Several things.
Some plastics love high speed drilling, others demand low speed. Generally the more fragile the blank is the higher the speed you need.
When you drill plastics the act of drilling will put bubbles into the material as it is drilled. Many shops will drill undersized and ream to the proper size to avoid this. The type and style of drill bit matters greatly in this regard. A forstner bit is very good on plastics that has bubble problems when drilling.
I would say drill first then make it round. By rounding it you could have imposed stress fractures in the blank which made it ill fated when drilling. Depending on the material mind you some blanks are very prone to this problem.
Heat is your enemy. As you withdraw the drill bit if it's to hot to the touch then you are in the 'no no' zone, the goal is to keep it cool to the touch and fluid may be needed at times. Also metal expands when it is hot, the hotter it gets the more inaccurate the drill bit becomes.
Explosions are often the result of bad catches and you have it under load so something has to happen, the blank being the weakest point in the chain and it goes SNAP. The events leading up to the explosion will tell you exactly what the cause was. If you look at it you have the feed rate and the stress forces applied to the blank when drilling or turning and the bonding of the blank material can only withhold a certain amount of stress before it yields. This is a very fine line to walk.
As for the scraper vs carbide tool a good and sharp traditional chisel will win hands down vs a carbide tool any day of the week simply because the carbide tools are *NOT* sharp. Even being brand new they are not that sharp.
