I think there's two kinds of spell here, which would probably work in quite different ways.
- A spell cast during a fight, to turn one or more convenient jointed objects into minions. This is comparable with a summoning spell with the inconvenience that it only works if there are appropriate objects available. So it should be a little easier to cast than a corresponding summoning spell.
- A spell cast before a fight, to create a minion. This is comparable with a zombie creation spell, so the animated creatures should lose power gradually, or else it should cost money or require a magic item like the zombie ring to keep them going.