| 摘要 |
A tool which makes an undercut in the surface surrounding a drill hole in a layer of coal has a head plate which abuts against the surface at the inner end of the hole when the tool is inserted and which is installed at the front end of a rotary tubular guide for two bit holders constituting one-armed levers and having front portions provided with bits and being movable radially outwardly through windows in the guide. The rear portions of the holders are coupled to the front part of a reciprocable rotary shank which can be moved forwardly to expel the bits from the guide while moving the holders forwardly. When the shank is retracted, the bits are withdrawn into the guide. Such movements of the bits are caused by a transverse pin mounted in the guide and extending through elongated cam grooves in the median portions of the holders. The configuration of surfaces flanking the cam grooves is such that the bits move outwardly during a first stage of forward movement of the shank relative to the guide and the bits thereupon remain in extended positions and make a cylindrical portion of the undercut in the surface surrounding the hole during the last stage of axial movement of the shank into the guide. The tool is thereupon withdrawn and replaced with an anchor bolt which can be expanded against the surface portion surrounding the undercut and serves as a supporting or holding device. |
