Opening manhole covers, which weigh up to 400 lbs., is difficult at best and dangerous at worst. Dave Roberts, 51, who spent 25 years lifting them while working for waste-water authorities in Minnesota’s Twin Cities, decided to find a better way. A perpetual tinkerer, Roberts designed a machine to remove the heavy cast-iron lids using a powerful magnet and a hydraulic hoist that attaches to a back of a vehicle. He assembled the first device in his garage five years ago and sold seven of them to his employer.