Best Coordinate Measuring Arms: How to Choose a Portable CMM

The best coordinate measuring arm is the one whose accuracy, reach and probing options match the tolerances you actually inspect, not the one with the biggest spec sheet. Portable arms from established makers such as FARO and Hexagon dominate the professional market, while hobbyist and small-shop buyers often start with entry-level articulated arms or alternative tools. This guide explains the specs that matter before you request a quote.

What a Portable Measuring Arm Actually Does

A coordinate measuring arm is a multi-jointed device with rotary encoders at each joint. When you touch a probe to a part, the controller calculates the XYZ position of the tip. Because the arm is portable, you bring it to the part on a bench, a machine tool, or a fixture instead of moving the part to a granite-table CMM.

Typical jobs include first-article inspection, reverse engineering, fixture verification, and checking large weldments or castings. If your parts are small and your tolerances are in the low microns, a bridge-style CMM may serve you better. Arms shine when flexibility and speed matter more than the last increment of accuracy.

Specs That Matter Most

Start with the manufacturer’s stated volumetric accuracy and the standard it follows. Many makers cite ISO 10360-12 test results, which describe how the arm performs in a structured test. Be careful comparing numbers across brands unless they reference the same standard, and ask for the full test report rather than a single headline figure.

• Reach: arms are commonly offered in several reach classes, often from roughly a meter-class to multi-meter models. Longer reach generally means lower accuracy, so buy the shortest arm that covers your parts.
• Axes: six-axis arms handle touch probing and are the common choice. Seven-axis models add a rotating wrist that makes scanner work easier.
• Weight and counterbalance: a well-balanced arm reduces operator fatigue and improves repeatability.
• Repeatability: ask how it is specified, since probing force and technique influence it.

Comparing Arm Types by Use Case

Probing and Scanning Options

Touch probing remains the baseline. Hard probes with interchangeable styli handle most feature measurements such as holes, planes and cylinders. Check that the arm supports the stylus lengths and ball sizes you need, and that probe calibration is quick enough to repeat often.

Laser line scanners capture dense point clouds that suit freeform surfaces. They add cost and learning curve, and the scanning accuracy is typically different from the probing accuracy. If you only measure prismatic features, a scanner is often unnecessary. If you reverse-engineer sculpted shapes, it may be the main reason you buy an arm.

Software and Workflow Fit

The arm is only half the purchase. Inspection software translates raw points into reports, GD&T callouts and CAD comparisons. Confirm that the software imports your CAD format, exports reports your customers accept, and runs on the computers you already own. Ask whether the vendor locks features behind annual subscriptions, since that changes the long-term cost.

Some shops pair an arm with a handheld tablet or mount it on a rolling stand with a magnetic or vacuum base. Stability of the mount matters as much as the arm: a wobbling base ruins measurements regardless of the arm’s specifications.

Environment, Calibration and Ownership Costs

Temperature swings affect both the arm and the part. Metals expand with heat, so measuring a part fresh off a machine can mislead. Many arms include temperature compensation, but a stable shop environment helps. Plan on periodic calibration or certification, typically annual, and ask the vendor what that costs and how long the arm is out of service.

• Ask about warranty length and what it covers, especially encoders and the counterbalance.
• Check local service availability; shipping a large arm for repair is slow.
• Budget for stylus kits, carrying cases, mounts and training, which are easy to forget in the quote.

If you are comparing accessories before buying, browsing CMM stylus kits and granite surface plates can show what a complete inspection station involves.

Common Buying Mistakes

Buyers frequently oversize reach, paying for accuracy loss they do not need. Another frequent error is judging an arm by its best-case spec while measuring parts with tolerances tighter than the arm can reasonably verify. A common guideline is that the measuring system should be several times more accurate than the tolerance being checked, so match the arm to your tightest requirement with margin.

Others skip a demonstration on their own parts. Request a trial measurement of a representative part, compare the results with a trusted reference, and watch how long setup takes. Operator comfort is also worth testing, because a tiring arm gets used less.

Frequently Asked Questions

Are coordinate measuring arms as accurate as a bridge CMM?

Generally no. Bridge CMMs in controlled rooms reach tighter tolerances. Arms trade some accuracy for portability and speed on the shop floor.

How often does an arm need calibration?

Many makers recommend annual verification, though your quality system or customer requirements may dictate a different schedule.

Can a beginner learn to use one?

Yes, with training. The hardware is approachable, but good technique, probe calibration and understanding of datums take practice. Vendor training courses are worth the cost.