Can I publish photos or reconstructed drawing of the internal part of a patented instrument?
Following up on the answer of HairyBlob, you are also safe from copyright issues (a separate thing) if you are using your own drawings, especially if they are an abstraction of the ideas in the patent. 

There is no restriction on public discussion of patents (most places). In fact the patent process requires disclosure. But you can't republish copyright material. And your description suggests you aren't doing that.