1. Structural Characteristics of Planer-Type Milling and Boring Machines Long hole boring, exemplified by coaxial hole systems in box-type parts, is one of the most important aspects of metal cutting. Although there are still instances of using boring dies, guide bushings, long boring bars supported by the rear column of a planer-type milling and boring machine, or manual workpiece rotation to align the workpiece 180°, in recent years, on the one hand, the widespread use of CNC milling and boring machines and machining centers has significantly improved the coordinate positioning accuracy and table rotation indexing accuracy of various horizontal milling and boring machines. Long hole boring has gradually been replaced by efficient table rotation with 180° self-positioning turning boring (Figure 1); on the other hand, the mass production and application of conventional or CNC planer-type milling and boring machines with a planer-type bed layout has made table rotation with 180° self-positioning turning boring almost the only method for boring long holes on these machines. Figure 1 shows a table-mounted milling and boring machine with a 180° self-positioning worktable. In terms of tool system, the table-mounted milling and boring machine is functionally identical to a table-mounted milling and boring machine in terms of its motion mechanism. The main difference lies in the structural layout of the x and W coordinate movements in the five coordinate feed movements (x, y, z, W, and B) that form various workpiece profiles. Unlike the three-layer structure of the table-mounted milling and boring machine, which completes the x (horizontal), W (vertical), and B (rotational) coordinate movements respectively, the table-mounted milling and boring machine with a planer-type milling and boring machine only completes the x and B coordinate movements. The W coordinate movement is achieved by the column carrying the spindle head moving along the longitudinal section of the bed. Its coordinate system is shown in Figure 2. Obviously, when boring on a planer-type milling and boring machine, the movement of the tool relative to the workpiece (including the main motion and feed motion) is all driven by the tool, while the workpiece remains stationary. The movement that completes the boring tool feed can be achieved by the boring spindle moving linearly along the z-axis when the column is fixed at a certain position on the longitudinal bed, or by the column moving linearly along the W-axis after the boring spindle has a certain extension value. Figure 2. Coordinate system of a planer-type milling and boring machine. 2. Coaxiality error and compensation of column feed during turning and boring. The coaxiality error D3 of the column feed during turning and boring is similar to that of a table-type milling and boring machine. It is the indexing error da of the table rotating 180° and the positioning error dx2 of the table's horizontal (x) movement Lx=2lx (Figure 1) required to make the axis of the half-hole d1, which was bored before turning, coincide with the axis of the boring spindle again after turning to bore the other half-hole d2. Furthermore, the tilt angle error df generated by the table surface in the xy coordinate plane, the tilt angle error dy generated in the yz plane, and the translation error dy generated in the y direction before and after the table rotates 180° are also important factors affecting the coaxiality of the turning and boring of the planer-type milling and boring machine. However, the influence of the spatial position of the boring spindle axis on the coaxiality of the hole during turning is significantly different between a planer-type milling boring machine that uses a column to feed the bore to complete the boring of the entire length of the hole and a bench milling boring machine that uses a table to feed the bore. If we define the longitudinal (or transverse) trajectories of the table center on a bench milling and boring machine as the trajectory of a point on the worktable axis that is at the same height as the axis of the hole being bored during the longitudinal (or transverse) trajectories of the worktable, and define the trajectory of the boring tool tip's rotation center point O1 on the boring spindle axis that is at the same height as the axis of the hole being bored during the vertical movement of the column along the W coordinate as the column longitudinal trajectories, then the feed trajectory of the boring tool head mounted on the boring spindle bar, i.e., the axis of the hole actually bored by the boring tool, should be parallel to the W coordinate axis, regardless of whether the nominal axis of the hole being bored is parallel to or intersects with the column longitudinal trajectories (the case of the two coinciding is extremely rare). Therefore, in this type of feed-type turning boring, the straightness error of the half-holes d1 and d2 bored before and after the turning is not inherent in the axis itself. The key to whether the entire hole length is coaxial depends on whether the two axes of holes d1 and d2 coincide into a straight line. Whether the axes of holes d1 and d2 coincide mainly depends on whether the rotation center O1 of the boring tool tip is exactly on the nominal axis of the hole being bored. If the rotation center O1 of the tool tip is exactly on the nominal axis of the hole being bored, it indicates that the longitudinal movement line of the column coincides with the nominal axis of the hole being bored. At this time, the two starting points of boring holes d1 and d2 before and after the turning are both on the nominal axis of the hole, and the feed distance is the same segment of the column along the W axis. Therefore, the axes of holes d1 and d2 coincide into a straight line, and there is no coaxiality error. If the rotation center point O1 of the tool tip is not on the hole axis, but deviates by a linear error dy3 along the y-coordinate direction, then although the starting point of boring sections d1 and d2 is not on the nominal axis of the hole being bored in both cases, since these two starting points are still the same point, and the feed distances in both cases are the same segment at the same height in the y-direction, the axes of holes d1 and d2 still coincide as the same straight line, and there is no coaxiality error. When the rotation center O1 of the tool tip is not on the hole axis, but deviates by a linear error dx3 along the x-coordinate direction, the situation of boring after turning the tool tip is shown in Figure 3. It can be seen that the two axes of the two parts of the long hole d1 and d2 bored before and after turning the tool tip are respectively located on both sides of the nominal axis of the hole being bored, and the distance between each coaxial axis is dx3, thus causing a coaxiality error D3 in the long hole. According to the concept of coaxiality, we have D3=4dx3=4ltgdb′ (1) where db′ is the angle error of the intersection of the nominal axis of the hole being bored and the longitudinal line of the column. The distance from the center point of the tool tip rotation to the intersection of the two lines is the distance between the two lines. If the longitudinal line of the column is not intersecting with the nominal axis of the hole being bored, but is parallel, and the distance is also dx3, then it has the same effect as the intersection of the two lines shown in the figure, which results in the angle error db′. Figure 3 Determination of the D3 compensation value Dx3 of the column feed boring of the planer-type milling boring machine. This coaxiality error is a fixed value error. After boring the d1 hole, the worktable can be rotated 180° and moved horizontally by Lx=2lx. Based on the determined compensation value Dx3 and its direction, the worktable can be moved horizontally by this Dx3 value to eliminate this coaxiality error. As shown in Figure 3, Dx3 = 2dx3 (2). If the nominal axis of the hole being bored is exactly within the yW coordinate plane (vertical plane) passing through the axis of the worktable, then the hole being bored after turning around does not need to be moved laterally by 2lx. This eliminates both the coaxiality error D3 and the influence of the positioning error dx2 of the 2lx lateral movement on the coaxiality of the hole being bored. Therefore, under the premise that the workpiece is clamped in a reasonable position on the worktable, the optimal clamping position of the worktable on the horizontal bed of the planer-type milling boring machine is to place the rotation center point of the worktable at the intersection of the nominal axis of the hole being bored and the longitudinal movement line of the column. When the nominal axis of the hole being bored cannot pass through the rotation center of the worktable due to the structure of the workpiece, the reasonable position of the worktable on the horizontal bed should be to move the worktable laterally so that the rotation center point O1 of the boring tool tip is just above the nominal axis of the hole being bored on the workpiece. 3. Determining the Optimal Longitudinal Position of the Column During Boring Spindle Feed Determining the optimal longitudinal position of the column is crucial. In certain situations, planer-type milling and boring machines must fix the column in a suitable position on the longitudinal bed. When the boring spindle, carrying the cutting tool, extends out as the feeding method for boring, the angular error (db) between the boring spindle axis and the nominal axis of the bore being bored in the xz plane and the angular error (dg) in the yz plane, similar to that of a bench milling and boring machine, significantly impacts the coaxiality of the bore during reversal boring. Furthermore, as the boring spindle feed length increases, the downward deflection of the boring bar caused by the spindle's own weight also significantly affects the coaxiality of the bore during reversal boring. Unlike bench milling and boring machines, in planer-type milling and boring machines, when the boring spindle extends out of the bore, the longitudinally movable column must be fixed in a specific position on the longitudinal bed, and importantly, this specific position can and should be selected. According to the design specifications, the travel distance of the boring spindle along the z-axis should be parallel to the travel distance of the column along the W-axis; during boring, the nominal axis of the hole to be bored should be placed in a position parallel to the W(z) axis. Before boring, the worktable has been moved along the x-axis to align the nominal axis of the hole to be bored with the boring spindle and is fixed and clamped. The boring spindle has been retracted to the zero position, and the tool holder for clamping the boring tool is inserted into the inner tapered hole of the boring spindle (Figure 4). When there is an angular error db between the nominal axis of the bore and the axis of the boring spindle in the xz plane, the intersection point O of the two lines moves along the longitudinal line of the column as the column moves on the longitudinal guide rail of the bed, causing the value of l′ in Figure 4 to change accordingly. However, the angle db between the two lines remains unchanged. Therefore, no matter where the column is moved to, the formula for calculating the coaxiality error D4 of the turning bore caused by db can be expressed as D4=2dx4=4（lW+l′）tgdb (3) where dx4——the x-direction linear error of the axis of d1 to d2 after turning bore caused by the angular error db, lW——the distance from the front end face of the bore to the rotation center of the worktable, l′——the distance from the starting point M of the bore to the intersection point O of the two lines. The value of l′ can be positive or negative. When point O is inside point M, the value of l′ is negative, and when it is outside (as shown in the figure), l′ is positive. Obviously, D4 can be compensated by micro-moving the worktable in the x direction before boring the d2 hole. Its compensation value is Dx4=dx4=2(lW+l′)tgdb (4) Figure 4 Boring spindle feed boring equation (3) shows that, in the presence of db, the necessary and sufficient condition for eliminating D4 is lW+l′=0. As can be seen from Figure 4, under the premise that the workpiece is already in a reasonable position (the transverse line of the table center bisects the entire length of the hole), in order to satisfy the equation lW+l′=0, the intersection point O of the boring spindle axis and the bore axis must be located on the transverse line of the worktable rotation center. Since the intersection point O of the above two lines moves along the column longitudinally with the column longitudinally, the condition lW+l′=0 can always be satisfied. This means that when the boring spindle of a planer-type milling and boring machine is fed into the boring hole, in order to minimize D4, the optimal position for clamping the column on the longitudinal bed should be such that the intersection point O of the boring spindle axis and the nominal axis of the hole is exactly located on the transverse line of the rotation center of the worktable. The determination of angular error db and intersection point O: As mentioned above, obtaining the optimal clamping position of the column and accurate compensation requires knowing the positions of db and intersection point O on the boring spindle axis. This can be determined by actual measurement, as follows: Place the worktable in the transverse middle position, place a straightedge transversely on the table surface, and place a gauge block on the upper part of one side of the straightedge, off-center from the rotation center of the worktable (Figure 5). The column is stopped at a suitable, tested position. A dial indicator is fixed on the boring bar, with its head resting against the inner side K of the gauge block. A reading A1 is taken. The boring bar and the worktable are each rotated 180° (assuming no graduation error), so the indicator head is still resting against the original side K, and a reading B1 is taken. Using the boring bar axis as the reference line (O1～O1), a point Q1 on the column's longitudinal movement line is determined. The distance from Q1 to the boring bar axis is lQ1 = (A1 - B1) / 2. Pay close attention to the sign of lQ1, indicating which side of the boring bar axis point Q1 on the column's longitudinal movement line is located on. The column is moved longitudinally by LW2 distances, and the boring bar is correspondingly extended or retracted by LW2. Repeating the previous measurements, lQ2 = (A2 - B2) / 2 is obtained, thus determining point Q2. Connecting Q1 and Q2, we get the straight line Q1Q2 representing the longitudinal movement line of the column. The figure is shown in Figure 5b. We can directly measure the angle db between the boring axis and the longitudinal movement line of the column and the distance from the intersection point O to the front end face of the spindle box. If A1=B1 is measured, then lQ1=0, indicating that the point on this longitudinal movement line of the column is located on the axis of the spindle. If A2=B2 is measured, it indicates that the two lines coincide. If lQ1=lQ2 is calculated, then the two lines are parallel. If lQ1≠lQ2, then the two lines intersect. The angle db can also be calculated by the following formula: tgdb=（lQ2-lQ1）/lW2 (5) The value of LW2 can be obtained by recording the W coordinate indicator value. Figure 5 shows the adjustment of the position of intersection point O and db on the boring spindle axis. The position of the intersection point O between the boring spindle axis and the column longitudinal movement line (i.e., the nominal axis of the hole being bored), obtained through actual measurement, may not achieve the condition lW+l′=0 for the boring spindle's feed into the hole. For example, if intersection point O intersects the axis behind the front bearing of the boring spindle, even if the column moves forward to its longitudinal limit position, it is impossible for intersection point O to be exactly located on the horizontal movement line of the worktable rotation center. Therefore, to ensure the column's longitudinal position is achieved without failure, it is necessary to ensure that intersection point O specifically intersects on the workpiece side of the front bearing of the boring spindle, and to determine the specific position of intersection point O that guarantees lW+l′=0. Once the tool holder is determined, the distance from the tool tip rotation center to the front bearing is known, making it easy to determine the specific value of the reasonable position of intersection point O. Since the size of db and the specific position of the intersection point O on the boring axis are determined by the relative positional accuracy of the spindle box when it is mounted on the column and the column is fixed on the slide, after determining the appropriate position of the intersection point O on the boring axis, it is necessary to adjust the assembly position of the spindle box on the vertical guide rail of the column or adjust the position of the column relative to its slide (so that the column rotates slightly around the y-axis on the slide) to meet the requirement that the intersection point O has a suitable position. 4. Reasonable Determination of Tool Position on a Planer-Type Milling and Boring Machine When using a column-feed reversing boring machine on a planer-type milling and boring machine, the reasonable position of the boring tool clamped on the tool holder of the boring spindle along the Z-axis should, on the one hand, satisfy the requirement that the distance from the center of rotation of the tool tip to the front end face of the spindle box is slightly greater than half the total length of the hole (too small and long holes will not be able to be bored through, too large and the rigidity of the boring spindle will decrease); on the other hand, it should also satisfy the requirement that the center of rotation of the tool tip is placed at the intersection point O of the boring spindle axis and the longitudinal movement line of the column.