Centre drilling should be done at higher rpm with a slower feed.

Incorrect pitch of thread during thread cutting with single point tool due to
Incorrect selection of change gears
Lead screw worn out
Half nut worn out

Reason for torn threads while thread cutting on lathe can be due to
Blunt tool
Heavy depth of cut
Insufficient side clearance

For setting of tool for threading, it is aligned with the work by using centre gauge.