Stepping up to CMM

Portable arms are now a valid technology for entry-level users. The FaroArm is a portable coordinate measuring machine that allows manufacturers easy verification of product quality by performing 3D inspections, tool certifications, CAD comparison, dimensional analysis, reverse engineering, and more. With the ScanArm, the addition of the FARO Laser Line Probe to the FaroArm adds non-contact 3D scanning capabilities for detailed measurement of surface form, making the ScanArm the perfect combination of a contact and non-contact portable CMM.

If there’s one universal truth in machining, it’s this: if you can’t measure it, you can’t make it. Metrology in one form or another predates mechanized metalworking, but it’s really the drive to mass production and the need for standardized parts that drove the need for better ways of understanding part dimensions. Metrology has become a science in itself, which creates a natural conflict with production and custom machining processes alike. Fast, cheap and good are mutually exclusive properties in most job shop and production environments.

Coordinate measuring machines (CMMs) are a great equalizer, allowing smaller job and production shops the ability to monitor and control part quality in terms of accuracy and repeatability. CMMs are cost effective, but they are not cheap, making the decision to make the move to machine metrology an important one. There is almost no upper limit to the amount of time, money and energy that can be spent in part inspection and often, it’s the customer that drives the process.

Regardless of where the motivation begins, that first machine is the essential starting point for advanced production quality.

A basic bridge-type coordinate measuring machine uses a software-controlled motorized contact probe operating in the three Cartesian coordinates (x,y,z) to build a user-defined profile of a part’s critical dimensions.

WHY COORDINATE MEASURING?

For production shops, CMM technology is an obvious necessity, but more and more smaller job shops are embracing the technology. According to Rob Johnston, Carl Zeiss product manager, Elliott Matsuura Canada, the productivity argument for machine measuring can be as compelling for a small shop as it is for an OEM or Tier One part supplier.

“If you’re a small shop, take a step back and quantify the time you’re spending manually gauging parts and documenting parts”, says Johnston. “Big shop or small shop customers need documentation so without a CMM you’re buying multiple, often custom gauges, many of which may be undocumented. [Manual measuring] is also error-prone and can take hours to document a simple part.

It’s possible for a third of a machinist’s time to be spent measuring rather than cutting and running parts. In many cases it’s difficult to see how much time is wasted.

Job shops often also don’t realize how much money they spend on custom gauging. Often they may be halfway through a job and then realize they don’t have a gauge to measure the part. Then they’ll spend a great deal of money to buy a gauge used for a single application that then goes on the shelf and is never used again. And every individual measuring tool has to be recertified and recalibrated periodically. In many cases the overall cost is the same or more than owning a CMM.” Johnston notes that CMM ownership often generates significant savings by the third year of ownership, but the technology also allows the shop to change the way they think about part measurement. The obvious benefit is the ability to check more attributes more often, but a hidden benefit is the ability to use process control strategies for short runs that are usually reserved for mass production. Instead of a “first off, last off” approach, a CMM allows a thorough check of several early run parts to solve potential problems earlier. There’s also a significant marketing benefit to machine metrology, says Johnston, “It also allows them to show the customer their commitment to quality and it provides an internal yardstick to measure their how good their process is. CMM is as much of a sales tool of how good you are and your commitment to quality as it is a measuring instrument.”

The Mitutoyo Crysta-Apex S500/700/900 series Coordinate Measuring Machines can be upgraded with Renishaw’s PH20 probe head. The Crysta-Apex series machines are affordable and support a wide range of inspections solutions including laser scanning, optical, and surface roughness inspection. With the addition of the high-speed scanning capabilities of the PH20 the Crysta-Apex CMM adds wide ranging measuring capability in the quality lab or on the shop floor. The PH20’s unique ‘head touches’ allow measurement points to be taken by moving only the head, rather than the CMM structure. Using only the rapid rotary motion of the head, points can be taken faster, and with improved accuracy and repeatability.

MANY MACHINES, MANY CHOICES

It’s easy to justify the step up to coordinate measuring technology, but starting the process can be challenging. Mitutoyo Canada CMM sales team coordinator Gord Homann explains, “When making that first CMM purchase, a shop should look at the type of work they’re doing.

Never buy a machine just based on price, it should be the right tool for the job. It’s essential to meet the needs of what you are manufacturing. When buying a machining center, shops buy on the specifications they need....they should do the same thing with the CMM. It’s important to know what features must be measured, what tolerances exist, how tight they are and the volume of parts manufactured. Is it one million pieces or a ‘one-off’”?

Just like machine tools, there is a bewildering array of features and options to choose from. To start, Homann recommends starting with the basics, “In terms of CMM specifications it’s important to look at measuring volume and the accuracy specifications of the CMM compared to the tightest tolerance on the part being measured. Not all CMM’s are equally accurate. Is a manual machine good enough? Is it necessary to go to CNC to achieve higher accuracy?

Traditionally, a CNC (coordinate measuring) machine is twice as accurate as a manual machine. Volumes are important. A manual machine may easily suit the need if shop is doing series of ‘one offs’.”

Simple machines aren’t expensive by machine tool standards. A small manual CMM can be purchased for as little as $20,000 for a unit with a basic probe system, a computer and software. The automated solution can be bought for $50-60,000, depending on options and the measuring volume of the machine. Bigger costs more, as does measuring speed and accuracy.

THE ARM OPTION

While bridge-type CMM’s are used industry wide, there are other options. Laser scanners and other non-contact devices are widely seen as the future of metrology, but for entry-level operators, contact type portable arms are a relatively new option with several advantages in a smaller shop setting.

Compared to conventional CMM’s, arm type instruments appear much less sophisticated. In fact it’s a highly advanced technology that measures movement in articulated joints in multiple dimensions to define a series of points in three-dimensional space. The arm analogy is apt; the user leads the instrument by the hand touching off desired points in the measured object while the software builds a picture of the part form. For a shop just beginning with machine metrology, is an arm the right first machine? According to Canadian Measurement and Metrology president Elliott Foster, “most small shop owners from fabrication to machining are looking at space, expense and employing the resources to operate the equipment. It can be an onerous cost up front. There are several types of equipment a small shop owner should look at. One is the portable arm. Today they’re inexpensive; the accuracy has increased by 70 per cent in the last 10 years and are able to measure machined parts as well as other structures such as fixtures, castings and weldments and they’re entirely portable.

Many have built in temperature compensation. For a probing only system, the price can be below $40,000 for a new system. The skill level is lower to run a manual device; you still have the ability to read the CAD file, but can also add a laser scanner. For first machines we’re seeing interest in portable arms. Today, we sell as many portable arms as CMMs. They’re a great tool; you can bring them to the machine tool and measure in the chuck, you can mount them to a plate, you can measure fixtures and the complexity is less so there’s less training. For inspection at 30 micron and less an arm is an ideal solution. At the 10, 15 micron level it‘s different.”

Foster notes that it’s more important to match the measuring technology to the process, rather than the other way around. “It’s all about ‘horses for courses’” he states, adding, “if I am measuring batches of 20 or 30 parts, perhaps hundreds of parts it’s painstaking work generally to do that with an arm. For one-offs, fixtures, weldments, castings and short run work the arm is ideal. We’ll see machine shops making batches of 500 parts requiring 100 per cent inspection using an arm. It’s painstaking work. The other factor is precision. They don’t look at the CAD file or blueprint and miss the true positional tolerance of 10 μ for example.

The arm may be repeatable at the 10 μ level, but unless you know the calibration procedure used to certify that performance, it’s difficult to know if it’s usable for a given application. It’s important to talk to experts, regardless of the machine brand. If you’re going with an arm, make sure it fits the application.”

ACCURACY VERSUS REPEATABILITY

Many entry level buyers of CMM equipment focus on accuracy when really precision (repeatability) is the more important performance metric. A machine with proven precision can produce reliable data with fewer measurements and less time in inspection.

An important factor for first time buyers is the machine builders’ stated performance, but there’s a catch, testing methodology. Machine builders use internal procedures to qualify machines, using standard geometric shapes; real world parts have a multitude of possible quality issues such as flatness, roundness, straightness and others that are important to the end user but aren’t addressed by the manufacturer. Real parts create less than ideal testing conditions, making it important to try a new machine with actual parts to verify repeatability.

The precision issue is also a reason to buy a machine from a dealer with technical assets on the ground in Canada. Bankruptcies and auction houses can offer tremendous deals on used equipment, but CMM’s are not punch presses … a cursory inspection simply isn’t enough to weed out a “lemon”. It is possible to find a bargain, but for a first time buyer, the unit should have an extensive service history, be supported by a Canadian dealer with technicians familiar with the brand and have up to date calibration logs.

While the auction route carries risk, used equipment, if bought from a reputable dealer, can offer considerable savings. Foster relates, “For a mid-sized machine there are lots of great machines available from $20-25,000. Many mid-90’s machines are excellent and can generally be upgraded with new software. Entry level machines may have five year old software, but the rule of thumb is that a used machine is 50 per cent cheaper than new. For mid-90’s machines replacement parts are available and they’re reliable. If you stick with major name brands you can’t go wrong. FARO, Zeiss, Hexagon, Mitutoyo and others can be supported.”

MAINTENANCE: NO PROBLEM

Maintaining modern CMMs are not an issue for shops using modern machining equipment, as the basics still apply. Standard housekeeping regarding fluids, vibration and dust apply, although many units can tolerate shop floor environments, reducing or eliminating the need for a clean room.

Temperature compensation is important if the unit is portable, since the ambient next to five-axis mill is unlikely to match the QA room or engineering office temperature. The air supply is important, and is often overlooked.

Many shops routinely add oil downstream of the desiccation/filtration pack, which is great for cylinders and motors, but is a contaminant to a CMM. Air consumption is moderate; a dedicated supply from a small compressor is an easy solution.

In a five-axis CNC capable shop, it’s not a question of “why” CMM, it’s a question of “when”. Canadian shops have options at every price point, multiple brands to choose from and a qualified service infrastructure to keep the equipment calibrated. If you’re using hand instruments to measure parts made on CNC machine tools, you’re ready for a CMM.