can i modify a patented product that is owned by another person and charge for my labor ? for example if a person wanted there crossbow modified can i charge them for the modification ? is this patent infringement ?
To infringe a patent one must make, sell, offer for sale, use or import a claimed device in the jurisdiction of the patent, without permission of the patent owner. Or induce anyone else to infringe or sell or import a customized part whose purpose is to be a component of an infringing device. If you do not do any of these things without the patent owners permission, you should be OK. If you are buying a crossbow in the U.S. that was made and sold with the patent owners' authorization, the patents of that patent owner which cover the crossbow are exhausted. That owner, or others might have other patents that cover the modification you are planning to do. The issue isn't charging money to make a modification. If the modifications were covered by someone else's patent, making them would be an infringement whether or not you charged money.