Recently I've seen quite a number of posts on the forum discussing why sometimes the cmm software flips the primary axis direction (alignment rotated 180 degree to what it's meant to be) and how to fix it. Many experienced users have provided various solutions that will work indeed, which really shows the good spirit from an open technical forum.
It's still my interest to compile an article on this very practical issue along with a few methods so new cmm users can learn from there. Flipping on the primary axis (mostly measured as a plane) is not something new, it actually happened to me long time ago when I was programming in Geopak, MM4, LK-Dmis, typically old, non-cad system. And this issue is also software oriented because I still remember that one very old dos based DEA system will flip the primary axis depending on whether your plane points were probed clockwise or counter-clockwise, which is so unique that you don't find it in any other cmm system. With modern CMM system, usually equipped with powerful CAD interface, it's much easier to prevent your primary axis being flipped. But it could still happen due to various reasons, therefore I'd like to point out a few methods to fix it if that happens.
- If you're programming with pure DMIS based software such as Camio, Open Dmis or Virtual Dmis etc., make sure you have the correct nominal definition line right before your alignment construction, especially when the plane is taught by joystick, rather than picked from cad.
- If your software is dynamically updating your codes, for example in PC-Dmis you get prompted for yes/no when editing your alignment, a good practice is use RECALL/ALIGNMENT from time to time to lock nominal values.
- If you can measure a plane as an entity, try not to construct it from single points, or from buffered points (e.g. hits from other features). Some software are smart enough to re-comp your points and still get the correct plane direction from these hits but some are not. And also make sure proper settings related to this has been enabled. For example, in Mcosmos there's a magic button in plane construction that allows the software to calculate plane direction from measured value of individual points (see captured image):
- In Mcosmos you can also enable pre-define options to pre-define your plane direction because when you define your primary axis, it doesn't give your plus or minus option, but if your plane is pre-defined then it will always use your pre-defined vector for axis direction (see image):
- Variable method works all the time (assuming your logics is correct). When you have to deal with datum targets with different offset values (e.g. in die casting industry), in older software like MM4, Geomeasure or LK-Dmis, etc. you have to construct your plane from these points. To prevent the primary axis flipping, you can recall one of the target points after your alignment to see if it's in the right direction (+/-) or not, if not just rotate your axis by 180 degree. Using Mcosmos as example, the codes are like this (see image):
- RPS alignment for modern CMM software: almost all CMM software has RPS alignment now, or very similar (e.g. iterative alignment in PC-Dmis) and when used correctly, no flipping issue at all. RPS alignment is not just for automotive body line co-ordinate system (even though it was designed to handle that at first place), you can use it for simple alignment like plane-line-circle (but of course you must measure plane/line as points). The only drawback is that RPS alignment will not work if your primary datum feature is a cylinder.
or just simply scan this code from your smart phone:
Ray Xing GDTP S-0605
CGP Certified (ITAR equivalent)
CMM Application Specialist RX Metrology Solutions Inc.
One stop solution for CMM and GDT training www.rxmetrology.com