Datum Align to Negative side of an axis

[John Lankford]

In all versions of OGP measurement software, the Align X and Align Y checkboxes under Construct -> Datum Axis Align will align a selected feature to an axis, but only to the positive side of the axis.  In other words, if you have a datum origin established on the right hand side of your part, and you measure a feature to the left (negative X-axis side) of your part, and you click Construct -> Datum -> Axis, and click the checkbox for "Align X Axis", it will align the POSITIVE side of the X axis to that feature, and flip everything 180 deg around so that it is upside down from when you started.   



If you want to align the X axis to a feature to the left of your origin, then DO NOT check "Align X Axis".   Instead, type in 180 under "Align Angle".  Since the negative X axis is 180 deg from 0, this will do the job.

Same thing applies for Y.  If you wish to align a feature to the negative side of the Y axis, then you would type in "-90" or "270" under "Align Angle".

To understand this, it is helpful to better understand how the Align Angle option works for Datum Alignments.  Angles start at the positive X axis (0 deg) and positive angles go counter-clockwise.

+X	=  0 degress
+Y	=  90 degress
-X	=  180 degress
-Y	=  270 (-90) degress

[nita]

If I understand you correctly, the 180 degrees is an option, not a requirement.  It doesn't affect measurements at all, just the way the image is seen.

[John Lankford]

Quite the contrary, an alignment is a VERY important step to getting accurate measurements.  An alignment step not only changes the look of the model window, it will change the location of EVERY feature (except 0,0).   This will obviously change X & Y locations, True positions, and especially X and Y distances.   It would not affect the diameters or widths of slots, for example, angles, Z distances.

As far as the 180 degrees, it is necessary when the  feature you want to align to is to the LEFT of the origin.  Optionally, instead of setting the angle to 180, you can set the Y axis to Zero.  But I prefer the 180 technique.  If aligning to the RIGHT, obviously you would just check the "Align X Axis" option.

In conclusion, I might point out that I ALWAYS set at least ONE alignment step in a program.  The only reason I wouldn't would be if I'm checking a round part with nothing to align to (like a washer).  As mentioned before, it does affect measurements, but can also make your programs run better by helping locate the search windows more accuarately.

If you want, I'd be happy to take a look at your application and give you some advice.  Just email me with a drawing or you can fax it to 215-283-0159.

Hope this helps!