AutoCAD, Stop Helping Me!
Controlling the Insertion Units
New system variables were introduced with the advent of the AutoCAD Design Center (ADC) in AutoCAD 2002. When you drag and drop a block or drawing from the ADC into your current drawing, AutoCAD examines the values of INSUNITS in both the block and the current drawing and, depending on the value, automatically rescales the block appropriately.
For example: Using the ADC, if you drag a 2x4 light fixture symbol with an INSUNITS setting of 1 (Inches) into your current metric drawing which has an INSUNITS setting of 4 (Millimeters), the light fixture block is automatically rescaled by 25.4, converting it to the metric equivalent of this English sized light fixture.
Autodesk, in their finite wisdom, decided that this was such a clever feature they would extend it to all insertion operations in AutoCAD 2006. Now, when you INSERT a Block, or XATTACH a drawing, AutoCAD reads the INSUNITS of the source and target drawings and calculates a rescale factor for you. This rescale factor is then multiplied by the Scale you enter to yield the final insertion scale of the Block or Xref.
This can be handy, if you know about it and know how to make it work for you, but frustrating if you don't. So pay attention to the Block Unit area of the Insert and Xref Attach dialogs to see if AutoCAD is "helping" you determine the proper insertion scale factor. If the resultant scale is incorrect, adjust the setting of INSUNITS in both the source and target drawings before Inserting or Attaching it.
Some of the more common INSUNITS settings include:
0 = Unspecified (No units)
1 = Inches
2 = Feet
4 = Millimeters
6 = Meters
AutoCAD uses the values stored in system variables INSUNITSDEFSOURCE and INSUNITSDEFTARGET if INSUNITS is Unspecified.