为提高凿岩台车施钻时超欠挖控制的精度，通过对一种三臂凿岩台车推进梁上的顶盘结构进行分析，发现原顶盘的结构设计存在一定的缺陷，即顶盘钻杆导向孔与隧道纵向水平线之间的距离过大，导致施钻时外插角难以控制，从而影响超欠挖控制的精度。通过分析和总结，对顶盘重新进行了设计，通过减小顶盘钻杆导向孔与隧道纵向水平线之间的距离减小了施钻时的外插角，从而减少了对超欠挖的影响。

In order to improve the control precision of overbreak and underbreak of tunnel when using rockdrilling jumbo, the top plate structure on propulsion beam of a kind of threeboom rock drilling jumbo is analyzed. The fault, overlarge distance between guide hole of drilling pipe of top plate and longitudinal horizontal line of the tunnel, exists in the structural design of the original top plate, which leads to difficult control of extrapolation angle of the drill and further would affect the control precision of overbreak and underbreak. [JP2]As a result, optimization, i. e. reducing the extrapolation angle of the drill by shortening the distance between guide hole of drilling pipe of top plate and longitudinal horizontal line of the tunnel, is carried out; so that the overbreak and underbreak of tunnel have been effectively controlled.