|
|
Wireless probe performance
I'm frequently asked about the relative performance of
the various probes on the market and the impact that the wireless probe has
on that performance. I added this page to try to explain what the
limiting factors are give you an idea of what you can expect. Below
you'll find downloads of the test results and the Fusion model I used to
generate g-code that produced the reports. You'll need my
post-processor to generate the g-code for yourself. The macros required to
run it are already part of PathPilot.
If you want to know more about how the pdf reports
were created, see this page.
This test was performed using a 2020 model Tormach
passive probe, converted to wireless operation with one of my wireless kits.
The stylus I was using was from Renishaw. It's 50mm long (carbide)
with a 3mm ruby tip. At that size, it's likely to be one of the
bendiest stylii you'd use, so the results presented here are at the poorest
end of what you can reasonably expect.
The test
As a test, I chose to use a 1-2-3 block and used the
inspection report generation
feature of my post-processor to create pdf files with reports of the
results. The 1-2-3 block was mounted on the moving section of my vise,
retained by 4 small magnets. The 1-2-3 block was pressed against the
rear of the moving jaw, so the alignment with the mill's axes was only
approximate. Also, since I don't have access to any equipment capable
of measuring the block's dimensions accurately, I can't assess the absolute
accuracy of the measured dimensions. In any case, that would only
indicate the accuracy of the machine's ball screws rather than anything
about the probe.
The test then consisted of the following steps.
- The effective diameter of the probe tip was
determined using a 3/4" gauge ring and the tip calibration routine in
PathPilot. The effective diameter was measured at 2.839mm. The
difference between that measurement and the nominal 3mm is a mixture of
the mill's lost motion and flex of the stylus and probe mechanics.
As noted above, this probe's flex is likely to be comparatively large and
a shorter larger diameter stylus would likely produce results closer
to the nominal size.
- The mill's coordinate system was aligned to the
centre of the 1-2-3 block using Fusion's probing routines to probe both
sides in X and both sides in Y and take the centre.
- The program then made 3 separate measurements of X
size and centre and similarly 3 measurements of Y size and centre,
distributed across the length and bredth of the 1-2-3 block.
- The measurements in 3 above were repeated 3 times
and tabulated as if 3 separate 1-2-3 blocks were being tested. In
fact it's the same 1-2-3 block being measured 3 times. The three sets of
results should be identical and any variation indicates the repeatability
of the probe system.
- The probe was rotated 90 degrees in the spindle and
steps 2, 3 and 4 were repeated, creating a second report of 12
measurements repeated 3 times.
Downloads
You can download the following documents
- The Fusion
model that generated the nc code and report results.
- Report with the
Tormach probe's cable gland pointing in the -ve X direction.
- Report with the
probe's cable gland pointing in the -ve Y direction.
Results
Things you should know and things you should be able
to see from the reports.
- The smallest movement you can ask a 440 to make is
0.0025mm. That's just a fraction under 0.0001". So the reports
can't show anything smaller than that.
- The lost motion in a decently adjusted Tormach mill
is in the region of 0.001", so very roughly 10x the smallest move you can
ask it to make.
- The 3 tables in each of the reports should be
identical. If you compare the tables in either one of the reports,
the variation in any single measurement is at worst 0.0003" or around 1/3
of the mill's lost motion! Given that that variation is the sum of
the mill's repeatability, the probe's repeatability and any variation
caused by the wireless system's latency it's hard to conclude that any of
those components could be usefully improved. The repeatability
results are better than you could reasonably expect.
- If you compare the 2 reports, you can see that as
you go from one probe orientation to the other, the x measurements shrink
by about 0.001" and the y measurements grow by the same amount. This
is caused by the flexibility of the stylus and the fact that the force
required to activate the probe varies depending on the direction you
approach it. That's a function of the 3 point suspension used in the
probe and the strength of the spring that holds the probe in the central
position.
Conclusions
The conclusion is that the biggest error in the whole
system is caused by the amount of stylus flex you see varying because of the
activation force of the probe being different at different probing
directions. None of that error is caused either by the mill or the
wireless system and it's inherent in the 3 point suspension used in probes.
The things that would reduce that error are
- Use a shorter probe tip or one with a larger
diameter shank so there's less deflection to vary.
- Use a probe with a weaker spring, again reducing
the stylus deflection.
The downsides would be
- Fatter, shorter probe tips are limited in where
they can reach, so less convenient to use.
- If you weaken the probe's spring too much, then the
probe will start to lose reliability as the contacts won't always be
re-established when the probe moves away from the contact point.
Home | Performance | Firmware
This site was last updated
06/05/21
|
