I won't be surprised at all if a new CMMer starts to ask about how to use "read point" for initial alignment after he/she has run or even program the cmm for a while. Just think about how easy it is if you don't have to manually pick 4 points off a plane, 4 points off a circle and another 4 point off another circle (or, 4 points on each of 3 tooling balls, etc.). "Read point" method is really handy not just for creating initial alignment, but also for many other purposes. In this article I'll just give you some quick ideas on how to use this function for initial alignment in a few different popular software.
PC-Dmis seems to still have largest user base, so let's start with that. In pc-dmis, you can go Insert->Feature->ReadPoint to create some codes like this:
STARTUP =ALIGNMENT/START,RECALL:, LIST= YES
TIP/T1A0B0, SHANKIJK=0, 0, 1, ANGLE=0
COMMENT/OPER,Place your probe at the 6 mm hole near corner ..
A1 =ALIGNMENT/START,RECALL:STARTUP, LIST= YES
COMMENT/OPER,DCC WILL START !!!
I won't explain it in details but the basic idea is find a location where the operator can conveniently place the probe center in roughly and then CMM will start in DCC mode from there and the operator can just walk away to get a cup of coffee. When you use this read-point method, the theoretical nominal of the read point is not important (the actual values are derived from your ruby center position). What's important is your translational offset (or even rotational values) must be able to set your origin to part (or cad) origin, unless you're luck that you can always place your probe right at the origin.
Now let's take a look how this works in pure DMIS packages. I know the icon/tool to activate read-point is slightly different in various DMIS software but good thing is the final DMIS codes looks exactly same:
TEXT/OPER, 'Place your probe at center of the 6 mm hole near corner ...'
F(PT1) = FEAT/POINT,CART, 0,0,0, 0,0,1
MEAS/POINT, F(PT1), 1
D(CRD1) = TRANS/XORIG, F(PT1), YORIG, F(PT1), ZORIG, F(PT1)
D(CRD2) = TRANS/XORIG, -50, YORIG, 60, ZORIG, 100
TEXT/OPER, 'DCC WILL START !!!'
So this will work same way in different software even though sometimes you need to find a work around. For example, there's no "Read point" in Mcosmos. But what you can do is your use the built-in variable functions to grab the current machine position then assign these variable to your origin translation, just like this:
And if you're using CMM Manager, you'll need activate Free Position alignment. You can find very detailed instruction on their forum for this.
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