only then should budget considerations be addressed.

Companies are always in pursuit of the next big thing to help cut costs, boost production or increase sales. This leads some to follow industry trends and jump on the bandwagon every time a new technology is introduced.

However, just because the competition is doing it does not necessarily mean it is right for every company. There is a difference between doing things and doing them well. Narrowing down the best option takes work, but with proper foresight and investigation, the ideal solution soon emerges.

Industry’s move toward scanning solutions is partially because of improved throughput, but also due to the way in which part prints are created. In the past, parts were referenced with width and length dimensions.

Today, product lifecycles continue to shrink. To iterate an existing part, using width and length, is too complex and takes too long to change. When the entire shape of the part is controlled by profile, the time required to change the CAD model drops drastically. A part that is controlled by profile also lends itself to embedded GD&T profile controls. Scanning is especially useful when the location of features is contained within the part’s profile.

Blurring the lines with 5-Axis Touch Probing:

What is 5-axis measurement?

Based on advanced head, sensor and control technology, Renishaw, the world’s foremost manufacturer of CMM probing systems has created 5-axis measurement technology which delivers unprecedented measuring speed and flexibility, while avoiding the speed versus accuracy compromises inherent to conventional techniques. It boosts measurement throughput, minimizes lead times and gives manufacturers a more comprehensive appreciation of the quality of their products.

Unlike systems based around indexing heads or fixed probes, 5-axis motion enables the stylus to follow a continuous path around complex components without having to leave the surface to change stylus cluster or index the head. Controller algorithms that synchronize CMM and head motion produce an optimal tip path and minimize CMM dynamic errors.

Increase throughput (but with Touch Probing)

The ultimate scanning speed of a CMM is limited by machine dynamics, typically to between 80 mm/sec and 150 mm/sec. However, long before we reach this limit, measurement accuracy falls away; often limiting the effective maximum measuring speed to between 10 mm/sec and 25 mm/sec.

How?

Non-linear motion on a Cartesian CMM induces accelerations and decelerations that twist and deflect the machine structure, and these dynamic deflections result in measurement errors that increase with measurement speed and acceleration.

To avoid dynamic deflections, Renishaw's 5-axis measurement minimizes machine accelerations, while moving the stylus very rapidly over the component surface.

Simply put, with the introduction of a 5^th axis to the probe head itself, rather than have the bridge of the CMM move to take data points, the probe head moves with rapid infinite head positioning which creates a much more efficient inspection routine environment and therefore three times the throughput of your traditional 3-axis touch probe solution. This and competitively priced with any traditional 3-axis touch probe solution on the market today.

In Conclusion:

The importance of throughput vs. accuracy is the key consideration in developing a probing strategy. Large amounts of data may be critical for process control purposes, but gathering that data can increase inspection cycle time, ultimately affecting throughput. For other inspection purposes, such as determining the location and position of a work-piece feature, fewer data points can adequately provide the necessary dimensional information.

Taking the time to do your own thorough investigation will pay huge dividends down the road. One thing that is guaranteed, with all the options available to manufacturers today in the CMM and probing world, if you take the time you will be able to identify a solution that will suit your specific needs and budget!