Hole Saws 

The fastest and easiest way to cut accurate holes for installation of inspection ports is with powered hole saws, and IPS supplies the best.These aren't cheap hardware store saws, but heavy-duty industrial saws by Greenfield Industries. The mandrel separates from the cutting cup so you dont have to throw out the whole thing when the edge wears out; simply thread on a new cup and you're back in business.

RemGrit Carbide Grit Edge Hole Saws are made of tungsten carbide particles bonded to alloy steel for cutting extremely hard and abrasive materials. Recommended for stainless steel jacketing. Resists snagging.

Blu-Mol Bi-Metal Hole Saws are of heavy-duty bi-metal construction for longer service life. 4/6 variable tooth provides faster, smoother cuts. Recommended for aluminum jacketing.