Performing a fast interim check with a tetrahedron costs only some 10 minutes.
Length measurement errors are verified in 6 different lines in the machine volume.
The evaluation of tetrahedron measurements is done by the application "QuickCheck-T". This application keeps track of the machine´s geometrical performance over time. Parameters to whatch are normally the measured length errors in comparison with the specified maximum length errors.
In the evaluation routine as well the 3 squareness errors and the 3 scale factor errors of the machine are calculated for the volume where the tetrahedron was placed.
This is a tetrahedron without steel components.
Only carbon fibre composite and alumina ceramic balls are used.
This "Mini Tetrahedron"is made of Vitro-Ceramics (Robax from Schott, zero thermal expansion).
The balls have 2 mm or 3mm diameter and are good for actile as well as optical (video) measurement.
This is the standard tetrahedron with edge connectors of steel. Being very compact, these edge
connectors don't contribute much to the thermal expansion coefficient of the tetrahedron, and
the accessibility for probing is very good.
On the left we see a disassemblable tetrahedron in "normal" tip-up- arrangement
This tetrahedron is composed of 6 rods and 4 steel spheres.
The 6 rods are connected with the spheres by magnets inside their ends.
These rod ends are conical and precicion-lapped to match the spheres.
Our disassemblable tetrahedrons are secured against ac cidental disengagement of rods and spheres (see photograph on the right).
Such a tetrahedron allows to perform very fast interim checks on large CMMs too.
It is just the large CMMs that suffer from rapid changes due to influences from
soil and temperature environment.
Disassemblable Tetrahedron "2nd Generation"
2 rods with ceramic balls, 4 rods with precision-lapped centering cones of ceramics
Easy to calibrate: just send to the calibration lab. the 2 rods with fixed spheres (i.e. the number 1-2 and the number 3-4). Many laboratories are accredited for ball bar calibration.
Then perform a calibration of the other rod lengths on your own CMM by comparison: measure the tetrahedron in its coordinate system in three orientations, rotating the assembled tetrahedron in increments of 120°; average the results for each rod length. Apply correction. The correction is the difference between calibrated length and measured length.
For best accuracy repeat these 3 measurements all 3 times and disassemble and assemble in between the 3 sequences. In all make 9 measurements.
- Assembly Instructions
- EXCEL application for CMM Interim Checking with Tetrahedrons (requires password)
- Calibration Spreadsheet.
Miniature tetrahedron (10 mm balls) . The 4 ruby balls are pressed together by spring force.
They form a very reproduceable tetrahedron array. No glue is used.
The calibration is performed on the individual balls.
Only carbon fibre composite, plastic screws, tubes and ruby balls are used.
This is an excellent resolution test artefact as the balls' distance starts slowly growing from zero.
Optical measurement on a miniature tetrahedron (courtesy: Quality Vision Japan)
Tactile measurement of miniature tetrahedron (courtesy: Quality Vision Japan)
Multi-sensotr tetrahedron in transport box
With QuickCheck-T one may evaluate the length measurement errors and
machine geometry parameters, using measurement data obtained with the tetrahedron.
Length measurement errors are recorded as the difference between measured and
calibrated ball distances (with sign). The machine geometry errors are: scale factors X,
Y, Z as well as squareness errors between the axes YX, ZX, ZY.
One may store these results in a history sheet and review the changes of the CMM
over time (similar to a process control chart).
user surface to the QuickCheck-T user surface.
Note that usually only the last digits change from inspection to inspection.
Once all data have been entered, we press the button "Add results to history"
and the history sheet is amended by a new record line with all data needed for a thorough
"traceability".As well the three graphics are amended (position, squareness, maximum length error).
Data are normally entered by copy-pasting as there are only 6 values to be transferred from the CMM
Caps are available to cover some or all the balls of the tetrahedron so that the user is forced to use different offset styli. This is sense making for proficiency testing ("inter-laboratory comparisons") ... it makes differences in measurement accuracy bigger.
The caps can be removed by the reference laboratory for better calibration and then re-placed and sealed.