[ Pobierz całość w formacie PDF ]
20-16 Chapter Twenty
Once the machine stabilized (about 2 hours), the largest drift within any two hour segment in a
single axis was approximately 0.4 µm, with individual spikes of 0.3 µm over a 30-minute time
frame. Two additional 24-hour versions of this test were run with the same level of results. It is
critical to note that the charts clearly display a direct correlation between temperature change and
displacement, very close to a linear relationship.
Tolerances on Tooling Components and Assemblies
What needs to be kept in mind on this issue is that the enhanced-accuracy CMM was justified
principally to measure critical features on tooling components and assemblies. In addition, we
were clearly aware (up front) that this CMM (or any CMM) was not capable of measuring every
feature we considered critical to process or function. For example, one of the restrictions on a
contact CMM is probe diameter. The smallest standard probe tip available is 0.3 mm, which
restricts measurements on an inside radii or diameter.
A large percentage of the features of size have tolerances of 1.25 µm to 2.5 µm with feature
location tolerances of 5 µm. I believe I would be conservative in saying that greater than 50% of the
features that are measured on this CMM are
tooling drawings at this time.
If we look back at one of the original assumptions (#1. 0.5 µm is accurate enough to tell us
what effects the tool shapes have on the forming process), this was a worst-case statement
which included accuracy and repeatability of the measurement system. What has been discussed
so far has been only repeatability.
Miscellaneous Feature-Based Measurement Tests
It is essential that the results from the thermal drift test are understood to be based on a simple
measurement within a small known envelope of 25 mm, so accuracy and repeatability are at their
best. Where it starts becoming more difficult is in measuring other types of geometric features
within a larger envelope, such as perpendicularity, cylindricity and profile, to name a few. It takes a
significant number of points on a given feature to get an accurate representation of its geometry. A
general rule to note is that as you increase the number of points, the better the accuracy and
repeatability. There are exceptions, but in general this holds true.
(6) Miscellaneous Variables Aid in Decreased Confidence of Measured Results
In addition to temperature, there are many other variables that influence accuracy and repeat-
ability. Some of these variables are humidity, contamination, types of probes due to stability (stiff-
ness) such as the difference between steel shafts versus ceramic and carbide, probe speed, and
fixturing. The list goes on and on. The key item at this time that is restricting our leap into the
sub-micrometer capability we need (and have been striving for) is temperature.
(7) Summary
The great part about our CMM is that it is exceeding the specifications committed to by Brown
& Sharpe/Leitz. They were aware from the beginning that our expectations of their system was to
push it well beyond their stated capability. They also mentioned that tight temperature control would
be necessary to accomplish this task.
I sincerely feel the level of temperature control I m stating here is also needed in many other
measurement applications at our site to reduce current inaccuracies. I hope I have convinced the
readers of this memo on the need for tight temperature controls to achieve sub-micrometer mea-
surement capability on this type of measurement system. I will need approval for additional ex-
penses of $35K to achieve the defined controls for the CMM room.
If there are any questions, I would be happy to address them as best I can.
END of MEMO.
All funds were approved based on this presentation.
Measurement Systems Analysis 20-17
20.3 CMM Performance Test Overview
The testing was done on a Brown & Sharpe/Leitz PMM 654 Enhanced Accuracy CMM to determine the
machine s capability and the confidence with which various features could be measured.
There are a variety of parameters affecting the repeatability of measuring a geometric element on a
CMM. These parameters can be separated roughly into three categories: environmental, machine, and
feature-dependent parameters. These include, but are not limited to, the following:
1) Environmental
" Room (and part) temperature stability
" Room humidity
" Vibration
" Dirt and dust in room
" Airline temperature stability
2) Machine
" Settling time (probing speed, probing offset, and machine speed)
" Probing force (upper and lower force, trigger force, and divider speed)
" Flexibility of probe setup (probe deflection)
" Multiple probe tips (star probe setups and magazine changes)
3) Feature Dependent
" Size (surface area) of feature
" Number of points per feature
" Surface roughness (form) of the part
" Scanning speed
[ Pobierz całość w formacie PDF ]