I did this exact job at my work for a year and a half. I still assist that group from time to time. Where I work we destroy hundreds of drives a day, so obviously we had a machine that would do the job.
For remote sites, however, it took us a while to come up with a good solution, so here's what we did in the meantime, and what you can do also, at home:
Get yourself a technical screwdriver set... yes, one that will work on those screws on the hard drive. Their exact name escapes me at the moment, as I just woke up. Since you know what I'm talking about anyway, get that. Exactly that.
Anyway, open up that puppy, and remove the platters from the spindle.
Generally your platters should be metal. If this is a rare case where they are glass, smashy smashy. If not, get some vice grips, channel locks, pliers, whatever is handy, and fold that platter over like a taco.
We sent a sample like this around to all the major data recovery outfits and they all quoted 5 figure sums with zero guarantees. They probably could get some data off it, but who is going to pay $10k+ to find out?
At that point, is the data secure? About as secure as you can make it without some crazy.
We would do other things after that point, but it was mostly out of due diligence/paranoia than actual data security.