Question

expansion on a bad workman blames his tools

Answer 1

A bad workman blames his tools. If someone performs a job or task poorly or unsuccessfully, they will usually lay the blame on the quality of their equipment, or other such external factors, rather than take responsibility for their own failure.

Answer 2

It means that suppose if a mechanic was given a work by his master but he didn't know the work properly then by chance if the work was not done properly then he will blames his toolkit that his toolkit was not so good, but he will never say that he didn't know the work that he was given by his master.