How do portable CMMs work? Find out in our new user guides: Fixed CMMs, including bridge cantilever and gantry types, offer a high level of precision but require the object to be brought to the machine for inspection.
A portable CMM is lightweight and provides all the benefits of a traditional coordinated measuring machine with added flexibility. The portable machine can be used anywhere as the machine goes to the part and not the other way around. A controlled environment is not required and the operation is kept simple.
Portable CMMs are typically less accurate than traditional Cartesian CMMs.
As the assemblies and subassemblies in manufacturing increase in size, they become less portable. This is where the portable CMM comes into its own for measuring accuracy, efficiency and convenience.
What types of CMM are portable?
Types of portable CMM include the Articulated Arm, which has different radial reaches depending on the size of the arm. The rotation of each joint and length of each segment is known to the software to combine with calibration to produce accurate results.
The angle of rotating joints within the arm structure is determined using optical rotary encoders. The encoders count rotation incrementally via the detection of accurately spaced lines on a glass grading disc. The software converts these counts into angle changes.
Articulated arms usually have 6 or 7 axes of rotation, resulting in a wide range of orientation.
Typical uses for the portable CMM include:
- Dimensional analysis
- CAD inspection directly against the CAD data in real time.
- On-machine inspections to inspect the parts whilst they are still on the machine that is producing them, to improve time efficiency.
- First article inspection to measure the individual parts and compare them with nominal data.
- Alignment to align the parts and assess any variation in relative position.
- Reverse engineering to digitise parts and objects, creating CAD models for production.
There are also portable laser trackers that measure two angles and a distance.
How do they work?
The tracker sends a laser beam to a retroflective target that is held against the object that is being measured. The light reflected off the target retraces its path, re-entering the tracker at the same position that it left. Retroflective targets vary but the most popular is the spherically mounted retroflector or SMR. As light re-enters the tracker it goes to a distance meter that measures the distance from the tracker to the SMR.
The distance meter may be either one of two types: an interferometer or an absolute distance meter (ADM).
Typical uses for this include:
- Alignment with real time feedback for object positioning
- Installation or layout/level machine foundation
- Part inspections to create a digital record of the actual versus nominal data
- Tool building to set up and inspect tools with just one operator
- Reverse engineering to create high accuracy digitally scanned data
How accurate are they?
Portable CMMs provide highly accurate results and are robust enough to work in a wide range of environments. The important element for accuracy with portable CMMs is the setup and calibration for each project. Just like with a fixed coordinated measuring machine, the probe must be accurately calibrated to ensure accurate results during operation. Each time an element has changed in the environment and operation of the CMM, the results much be checked for consistency and to eliminate incremental errors.
What is the difference?
The main difference is that the accuracy must be checked more frequently with a portable CMM as it is moved far more often than a fixed position one. With adequate setup and calibration routines, you can expect no greater error margin than with a fixed position machine.
When compared with the alternative portable solution, which is to take handheld devices to measure the part or object, the accuracy is infinitely improved. A portable option is still the best choice for when oversized, immobile or inconveniently located items require measurement.
Of course a CMM is only as accurate as its last calibration, so the quality of the probe spheres and software used during the calibration process is vital in maintaining accuracy.
CMM machine tolerances
The tolerances of CMMs can be adjusted, depending on the project. Most industry recognised tolerances will be ISO accredited to ensure that coordinating parts and manufacturing process are compatible across the globe.
ISO10360 is a standard recognised by industries across the world that guarantees a level of accuracy that coordinating assembly lines and manufacturers can work to, ensuring perfect results and compatibility.
This standard of ‘acceptance and re-varication’ was established in 1994 and outlines the specific details for testing the applications of CMMs. It includes details about the length, form measurement data with and without a rotary table.
An accuracy formula is followed to ensure that coordinated measuring machines are performing to the same standards across the industry. The most recent and relevant ISO accredited standard is ISO10360-2:2009.
The formula consists of a thoroughly detailed calibration procedure that takes a series of 5 different measurements of the length standards at specific intervals to match the CMM axis. These are taken again at 7 specific angles through the volume and repeated 3 times to ensure accuracy.
When calibrating the probe, the measurements are taken in a setup that is angled and offset, if feasible, to ensure that the results are realistic.
The results of the accuracy formula will appear as follows:
The “error of indication for size measurement” is given by:
Error (Eₒ) = A + L/K ≤ Emax
- L is the length, measured in mm
- A is a fixed term in um (microns)
- K is the Length divisor
- Emax is the maximum error
This specification can be used by inserting the length that you are measuring.
e.g in this example A=5um , K=177 , Emax= 7.5um
Length = 200mm Eₒ = 5+200/177 = 6.13um
So if the length was 200mm long, the CMM could read between 199.99387mm and 200.00616mm
Length = 800mm Eₒ = 5+200/177 = 9.52um (as this is larger than Emax, then Emax is used e.g. 7.5um)
Using Eₒ ≤ Emax the error found will never be greater than the Emax.
For a CMM to retain its accuracy it should be serviced and calibrated regularly. A typical service and calibration interval is 12 months, depending on the CMM environment and accuracy levels required etc. the interval may need to be reduced to less than 12 months.
So now you know how do portable CMMs work, call QCT today for information on purchasing one: 01332 572882.
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