You're probably not hurting yourself, but you're probably not getting the full benefit of a squat either. Most of those standing calf raise machines don't drop down very far, which is the most effective part of the squat. You're probably better off doing squats with just body weight, holding something heavy (or dumbbells if available) or even one legged squats if you're strong enough. Bouncing up and down a few inches just really isn't doing a lot of good. Now, if this particular machine *does* go down far enough (thighs parallel to the floor) then go for it. Just keep your back straight and watch your form.