Hello there!
I’ve recently made a Plugin to help me in one of my current projects, and I thought that it could be useful for other developers as well, so I’ve decided to polish and publish it! 
Model Welder Plugin
Current Version: 1.0.0
Model Welder is a simple plugin that allows you to make fast welds to Models and Tools.
If you have any questions or features that you would like to see, I will happily read and respond to your comments! 
Features
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Create automatic Primary Parts for your models in a single click. The Primary Part will automatically fit to the bounds of the model!
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Weld all the parts inside a Model to its Primary Part in a single click!
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Weld all the parts inside a Tool to its Handle in a single click!
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Undo and Redo any changes with Ctrl+Z and Ctrl+Y!
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All the weld features can be done with multiple selections for faster working flow!
Video showcase & Tutorial
Making a Primary Part for a Model:
Making a Primary Part for many Models at once:
Welding all the parts of a Model to its Primary Part:
Welding all the parts of many Model to their Primary Parts at once:
Welding all the parts of a Tool to its Handle:
Welding all the parts of many Tools to their Handles at once: