Let us start with Axis system
The basic question when you starting any new part/Component.
why local Axis system is necessary even if you have a global axis?
Is it necessary to create Axis system?
Yes ! you must create Axis system when you starting new part.
Assume you are creating a parts for a Automotive or Aerospace or any heavy machine tool. The assembly is having thousands of components in it.
If you use a global axis system to build/Model all the parts.
1. Its very difficult to assemble in assembly, when you import these components to assembly.
2. When you import all parts, all parts will come and sit on global axis system in assembly
3. You need to separate/ move all the parts apart.
4. you need to Identify where the components will be fit in assembly.
To avoid all these errors you need to create axis system when you starting new part.
Assume you are creating a rare wheel of a car. first thing you should know where and at what distance from global axis in car it will be fitted.Take the Coordinate(X, Y, Z)Using these Coordinate values you fix axis system and create a wheel. when you import this part in Car assembly it will sit at its position. No need to give a constrain again.
I will give one more Example. assume you are creating table and your laptop or Monitor placed on it at a center of the table top. ( Assume here table is a single part and your Monitor also single part)Just I want to create assembly of these 2 parts.
1. Take lower left corner leg of table as Global origin. model the whole table as a single part.
2. Measer the co-ordinates(XYZ) for center point of a table top.
3. When you start creating new part say Monitor, Using those value of XYZ what you got from table part create a point using Coordinate method 3D point then place a axis system on it.
4 make the monitor base center point as symmetric about the axis, then create whole monitor
5. save these two files separately
6. Import these 2 parts to assembly, automatically monitor will be placed on your table top.
How to create this axis System in part design by two ways.
1. Go to pull down menu -->INSERT------> Axis system
2. Select a command Axis system from Tools tool bar.
It will ask you the origin in pop-up menu, you just right click and take an option Coordinate. enter the values for X, Y and Z. then click OK.
Local axis is placed now at required location. Some time you you don't know the axis system location just enter 0, 0, 0 . later when you get the co ordinate values you just edit the values the part will be moved to that location along with your axis system.