Selection of hobbing feed

When I visited the "European Machine Tools Exhibition-EMO'97" in Hannover, Germany in 1997, I learned that the hobbing parameters of high-efficiency gear hobs developed by several gear hobs in Europe and America for car gear processing are from the strength of the gears being machined. Starting from the performance of the gear hob material, calculate the maximum allowable load of the top edge when the hob is cut, and then give the user a recommended value of "maximum allowable chip thickness of the hob top edge" to determine the optimization of the axial feed of the hob. select.
Foreign automotive gear materials are generally low alloy structural steel with hardness of 176 ~ 209HB. The materials of the gear hob are mostly high-speed steel and hard alloy. In recent years, the focus of each factory is to use a solid carbide hob.
In the rolling process of car gear machining, when hobbing, the tooth quality is required to meet the shaving requirements to select the hobbing speed and the maximum feed amount.

1 Hob feed

  1. Recommended values ​​for European tool factories

    2. The German saacke, Fette and sazzor gear hobs all recommend the maximum allowable chip thickness (h 1max ) of the gear hob tip to determine the hob feed.
    1. The h 1max of the carbide hob is classified according to the international standard (ISO): the P-type cemented carbide h 1max is 0.18 mm; the K-type cemented carbide h 1 max is 0.12 to 0.15 mm.
      Common grades of P-type hard alloys are P25-P40. Such solid carbide hobs are subject to TiN surface coating treatment in new knives, but are generally not coated after sharpening. K-type carbide hobs are often used for the tooth-shaped finishing of hardened steel gears, the grade is mostly K10, and the fine-die carbide is used for smaller-module hobs. In the fine hobbing, the new knife and the TiN coating treatment are performed after each sharpening. The pinion gear on the steering wheel of the car is hardened and then hardened by a carbide hob.
    2. The h 1max of the high-speed steel hob is specified for small and medium-sized car gears (m = 1.4 to 2.8 mm), and h 1max is generally selected to be 0.2 to 0.25 mm. For gears with large modulus, a larger value of 0.25 to 0.3 mm is available.

    The high-speed steel hob material of the above-mentioned European tool factory is usually made of 5% cobalt high-speed steel S6-5-2-5 (the number after S is the percentage of tungsten-molybdenum-vanadium-cobalt), and the quenching hardness is 65-66HRC. .
  2. Recommended value of the US Star Tool Factory

    3. The US Star Tool Factory is one of the major gear hob manufacturers in the United States. The factory also recommends the maximum allowable chip thickness h 1max of the top edge of the hob to determine the feed of the hob.
    1. The h 1max for the P-type cemented carbide hob is specified to be 0.15 mm.
    2. The h 1max of the high speed steel hob is specified to be 0.2 to 0.25 mm. The plant exchanged opinions with the German Liebherr gear hobbing machine factory. The maximum thickness formula of the hobbing chips derived by the two plants was obtained, and the obtained h 1max values ​​were similar.

2 The relationship between h 1max of the top edge of the hob and the axial feed amount f a of the hob


According to the maximum chip thickness h 1max of the top edge of the hob, the value of the axial feed amount f a of the hob can be applied in the hobbing process. The German Dr. B. Hoffmeister studied the maximum chip thickness of the top edge in the hobbing in a paper published by Achen University in 1979 and established a mathematical formula for calculating the workpiece from the chip thickness of the top edge. The axial feed of the revolution. This formula is validated at the top edge chip thickness h 1max = 0.35 mm for m = 1 hob and h 1max = 0.1 mm for m = 32 hobs. The hob factory in Germany uses the formula of Dr. Hoffmeister to calculate the f a value from the h 1max value. The calculation formula is
f a =F h 1 ·F m ·F z 2 ·F d ·F (N/z0) ·F a (1)
In the formula, F h 1 , F m , F z 2 , F d , F (N/z 0 ) , and F a are six influence factors, each having a calculation formula. Since the calculation equations of the various factors are more complicated, considering industrial applications, it has been simplified in Table 1 in recent years.
Table 1 Equations for calculating the influence factors of f a (simplified)
Impact factor name Computational equation
Cutting edge chip thickness h 1 factor F h 1 F h 1 =h 1max 1.9569
Gear normal modulus m factor F m F m =0.0446m -0.7730
Gear tooth number z 2 factor F z 2 For z 2 ≤120F z 2 =z 2 1.0607
Hob outer diameter d (mm) factor F d F d = (d/2) 1.6145 × 10 -2 × b 0.4403
F d = (d / 2) 0.4403 ( b =0 °)
Hob blade number N and number of cutter heads z 0 factor F N/z 0 F (N/z 0 ) = (N/z 0 ) 1.7162
Hob cutting depth a (mm) and gear tooth profile displacement correction factor c p factor F a F a = a -0.6243
The effect of c p is minimal and is calculated by c p =0
Table 1 h 1max - maximum chip thickness of the top edge of the hob, mm
f a ——Rolling axis axial feed / workpiece per revolution, mm/r
b ——The pitch of the gear is rounded, and the formula is the radians ( b angle in degrees, multiplied by p /180)

3 h 1max or f a selection

The raw data of the hobbing example and the f a or h 1max calculated according to the formula (1) are listed in Table 2, as explained below.
  1. No. 1 hob in Table 2

    2. This is the TiALN coated hard alloy overall hob extension structure promoted by the German saacke factory as a new product at the 1997 International Exhibition. Ø60 (D) mm × 150 (blade length) mm × Ø 27 (hole) mm, reflecting the small diameter, lengthening, multi-edge, multi-head modern hob, which represents the highest level of modern carbide hob hobbing . Used to process 20MnCrS5 sulfur-containing easy-cut carburized steel gear (containing 0.03% sulfur), hobbing speed V c = 395m/min (2095r/min), using f a =1.8mm/r (188mm/min), using formula (1) Calculate, we can get h 1max =0.14mm, which is consistent with the maximum allowable chip thickness recommended by the factory not exceeding 0.18mm. The tool life is 32m/blade and the entire tool can process 12600 gears.
  2. No. 2 hob in Table 2

    3. This is the insert carbide carbide hob developed by Chengdu Tool Research Institute. It has been in use since 1982. The gear materials were 40Cr and 20CrMnTi, the hardness was 176-209HB, and the hobbing speed V c = 175m/min (497r/min). The feed amount f a = 1.8 mm / r (31 ~ 23 mm / min). Calculated by the formula (1), h1 max = 0.14 mm can be obtained when machining a 29-tooth gear, which is within the maximum chip thickness recommended abroad. This knife was comparable to the world's advanced level in the early 1980s.
    Table 2 Gear, gear hob and cutting parameter data
    Hob number gear Gear hob Cutting parameter
    m(mm) z 2 b c p d(mm) z 0 N material coating structure V c (m/min) a(mm) f a (mm/r) h 1max (mm)
    1 1.7 40 31°46' - 60 2 16 P25 TiALN Overall multi-blade carbide hob 395 5.6 1.8 0.14
    2 3.175 29 - 0.3 112 1 12 S30(601Г) - Insert carbide hob 175 7.1 1.8 0.14
    3 2.8 36 - - 108 3 12 5% cobalt high speed steel TiN Round grinding gear hob 74 6.3 1.2 0.25
    4 2.8 36 - - 120 3 20 Overall multi-blade gear hob 74 6.3 3.0 0.25
    5 2 47 15° - 50 3 twenty two Overall multi-blade tooth re-grinding hob 120 4.5 4.4 0.23
    Note: For the calculation example, the factors of the No. 4 hob are calculated by the formula of Table 1 to obtain F h 1 =0.0663, F m =0.0201, F z 2 =44.747, F d =6.066, F (N/z 0 ) =25.94, F a =0.317, f a =2.97 take f a =3.0mm/r
  3. No. 3 and No. 4 hobs in Table 2

    4. This set of data is part of the economic analysis of the round grinding hobs and the overall multi-head, multi-blade hob machining of the car gears of the German Fette tool factory at the Nord gear factory in Germany in 1993. The tensile strength of the gear material was 650 N/mm 2 . The Fette plant gives both the f a value and the h 1max value in the data. Substituting the h 1max value into the formula (1), the obtained f a value is the same as the known f a value. This test is carried out given the same h 1max value, indicating that the plant uses equation (1) to determine the corresponding f a value (the six factor calculations in Table 1 are provided by the plant).
    The two hobs of No. 3 and No. 4 are mainly due to the large difference in the number of teeth. The former round grinding hob is 12 teeth, and the latter overall hob is 20 teeth. Since the relationship between F (N/z 0 ) factor and the number of hobs of the hob is 1.7162, the two knives are at the same h 1max = 0.25. Under the condition of mm, the corresponding f a values ​​are 1.2 and 3.0 mm/r, respectively, and the difference is 2.5 times.
  4. No. 5 hob in Table 2

    5. This is the test data of an overall multi-slot non-re-grinding hob machining car gear reported by the Italian SU Gear Tool Factory in 1995. Note that the hob diameter is small (Ø50mm), the number of cutting teeth is large (22 teeth), and the number of heads is large (3 Head), the full height of each blade is 5mm, but can withstand the chip load of h 1max =0.23mm. The hob material is cobalt high speed steel S6-5-2-5 containing 5% cobalt. Non-reground hobs only have a market in the United States.

As can be seen from the above five different types of hobs, it covers the different varieties and structures of modern hobs. The hobs 1, 4 and 5 are integral structures, representing the modern popular advanced hob types, 2 and 3 respectively. The hob is a stud structure with a small number of teeth. It represents the world's popularity in the 1980s, but it has been given a position in the US and Europe to give a more advanced overall PVD-TiN coating after each sharpening. Hob. The h 1max values ​​used for each of the hobs in Table 2 are in accordance with the recommended permissible values ​​of the tool factory. The formula (1) for calculating the f a value has been applied in actual production.

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