In Dragon Ball, the Z Fighters train under high artificial gravity to be extremely strong under normal gravity. I don't recall the Gravity Machine having an upper limit to its effect, though, and there have been a few cases where the gravity has been too strong for the person training. Is there a reason the Z Fighters haven't used it as a weapon, say by cranking it up beyond what the enemy is able to withstand? Or have they and it slipped my memory? 

I get that the point of Dragon Ball, visually, is fighting and training sequences, but it just seems to me that it would be easier and more resource-efficient to use the Gravity Machine as a weapon than putting the effort towards training under the high gravity and then fighting in person.