Design of round pull broach

Task 1. Design of round pull broach The broach for a cylindrical hole must be designed according to a given task’s parameters (Table 1). Table 1. Data of the task Variant No. Diameter of the hole D, mm Length of the work piece L, mm Diameter of pre-drilled hole, mm Material 12 16H8 28 14 Steel C45 Calculations In order to design broach, some geometrical parameters of the broach must be chosen and calculated. After calculations will be made, in order to select the optimal scheme of the broach calculations for single and group schemes must be made and compared. 1. Standard tolerance for hole Various grades of tolerances are defined using the ‘standard tolerance unit’, i, in µm, which is a function of basic size. i  0.453 X  0.001X where, X (mm) is the geometric mean of the lower and upper diameters of a particular diameter step within which our diameter lies. Diameter steps in I.S.I (a-b, where a is above and b is up to) are given in Table 2: In the given value of diameter 16H9, 16 lies between 14 & 18 in the Table 2 The fundamental tolerance unit (i) is given by; i  0.453 15.874  0.001* 15.874  1.146 m According to international standard, the hole tolerance for H9 or IT9 is – 40i Therefore the standard tolerance is – 40 i = 40*1.146 = 45.84 = 0.045 mm 2. Primary geometrical parameters of the broach The C45 steel hardness is considered to be in the range of 170-210 HB. For this task the hardness is chosen to be 200 HB. Selected broaching machine is able to create a maximum pull force of Pm = 204 kN; Due to Table 2 data, length of a broaching stroke is ; Table 2. Parameters of broaching machine tool Parameter and dimension Value Nominal pulling force, kN 100 250 630 1000 Length of a broaching stroke, mm 1250 1600 1600 2000 2000 D2, mm 125 160 200 250 The pull end with a round neck (Fig. 1) is chosen due to it’s quick locking and unlocking of the broach capability, simplicity in production, and because for round hole manufacturing broach can turn during the procedure. Fig. 1. Pull end with a round neck Table 3. Dimensions of pull end necks The diameter of pull end of the broach is chosen from the Table 4. Table 4. Broach diameter dependence on hole size d 14-15 16-17 18-20 20-21 21-22 23-26 27-28 29-32 33-37 38-40 41-47 48-60 62-71 75-90 92-105 d1 12 14 16 18 20 22 25 28 34 36 40 45 50 63 70 The broached hole diameter is 16 mm, therefore d1 = 14 mm is chosen. Area of the broach neck is chosen from Table 3. Diameter d1 = 14 mm, therefore Fx = 70,9 mm2. Calculated geometrical data is shown in Table 5. Allowance is calculated by formula: A = 16,045 – 14 = 2,045 mm The distance before the beginning of the broach (L0) was calculated from Table 4: L0 = L1 + L2 + L3 + L4 =120 + 20 + 20 + 12 = 174 mm Table 5. Geometrical data 100 1600 2,045 14 70,9 With all calculated parameters single scheme broach calculations can be made. 1. Single scheme case Pitch of teeth: , here m value is in the range of 1.25…1.5. m = 1.25 is chosen. Calculation of the maximum number of teeth working at the same time: The height of the teeth is taken from the Table 6 using the Fig. 1. Fig. 2. Teeth geometry Table 6. Chip grooves’ parameters t c r R t c r R 4.5 2 1.5 1 2.5 3.14 14 3 4 1.5 10 7.07 4 2 12.56 6 2 2 1 4 3.14 5 2.5 19.63 2.5 1.25 6 3 28.27 8 2.5 3 1.25 4.91 16 4 4.5 2 12 12.56 3 1.5 7.07 5 2.5 19.63 10 3 3 1.5 7 7.07 6 3 28.27 4 2 12.56 7 3.5 38.48 12 3 4 1.5 8 7.07 18 5 6 2.5 12 19.63 4 2 12.56 6 3 28.27 7 3.5 38.48 is chosen . Area of the chip is chosen from Table 6. . Chip groove filling coefficient K is chosen from Table 7. Table 7. Chip groove filling coefficient Processed material Cutting scheme Single Group Steel 4 3 Specified material is Steel C45, therefore K = 4. Feed per teeth: In order to check strength parameters of the broach, the maximum tenses in the broach shank and teeth must be found, assuming the broach is made of high-speed steel with an allowable tensile stress of 250 in the shaft and 300 in the cutting part. Maximum stress in the shank: Maximum stress in the cutting part: Feed per teeth by cutting force: Coefficient is taken from Table 8. Table 8. Coefficient according to material hardness Material Hardness HB N/mm2 Carbon steel 220 2860 Alloy steel 220 3610 Grey cast Iron 180 2040 Malleable cast iron >200 1670 Calculated single scheme data: Table 9. Single scheme data 7 5 2.5 4.91 4 2. Group scheme case In order to calculate parameters for group scheme, number of teeth in group must be decided. The teeth number in one group is marked as and can be between 2 and 5. Chosen value for Second, a pitch of teeth tp needs to be calculated. tp  where value of m is in the range of 1.45…1.95. Let it be 1.7 tp  1.7 Calculating the maximum number of teeth working on the same time Zmax The height of the teeth hk is taken from the Table 5. hk = 3 mm; Also the area of the chip (Fk) can be provided from the Table 5. Fk.=7.07 mm2; Chip groove-filling coefficient K will be provided from the Table 6. For steel, K =3. Feed per teeth: Allowable feed per teeth by cutting force: Calculated group scheme data: Table 9. Group scheme data 9 4 7,07 3 0,0841 0,075 3. Comparison between both schemes In order to compare the effectiveness of both schemes, calculations of broach cutting length must be made using both schemes. The smaller and less material requiring to produce broach should be chosen. Cutting length of single scheme broach: Cutting length of group scheme broach: Since lps > lpg therefore broach with group scheme should be manufactured, because it is more cost effective, since it uses less materials. Teeth height in group scheme Allowable feed per teeth in group scheme is Szp=0.0841 mm; First tooth of the broach is calibrating teeth and must be the same diameter as the drilled hole. D01 = D1 = 14 mm Now each tooth diameter in a group will be increased by 2*Szp=2*0.0841 = 0.1682 mm until it gets close to the diameter of 16H9 without going over the tolerance limit.

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