Glyph origin of 我 - How did 我's radicals 手 and 戈 create the meaning of 我? Tell me, please, how do 我's radicals 手 and 戈 create the meaning of 我  (ie: I)? 
  I know that 戈's pictogram (象形) means a rake-like tool or weapon, borrowed for sound to mean "me" (note 'Oracle bone script')." But then how did "rake-like tool or weapon" shift semantically to mean "I"?
Yes, 我 was a kind of weapon.
According to the book 字源谈趣, 我 was a kind of broadax originally in slavery society.


请看图（A），是甲骨文里的“我”字，象个什么东西呢？象把有柄有钩的古代武器——大斧，斧口有一排锋利的锯齿。原来，这是奴隶社会里一种用来行刑杀人和肢解牲口的凶器，叫做wǒ，古代也有人把这种锯斧叫做“錡”。

And then later, 我 was obsoleted by other new weapons little by little. From Han and Tang dynasty, 我 was borrowed to be used as first person, and the pronunciation doesn't change till today.

“我”这种奴隶社会杀人的凶器，到了战国时代，大抵被后起的更优良的凶器所淘汰；于是“我”字在汉唐以后便被普遍借用来作为第一人称使用，还是读作wǒ，字音至今不变。（先秦乃至汉唐以前，第一人称，是用“吾”“予”“余”等字来表示的。）但词性却已由名词变作人称代词，而“我”是杀人凶器的原义便早已不为人所知了。

This phenomenon is called 假借字, means select and use an old word for new expression and meaning, without creating a new word.

许慎：“假借者，本无其字，依声托事，令长是也。”即本来没有那个字，现在为了表达的需要，在不另造新字之情况下借用旧字来充当新义，就成了假借，“令”本义表命令的会意字，后来指县令。

Additional reference: 汉字“我”最早是杀人凶器