In order to solve the problem of high machining cost of high precision rack with special tool and special machine tool, the universal angle head on CNC milling machine is used to improve the machining method, and the random measurement is carried out by using bar instead of sample plate in the process of machining, which effectively guarantees the machining accuracy of tooth profile and pitch. The rack produced by TSW t-win has high precision. The standard grinding precision can reach din6, and the straightness is within 0.03. We can customize according to the gear drawing, and also provide technical drawing support.

Process parameters of workpiece

The material of the workpiece is 42CrMo, the hardness is 248-302hbw, and the tensile strength is 850mpa.

The corresponding technical parameters of the rack are shown in Table 1, where l is the distance between any two teeth on the rack (mm).

Machining difficulties and scheme of rack

As can be seen from Figure 1, the overall dimension of the rack is 2 895.6mm × 460mm × 210mm is a typical slender part. It is difficult to directly process the tooth shape from the forging blank and control the deformation, which requires that the workpiece processing must have a reasonable process sequence and an effective clamping method. The tooth thickness tolerance and the tooth distance error of the rack are not always small, and the accumulated error of the tooth distance in the whole length range is not more than 0.4mm. If the forming milling cutter or the rolling cutter is used With the continuous processing of the gear cutter, the cutter will inevitably wear, which will not guarantee the pitch accuracy, but also lead to the increase of the accumulated pitch error, which can not meet the use requirements of the rack.

Based on the above factors, the manufacturing process of the rack is formulated according to the equipment conditions of the unit: forging blank → rough machining → tempering → first semi finishing → artificial aging → second semi finishing → natural aging → finishing → assembly and splicing of each rack.

Rack manufacturing process and control measures

(1) Rough machining is the process of transforming from rough to semi-finished products. In order to reduce the cost and improve the machining efficiency, a φ 200mm cutter head, a φ 315mm three-sided milling cutter and a φ 25mm cutter head are arranged on the W200H digital display floor boring machine of Skoda, Czech Republic. First, use a φ 200mm cutter head to process six surfaces of the rack, leaving a machining allowance of 10 mm on one side, then use a φ 315mm three-sided milling cutter on the universal angle head to cut a straight groove along the tooth profile, and then stand the workpiece side by side, use a φ 25mm cutter head to process the tooth profile on both sides according to the method of line arrangement, leaving a machining allowance of 10 mm on each surface, and turn it to the next order, so that most of the machining allowance can be quickly removed, compared with the single use of φ The machining efficiency of 25 mm cutter head rough milling is increased by nearly three times.

(2) After the first semi finishing rack is tempered, it returns to the 3M × 8m CNC gantry milling machine for semi finishing. Place the workpiece flat on the workbench for two clamping processes. In the process of alignment, it was found that the deformation of each rack towards the tooth side concave center was 3-5mm. This is mainly caused by the removal of large amount of machining in the rough machining stage. After the machining allowance is considered evenly, the profile and tooth shape of semi finish machining shall be finished, and 5mm allowance shall be reserved on each side, and the next sequence shall be changed for artificial aging.

(3) After artificial aging, the internal residual stress of the second semi finishing rack will be fully released. The main processing contents of this sequence include semi finishing of tooth profile, profile and machining of each handlebar hole, etc. On the CNC gantry milling machine, according to the clamping method shown in Fig. 2, use the φ 250mm right angle cutter head on the universal angle head to process the tooth shape, carry out the cutting in 2-3 times, leave 1mm machining allowance on one side, turn the workpiece 180 °, process the bottom surface to 1mm machining allowance, and turn the next sequence to natural aging for 5 days. The rack can be processed in groups on the same machine tool in semi finishing process, which can shorten the overall processing cycle.

(4) The control method of finish machining the equipment and processing method used in this procedure are the same as the above sequence, the difference is the control of rack machining accuracy. First, clamp the bottom surface of the workpiece towards the main shaft of the machine tool, and lower the equal height sizing block. After alignment, clamp it gently. Process the bottom surface to a machining allowance of 0.3mm, then turn the workpiece 180 ° and place it on the equal height sizing block to process the tooth shape. First, process the tooth top surface to the drawing size. Because of the high precision requirements of tooth thickness and pitch, the method of measuring bar is used to control the accuracy of tooth profile in the process of finishing tooth profile. When selecting the diameter of the measuring rod, the contact point between the outer circle of the measuring rod and the tooth profile shall be placed on the graduation line and its nearby position. Generally, the diameter of the measuring rod d = (1.68-1.72) m shall be selected, where m is the end modulus (mm). After calculation and rounding, measure with a measuring rod of φ (80 ± 0.01) mm. The specific control method is: draw the distance h between the highest point of the top of the standard measuring bar and the tooth top according to the size required by the drawing, as shown in Figure 3. In the process of machining, first process one side slope of all teeth to meet the requirements of the drawing, then place the measuring bar in the first tooth Valley, use the dial indicator to check the actual distance H 'between the measuring bar and the tooth top, and the difference between H' and H is Δ H, then the machining allowance a of the cutter head along the length direction of the rack can be calculated according to the formula a = 2 Δ htan25 °. After finishing the first tooth, the machine tool ram moves 152.4mm along the length direction of the rack to process the second tooth, successively processing each tooth and the size of the length direction. The tooth surface is processed according to three times of cutting, and then the root is cleaned with R15 ball cutter. After the tooth shape is machined, the workpiece shall be turned over to remove the machining allowance of thickness and width, and then transferred to the next sequence for assembly.

epilogue

The processing accuracy of the rack is well ensured by adopting reasonable process methods. The deformation of each processing surface of the rack is ensured within 0.1M m, and the surface roughness Ra is controlled within 1.6 μ M. In the process of tooth profile machining, the measuring bar is used instead of the tooth pitch template to measure the tooth profile, which effectively guarantees the tooth profile accuracy. The accumulated error of the whole tooth pitch is 0.10-0.15mm. It provides a reference for the processing of similar products in the future.