I have always drilled my holes into my tubs, never used a soldering iron because I have many other uses for that rather than mucking it up with melted plastic. In all the tubs I have drilled holes into, never once have I cracked any of them. Then I use a de-burring tool that belongs to my hubby that he uses as a machinist.. to clean up the edges of the drilled holes. The "shavings" that come off from the drilling and de-burring land right back in the tub and are dumped out after I am done, then the tub is cleaned, which you should do anyhow. If you are cracking your tub when drilling, most likely you are putting too much pressure on the tub to begin with or your drill bit is dull.