All About Portable CMM Arm Accessories

Choosing the best, most appropriate add-ons makes your work faster and easier

Published: Monday, May 22, 2023 - 12:02

A very popular version of the coordinate measuring machine (CMM) is the portable CMM “arm,” not to be confused with the “robotic arm” or simply, robot—as it is called in the factory automation world. Robots are motorized, machine-driven devices that have tube segments connected by articulating joints. The portable CMM (PCMM) arm is not motorized but manually operated. The lack of motors is part of what makes them especially portable, lightweight, adaptable, and versatile.

Unlike robots and CNC CMMs, portable CMM arms don’t require programming. However, digital work instructions are often employed with PCMM arms to make them more productive, controlled, and repeatable. CMM arm accessories are an essential part of the everyday use of these systems, and employing the best and most appropriate add-ons will make the job go faster and easier. These accessories are used with CMM arms manufactured or sold by API, Hexagon, FARO, Kreon, LK, Master3DGage, Nikon, Romer, Tomileri, and Trimos.

The two primary benefits of PCMM arms are: 1) they make it quick and easy to check manufactured parts and assemblies in place, and 2) you don’t need a measurement lab to use them. It can be very useful to measure in the machine in which the item is being manufactured (such as a CNC machine), or on the assembly to which it’s mounted (such as a car or aircraft). There are numerous types of accessories for PCMMs, depending on the specific task and the many production scenarios that might be encountered.

Hardware accessories are used for important requirements such as mounting the arm to a firm foundation so it remains solidly fixed while measuring. Also, probes of all sizes and lengths are necessary to reach different features that might be near the surface or deep inside the inspected article. This guide is a general overview of the commonly used accessories with today’s PCMM measuring arms.

Mounting bases

The most common base includes a threaded 3½-in. adapter on a machined plate, which often includes a magnet or vacuum assembly for securing to a steel or granite table, respectively. Bases can also be clamped or bolted to a solid, sturdy table, such as granite or steel. The 3½-in. threads comply to a metrology standard that allows the base to be adapted to many types of stands, tripods, carts, or tables.

Tripods

For easy transport within the factory, and especially out in the field, folding tripods are an excellent option. A tripod’s three points provide geometric assurance that the structure will sit firmly on the floor but won’t rock if not perfectly flat. Many come with a transport case of their own. Also available are accessories such as extendable arms with magnets or clampable brackets to secure the tripod to the workpiece or table. Another good add-on is a laptop shelf.

Rigid rolling stands

Similar to tripod mounts, the rolling stand is highly portable, albeit not as lightweight and packable as the tripod. It’s best suited to use inside the factory, and because it has more rigidity than a tripod, it can be a better solution for those who need to move around the shop and don’t need to take the PCMM off-site often. The rolling stand typically has the 3½-in. interchangeable-metrology threaded ring to easily attach and dismount the arm. It has hard wheels that can retract after reaching the measurement location, leaving the arm and stand on three points of firm, nonrocking foundation. Three points provide geometric resolution that ensures the structure will be rigid but won’t rock if the floor isn’t perfectly flat. As with the tripod, a laptop shelf might be a good add-on for the rolling stand.

Granite tables

Common in inspection labs, and also made of the same material used on virtually all tables of stationary, programmable CMMs, granite tables are also good matches for CMM arms. In fact, it’s common to see arms mounted to a CMM’s granite table for quick or auxiliary checks.

Steel tables

Although steel has its downsides of weight and cost, it’s a good material to make a firm, rigid, and flat work surface. Steel also has the benefit of allowing magnetic mount bases, which make quick relocation and mount/dismount of the arm a breeze.

Portable carts

Commonly configured as a steel, robust cabinet with wheels and a granite surface plate mounted to the top, these make great workstations for rolling the arm around the shop. Optional monitor mount poles can be added to make room on the table for work, while the computer workstation can be housed inside the cart.

Scanners

The PCMM arm scanner attachment, or “laser line probe,” adds significant data-capture capability to the arm. Main benefits include its noncontact data-gathering ability and its fast acquisition of data. While many who sell arms and scanners promote the notion that one must have a scanner for reverse engineering (RE), this is absolutely not the case. Scanners can provide a significant advantage for RE on certain geometry types, such as organic contours, or for parts that require considerable, fine detail. But for simple, primitive shapes and even gentle contours, a hard probe can often do the job more quickly, more accurately, and at a lower cost. For inspection, contoured shapes that don’t require super-tight tolerances are well-suited to noncontact scanners. Good examples are stamped or formed sheet-metal parts, and plastic or composite parts that have been molded. Here again, for primitive shapes, such as machined parts with flats and small deep holes, especially with tight tolerances, a hard contact probe might be more efficient and cost effective.

Turntables

Turntables are most commonly used for 3D noncontact scanning because they allow the workpiece to be turned and easily scanned 360 degrees around the part. Typically, they’re designed with an integral encoder and communication support between the arm and control software. This is so the workpiece can be rotated and data measured or gathered while maintaining the 3D reference system, ensuring good quality data. A turntable makes the work go faster and prevents device leapfrogging or relocation to access all sides of the part.

Training, demonstration, and DPD1 artifacts

A machined part of stable material with a wide range of primitive features as well as organic contours makes a great tool for several useful purposes. It’s excellent for training new users, and also for a refresher learning session to help maintain knowledge of both the arm and application software. There are times a user may want to test out a feature or workflow, and having a familiar, known part is good for those tasks. As a test artifact, a machined part provides a tool to check the arm, software, and processes for accuracy and repeatability. Some companies, such as Boeing, have strict requirements for digital product definition (DPD) that require their suppliers to check their measurement systems against a known artifact periodically, or if any changes have been made to the hardware or software since the system was last verified.

Certified length standards and scale bars

Certified length standards are best-practice tools that are either required or highly recommended to check the arm’s accuracy and repeatability over the measurement volume for which it’s designed. These metal bars, made from reasonably thermally stable material, should be checked at least annually against a NIST-traceable standard by a qualified laboratory or metrology service provider. Certified length values are labeled on the bar so the user can check actual measured values against the certified numbers. Scale bars are similar to certified length standards in that they’re also made to be stable in length and must be certified by a lab to NIST standards. They are similarly used to verify that the measuring equipment is accurate and repeatable. Length standards and scale bars are among the most overlooked of the CMM arm accessories, and they definitely shouldn’t be.

Software

The choice of operation and analysis software and its capabilities, user-friendliness, and interoperability is critical. Software really does the “heavy lifting” and can be what makes the portable arm either a productive measurement tool or a boat anchor. The software must work cohesively with the arm, the baseline for measuring—e.g., the CAD model—and the user.

Many have chosen their software based on a salesperson’s demonstration on a rehearsed demo part, and based on promises that it will do everything it will be called upon to do. And, of course, it will do everything so easily a child could do it. The truth is that’s not often the case. The real test is for prospective arm users to test the software on their arm, in their shop environment, on their real parts, and to their real specifications. Although it isn’t normally realistic to stage a full inspection or reverse-engineering job from start to finish, the system must be taken through its full paces as much as possible. All software and hardware should go through the same apples-to-apples evaluation for true due diligence that won’t disappoint.

Follow these links to see Verisurf’s inspection and analysis suite, the tool-building and inspection suite, and 3D scanning and reverse-engineering suite.

Computer workstations

Recognizing that we’re talking about portable arms, it makes sense that the workstation that operates and gathers data from them should be a laptop. (If an arm is used regularly in one location, a desktop computer would be fine.) Some users roll their arm station around on a robust steel cart with a granite worktop, so if the desktop workstation fits in the cabinet and includes an optional post to mount the monitor, a desktop might bring more power and efficiency and is a bit safer from theft. Also important, whether a laptop or desktop, it should be an “engineering” quality computer—one that has been heavily tested by a reputable manufacturer so your expensive measuring machine isn’t rendered useless due to a failed workstation. By “engineering quality” we mean the PC’s processor for performance and speed, RAM, hard-drive space, graphics card, and monitor, which are all important. In the case of inspection, most work is done with CAD files or, in the case of reverse engineering, creating a CAD file. So a system with a graphics card optimized for CAD is the way to go. (Recommended workstation specifications.)

Custom probes

Probably the most important of all the CMM arm accessories are the probes. While “standard” probes are provided with arms at no extra cost, custom probes are will be required to measure certain shapes and features. Because each customer has different use cases as well as parts of sizes and shapes peculiar to their operation, each user will need to purchase or fabricate custom probes of the size and reach necessary for the different situations they encounter.

Probes will differ mainly by their lengths and stylus-ball diameter and material. The most common materials are ruby, stainless steel, ceramic, and silicon nitride. Also, the probe-mounting feature will vary by OEM. A good option is for the user to obtain a probe base piece that can adapt varying probe shafts and styli to meet requirements as they emerge. Then they can be customized as needed.

A word of caution about probes with small styli: They can be fragile and easily damaged if not handled gently. Maintain firm control of the arm’s wrist, taking care not to let the stylus drop on a hard surface.

Fixture plates and modular fixturing

A few manufacturers offer modular fixture plates, which are tools that can be quite useful, versatile, and economical. They have either threaded or slotted anchor points for spring arms, clips, and stacking elements that let you create quick mounting setups for your parts. These will hold the parts firmly in place but allow you to fix different shapes securely on them.

Probe calibration sphere or fixture

As with stationary, programmable CMMs (read “What is a CMM”), portable CMM arms must be calibrated or “qualified” to accurately locate the center of the probe ball. This is done when there has been a change in the probe itself, when a different probe is mounted on the device, when the probe’s accuracy comes into question, or as normal best practices at the start of a measurement cycle.

For the calibration, the device and probe need to measure a sampling of points; calculate a mathematical average, then locate, fix, and save the result to the measuring software. Certified qualification spheres are the common solution, usually about 1 in. or 25 mm in diameter, affixed to a base that can be clamped firmly to a work surface. Alternative to the sphere concept is a fixture in which the probe can be firmly nested while the arm is rotated about the fixed point to gather calibration point samples. Whether it’s a sphere or a fixture, probe calibration and regular use is an absolute must.

CMM arm accessories should be well maintained and protected from impact, accidental drops, adverse weather, and corrosive environments. All applicable items should be part of the arm’s annual calibration, and should also be assembled on the arm when accuracy checks are performed for each job or each work shift.

This article first appeared on the Verisurf blog.

About The Author

Scott Knoche

Scott Knoche is part of the marketing and technical sales team at Verisurf.