Splicing Design Method and Accuracy Analysis of the U-shaped Frame

The U-shaped frame is a crucial component of the theodolite. The coaxial accuracy of the bearing holes on both sides of the U-shaped frame directly affects the pitch-axis accuracy and further influences the angular measurement precision of the theodolite. The machining of the bearing holes on both sides of the U-shaped frame with a long span requires the use of methods such as a large-stroke boring machine or the assembly and adjustment of the frame's own structure. However, considering various factors such as the manufacturing cost, manufacturing time, and machining precision of the U-shaped frame, neither of the above methods is the optimal solution. This paper proposes a splicing design method for the U-shaped frame based on the principle of one side and two pins positioning. This method solves the problems of low coaxial accuracy and difficult machining of bearing holes in U-shaped frames with long spans. Through multiple splicing accuracy tests of the coaxiality of the bearing holes in a U-shaped frame with a span of 1500 mm, the average coaxiality error of the bearing holes is 0.023 mm, with a maximum value of 0.038 mm, which is less than the theoretical maximum coaxiality error of 0.058 mm. The coaxiality tolerance precision level is close to Grade 5, which meets the requirements for the use of high-precision theodolite.