Taking a valve body machining process
as an example, the process content and fixture scheme on the machining center are introduced. By adjusting the process route and recalibrating the positioning reference in the finishing process, the problem of flange bore position degree over-variance caused by V-shaped block positioning error is solved.
Valve body is the main parts of the valve mechanical products, generally play a role in bearing the pressure of the medium. Valve body parts structure is relatively complex, the blank is generally formed by casting, the material is generally cast iron and cast steel, a few stainless steel material. This paper introduces a company valve body material is cast iron, through the introduction of the valve body in the vertical machining center processing process and fixture, led to the actual processing of the valve body hole relative to the center of the reference position of the problem of ultra poor, in order to compensate for the V-shaped block will produce positioning error defects, by adjusting the process route to cleverly solve the problem.
1 valve body original process route
Figure 1 shows a company valve body parts, the material is HT250. technical requirements are: ① castings should be in line with the provisions of GB/T 12229-2005. ②Castings shall be annealed. ③ uninjected casting rounding R3 ~ R5mm. ④ uninjected tolerance processing size according to GB 1804-79 regulations H14 (h14) js15. ⑤ valve seat pile E410, hardness 33 ~ 38HRC after tempering, thickness ≥ 2mm after processing. ⑥ cast word according to 50J41H-160-01a / A.
Figure 1 φ88H7, φ110H7 hole, φ205h8 and φ215h8 outer circle of relatively high precision, is the important size of the part. The company uses one horizontal lathe, one vertical lathe and one vertical machining center (with fourth axis rotary table and disc tailstock) for machining.
Figure 1 Valve body parts
The machining process route of the part is as follows: rough and finish turning φ215mm outer circle and end face respectively by horizontal lathe → rough and finish boring φ88H7 inner hole → turning head rough and finish turning φ215mm outer circle and end face → rough and finish boring φ88H7 inner hole.
Change vertical lathe to rough and finish turning φ205h8 outer circle and end face → rough and finish boring φ110H7 hole → rough and finish turning φ53mm hole upper end face → boring φ53mm hole and φ50mm hole.
Change the vertical machining center, rotate the rotary table to 90° to drill φ25mm hole on the outer end surface of φ215mm to leave a margin → bore φ25mm hole → rotate the rotary table to 0° to drill φ22mm hole on the outer end surface of φ205mm to leave a margin → bore φ22mm hole → rotate the rotary table to -90° to drill φ25mm hole on the outer end surface of φ215mm to leave a margin → bore φ25mm hole.
2 Vertical machining center fixture program
The vertical machining center machine tool mainly processes 3 holes on the outer end face. In order to improve the processing efficiency, the machine tool is equipped with the fourth axis turntable and disc tailstock, so that the hole processing can be completed in one set-up. The center of φ215mm outer circle is selected as the positioning reference, and the workpiece is limited to 4 degrees of freedom by V-block positioning on the outer circle; the other positioning reference is selected as the center of φ205mm outer circle, and the workpiece is limited to 2 degrees of freedom by V-block positioning on the outer circle. In this way, all 6 degrees of freedom of the workpiece are restricted. The fixture is shown in Figure 2, and the clamping point of the fixture is selected on the outer circle. Figure 2 Fixture 1 - Fixture base 2 - Adjustment support nails (2) 3 - Short hexagonal screws (6) 4 - Non-standard support 5 - Flat platen B extension (2) 6 - Shouldered hexagonal nuts and double-head bolts (2 pairs) 7 - Fixed V-block 8 - Short locating pins (4) 9 - Non-standard support 10 - Long locating pins (4) 11 - V-block (2) 12 - Long hexagonal cheese head screws (4) 13-hexagon socket compression screws
3 Analysis of the causes of machining errors
The fixture shown in Fig. 2 was machined on a vertical machining center
equipped with a fourth-axis rotary table and tailstock, and when the machining was completed and tested, there was a problem with the position of the hole being out of tolerance. Previously, the machining center passed the JB/T 8771.7-1998 standard test material acceptance requirements. According to the accuracy of the standard test material, the position of the hole on the part diagram can meet the requirements. The problem may lie in the fixture. The positioning error of the fixture is divided into datum movement error and datum non-reconnection error. Analysis of the valve body original processing process route can be seen, turning φ215mm outer circle and φ88mm bore benchmark is the use of mutual benchmark principle processing, are the outer circle center as the benchmark, turning φ205h8 using special fixture, is also the outer circle center as the benchmark for turning. In the vertical machining center machine tool is also around the outer circle center as the reference for processing, according to the analysis of the drawing, the design reference and positioning reference is always overlap, so there is no benchmark non-overlap error. The positioning element of the fixture on the vertical machining center is the V-block, and the V-block positioning usually has the datum movement error, which is calculated as ΔY = δd/[2sin (α/2)], where ΔY is the datum movement error (see Figure 3), δd is the tolerance of the outer diameter of the workpiece, and α is the angle between the two limit surfaces of the V-block. Fig. 3 Schematic diagram of the base shift error
The V-block angle selected in this paper is 90°, and the tolerance of the outer diameter is 0.072mm, so it can be concluded that the reference movement error is 0.051mm, while the position tolerance value required on the drawing is 0.05mm, so it is obvious that the positioning error has exceeded the position tolerance, which cannot meet the processing requirements, and it can be judged that the position overrun is related to the V-block positioning error.
The cause of the position degree and verticality exceeds the tolerance has been found, at this time there are 3 solutions.
1）Program 1: Change the fixture positioning elements, and adopt the plane positioning method for the 3 outer circular surfaces, which can reduce the reference movement error, but it is not a cure, and cannot solve the problem of over-positioning very well.
2）Program 2: Increase the process to finish turning φ205h8 and φ215h8 outer diameter to φ205h6 and φ215h6 accuracy, which can reduce the datum movement error to 0.02mm, which can greatly reduce the influence on the position degree error, and is better than program 1. The disadvantage is that the lathe processing
accuracy requirements are higher, reducing the processing efficiency and increasing the manufacturing cost, and from the perspective of mass production, it is difficult to ensure this accuracy, the program is very "chicken".
3) Option 3: Change the overall process route of the valve body. The design benchmark on the drawing is the center of the bore, and the finishing boring process is added before drilling in the vertical process to recalibrate the benchmark movement error caused by V-block positioning, and the finishing boring process on all lathes is cancelled, so that the original process route becomes: rough and finish turning φ215mm outer circle and end face respectively with horizontal lathe → rough and finish boring φ88H7 bore → turning rough and finish turning φ215mm outer circle and end face → rough and finish turning φ88H7 bore → turning head 215mm outer circle and end face → rough boring φ88H7 inner hole.
Change vertical lathe to rough and finish turning φ205h8 outer circle and end face → rough boring φ110H7 hole → rough turning φ53mm hole upper end face → boring φ53mm hole and φ50mm hole.
Change the vertical machining center, rotate the rotary table to 90° to finish boring φ88H7 bore→drill φ25mm hole on φ215mm outer end face to leave the balance→boring φ25mm hole→rotate the rotary table to 0° to finish boring φ110H7 hole→finish milling φ53mm hole upper end face→drill φ22mm hole on φ205mm outer end face to leave the balance→boring φ22mm hole→rotate the rotary table to -90° to finish boring φ 88H7 bore→drill φ25mm hole on φ215mm outer end face to leave a margin→bore φ25mm hole.
In this way, the influence of datum movement error can be reduced to zero. The position and perpendicularity are all guaranteed by the accuracy of the machine tool. The reduction of two turning tools for finishing bore, the increase of two boring tools and one end mill, the increased tool cost is quite acceptable, little change in productivity, no impact on production capacity, is a feasible solution.
The final decision was made to choose option 3 to solve the hole position overlap problem.
In summary, it can be seen that there is an error in positioning with V-block, so the positioning reference was recalibrated during the finishing process, and the relevant processes were adjusted to solve the problem of hole positional over-positioning caused by V-block positioning error. Previously, we were always troubled by the misunderstanding that the outer circle and inner hole of lathe are one datum, and ignored the difference between chuck positioning and V-block positioning, which led to the shortage of valve body process route arrangement. The solution adopted in the paper provides a reference experience for future finishing work
with V-block positioning conditions.