Let me explain history free editing with a simple example. Make a model as shown in Fig A as per the sequence given below.
1. Make a cube (100x100x100).
2. Make a central hole (50 Dia. x 75height) from the bottom of the cube.
3. Make another cube (75x75x75) on top of the first cube.
1. Make a cube (100x100x100).
2. Make a central hole (50 Dia. x 75height) from the bottom of the cube.
3. Make another cube (75x75x75) on top of the first cube.
Fig A. Test model
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Now, try to modify the height of the cylinder from 75 to 150. In a history dependent modeler, the height of the cylinder after modification will only be 100 instead of 150 as shown in Fig B. Whereas, in a history free modeler, the height of the cylinder after modification will exactly be 150 as shown in Fig C.
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The faulty modification caused in history dependent modelers is due to the fact that model re-evaluation is based on the chronological feature creation order i.e. a new boundary model is created by sequentially re-executing the operations in the modified history. Hence, even though the modified height of the cylindrical hole is 150, on subtracting the cylindrical hole with a cube of height 100, the hole becomes a through hole in the first cube. Then the second cube is placed on top of the first cube.
Whereas in history free modelers, the modification is handled in a different manner that only the immediate faces connected to the modified face/dimension are modified. Even though, such modelers are powerful in history free modification, model modification requirements such as converting a through hole/pocket into a blind hole/pocket or vice versa is better accomplished using feature information.