I have 8 Ikea Billy bookcases and I would like to attach them together to make the seam betwen them as narrow as possible.
What sort of screw or fastener is appropriate for particle board?
I am not a big fan of screw threads in particle board.
They come in a wide variety of lengths and finishes. Pick the size and style that fit.
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I would talk to someone at Ikea about this. If you look on their site at Billy bookshelves, when they sell a wider version it is two bookshelves side by side and they do not share a single side piece. It appears to be two separate ones that they have tied together. So they must have a system for doing this and you can get the hardware from them.