How to reduce 36V 1.8A to 18V 1.2 A
What you are asking for is a buck converter.  That is a switching power supply that makes a lower voltage (and higher current) from a higher voltage (and lower current).  There are many chips available to do most of this for you.  You will have to provide the inductor and usually a few other parts.  Some chips require you to provide the switching element, others have them built in.

If this is meant to replace the battery in your drill, then it will probably work.  If you intend this to work together with the battery and even charge it, then it gets rather more complex and you can't just connect a fixed 18 V supply to the battery.