The work visa thing poses an interesting problem. One time coming back from the US I was sat next to a young chap from Europe (can't remember which country he was from, he might have been Dutch, spoke excellent English) who was being deported from the US because he told Immigration that he was going to "work" there. He had been going to visit a friend, a fellow musician, and had a bunch of self recorded CD's which he planned to sell on the street while he and his friend busked around California.
His mistake was being too open about what he was going to do. If he had said he was visiting a friend and the CDs were for his friend he probably would have been allowed in no problem.
Generally the question is, "What is the purpose of your visit?" or a translation of that. You are not travelling to work, but if you say you will be working, I can imagine the French would want you to have work papers. You probably know better than I, but going on what my friends in France say, the French sound quite strict about people having papers.
What no one cares or does much about are writers who work on books or articles while they travel, or artists who draw or paint and then sell the finished work when they get home. Knife making, with heavy tools, could be a hard sell. A suitcase of wood, blades and files would most likely not even get you stopped by Customs...I never got stopped coming to the UK from the Blade Show, and I carried handle materials, steel, knives, Kydex and tools. I didn't get stopped carrying such stuff into the US either.
Are there importation duties/taxes if you ship knives from France back to the US? How will your customers feel about paying these, compared to not paying them if you sent the knives after you returned?
