I am looking for a book I read when I was young on the recommendation of my father. While I read the book in the mid 90's I believe that it was published in the 60's or 70's.
It is about a teen who finds a time machine in his (I think..) grandfathers basement and accidentally time travels back to the stone age. He wonders off from the machine and gets lost. He finds a group of cavemen who he eventually joins and survives with. He slowly grows up and they become a substitute family. At the end of the book he finds the time machine again and returns to the date he left but now a man very different from when he left.
It was written in English and I have a strong feeling that the title was short, one or two words, perhaps relating to either time travel or cavemen. I was around 13 or so at the time and the book was available in the middle school library. I have not found any books that come close to this description as I know the main character was a solitary boy. Most books I have encountered are about multiple travelers.
I'm guessing that it was not a very popular book as I have been searching for the title on and off for a number of years now. As this book was recommended to me, I've often thought of sharing it with my own kids.