Is the aforementioned transaction considered usurious, and does the friend remain a partner after the full amount has been paid and the machine sold, and is he sinful for his procrastination in payment, and is insistence in demanding money considered a sin?

The machine belongs to the questioner, and his friend is a partner with him in the produce. This is a permissible type of ijarah (leasing/hiring). The employee (or lessee) is not liable for what is under his control except in cases of transgression or negligence. Therefore, the questioner has no right to hold his friend accountable for the price of the machine, because the questioner is its owner, and he must return what he took from him, considering it a debt.