I was wondering if anyone has any sort of rule-of-thumb about how many points to take on diameters. I know that varies for size and/or maybe what kind of tolerance on the feature. I mean, I know how many hits I usually take on certain features and what works for me, but is there any sort of stuff out there that talks about recommended numbers of hits for features? Just wanted peoples thoughts on this...
I learned a rule, long time ago, that says (3*number of hit which defines the feature) minimum :
Line defined by 2 points =>6 hits mini
Circle and plane defined by 3 points =>9 hits mini
Sphere defined by 4 points =>12 hits mini
cylinder defined by 5 points =>15 hits mini
Cone defined by 6 points =>18 hits mini
That rule doesn't give enough hits to define the form, only the size and the location.
Another rule that I read here is to take an odd number of hits.
To match with the previous it gives :
Line =>7 hits
Circle and plane =>11 hits
Sphere =>13 hits
cylinder =>17 hits
Cone =>19 hits
I was wondering if anyone has any sort of rule-of-thumb about how many points to take on diameters. I know that varies for size and/or maybe what kind of tolerance on the feature. I mean, I know how many hits I usually take on certain features and what works for me, but is there any sort of stuff out there that talks about recommended numbers of hits for features? Just wanted peoples thoughts on this...
From the National Physical Laboratory - National Measurement Institute in the UK.
One way to determine the correct minimum number of hits in a circle is to start off with the minimum and then increase the number of hits until the results stop varying enough to matter to you. This is going to be related to the circle size and process used to create the circle.
There are so many variables - type of probing system, uncertainty of the entire system (probe + CMM), tolerance you are trying to achieve, specific customer requirements etc...
For example, if you need to apply a UPR filter, you need to scan the complete circle (no partial arcs) and have at least 7 hits per undulation for the calculation to work correctly so, for a 50 UPR filter
MinimumPointsPerMM = 7 / ((diameter*pi) / 50)
I've also come across several OEM's that have their own guidance on number of hits - some provide a table where they might list the number of hits they expect you to take for certain ranges of sizes (up to Ø5, over Ø5 and up to Ø10, etc). I've seen others apply a formula - number of hits = 10% of the circumference of the circle, rounded to the nearest whole number for example.
I think kingsld1's idea is probably the most scientific and the only real "rule" you could follow.
I was told when I first started programming to always use a prime number for the numser of hits. Because this will will insure that no two hits are directly across from each other and you will get a better idea on the form of the circle. Of course with tigher tolerances you should use a larger prime number.
One way to determine the correct minimum number of hits in a circle is to start off with the minimum and then increase the number of hits until the results stop varying enough to matter to you. This is going to be related to the circle size and process used to create the circle.
One good practice is to study different methods on a ring gauge, number of hits, clockwise, anticlockwise, pre hit / retracts, touch speed's, probe size, touch trigger, scanning etc. Then check the results against the calibrated ring size.
