Abstract: A method for putting together a down-the-hole drilling apparatus for plastic pipe drilling. The down-the-hole drilling apparatus has a drilling device for carrying out drilling by using a drilling unit with a plastic pipe, which drilling unit includes in its drilling head a drilling arrangement. The plastic pipe is arranged to be pulled into the hole to be drilled by a power influence directed thereto from the drilling unit, being coupled with the second drilling means in a manner transmitting power at least in the longitudinal direction of the drilling device. The drilling unit and the plastic pipe are coupled by a skirt part that exists at an opposite end of the drilling arrangement, with respect to the drilling surface of the drilling unit, which skirt part has an internal protrusion assembly in radial direction.

Abstract: A drilling device for a down-the-hole drilling apparatus. The drilling device includes in a guide, a drilling member and a flushing mechanism configured to flush drilling waste. The drilling mechanism includes first and second drilling bit arrangements that are movable with respect to the drilling device and that are arranged to be expanded to drill a hole having a greater cross-sectional surface area than an end of the casing and arranged to be reduced to a size smaller than the end of the casing. Bit pieces of the second drill bit arrangement expand and reduce the second drill bit arrangement. The bit pieces expand the second drill bit arrangement by moving in a radial direction outwards in slide grooves, by rotating the guide in a drilling direction, and move in the radial direction inwards in the slide grooves, by rotating the guide in an opposite direction.