Numerical Control Transformation of X62W Milling Machine
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1 Infeed system design
In the transformation of the transverse feed system, the spindle nut mechanism of the original machine tool was removed, and a ball screw nut pair driven by a stepping motor and a pair of gears was replaced.
1. Workbench weight estimation
The longitudinal table weighs approximately 100kg, the saddle and slewing ring weigh approximately 200kg, and the total weight G = 100+200 = 300kg.
2. Cutting force calculation
Cutting power is
Nc=NhK (1)
Where: N - the main motor power, 7.5kW
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K - feed system power coefficient, take 0.96
According to formula (1) available: Nc = 4.32kW. Since Nc=FzV/6120, Fz=6120Nc/V, and V is the cutting line speed, taking 100mm/min, the main cutting force Fz=2.59kN. Usually: Longitudinal cutting force F = (0.6 ~ 0.9) Fz, vertical cutting force F = (0.45 ~ 0.7) Fz, transverse cutting force F = (0.5 ~ 0.55) Fz. Take F longitudinal = 0.6 Fz = 1.55 kN, F droop = 0.45 Fz = 1.165 kN, F transverse = 0.5 Fz = 1.295 kN.
3. Ball screw design
Axial force of table feed screw
F-axis = KF horizontal +f' (F vertical +W) (2)
In the formula: K - experimental factor considering the influence of subversion torque, K = 1.1
F transverse - horizontal cutting force f' - coefficient of friction on the guide, f' = 0.15
W——gravity of the table, saddle, and turntable, W≈10G=3kN
According to equation (2), F-axis = 2.049 kN.
Strength calculation
Ball screw speed: n = Vf / p, wherein the feed speed Vf = 60mm / min, ball screw pitch p = 6mm, you get: n = Vf / p = 10r / min. Taking the life time T=15000h, the life value L1=60nT/106=9. Take the operation coefficient fw = 1.2, the hardness factor fH = 1.1, then the maximum load Q = (L) & frac13; 1fwfHF axis = 5.625kN. According to experience, select the ball screw model ND3506-1 × 2 / E left, that is, the inner cycle, double nut gasket gap type, nominal diameter of 35mm, pitch of 6mm, two laps, E-class accuracy, left-handed, The ball diameter is 3.969mm. Its rated dynamic load is 12.847kN>Q, its strength is enough.
2. Efficiency calculation
Since the selected ball screw has a helix angle a=3°7′ and a friction angle f=10′, the transmission efficiency of the ball screw h=tga/tg(a+f)=tg3°7′/tg (3 °7'+10') = 0.95.
The design of the longitudinal feed system is similar to the design of the infeed system, and calculations are omitted.
2 Selection of Stepping Motors and Gears
1. Selection of Lateral Stepping Motors
1. Determine the step angle to take the system pulse equivalent dp = 0.01mm / pulse, choose the step angle qb = 0.75 °
2. Calculation of starting torque
Screw traction
Fs=F horizontal +1.414f when W (3)
Where f is - equivalent friction coefficient, f when = 0.01
Substitution (3)
Fs=1.337kN
Set the stepper motor equivalent load torque M, load force is F negative, according to the principle of conservation of energy, the work done by the motor and the work done by the load is equal, that is, Mfh = F negative L, then
M=F negative L/hf (4)
F negative = Fs + μ (W + F1), the numerical control device sends a pulse, the displacement of the table L is a pulse equivalent 0.001cm, then the motor turns the angle f = 2pqb/360 °, into the type ( 4) Get
M=F Negative L/hf=360°dp [Fs+μ(G+F1)]/2pqbh=167N·cm
Where: μ - friction coefficient of the machine guide, take μ = 0.18
F1 - consistent with the direction of gravity, the load force acting on the moving part, F1 = F vertical = 1.165kN
If the influence of the inertia of the moving parts at startup is not considered, the starting torque Mq=M/0.3 to 0.5, the safety factor 0.3, and Mq=556 N·cm. For a stepping motor with four phases and eight beats, Mmax=Mq/0.707=786N·cm.
Stepper motor maximum frequency
Take the maximum horizontal feed rate Vmax = 1m/min, then the maximum frequency of the stepper motor is fmax = 1000Vmax / (60 × dp) = 1667Hz. Based on the above data and experience, the 110BFG type stepping motor is suitable. The step angle of the motor is 0.75°/1.5°, the maximum static torque is 800N·cm, and the maximum no-load starting frequency is 1800Hz.
2. Gear design (horizontal)
According to the step angle qb, ball screw pitch p, and pulse equivalent dp, the gear ratio of the pair of gears added between the stepping motor and the ball screw is: I=Z1/Z2=dp×360°/( Qb·p)=0.8, choose Z1=48, Z2=60, according to experience, take 1.5mm gear module.
3 CNC System Hardware Design
The numerical control system used is the BKC2-VIII type numerical control system that we recently developed, mainly consists of the main control system and the display system. The main control system is used to control the input and output of the keyboard, the overtravel of the worktable in the X and Y directions, the closing of the side doors, the query, the phase of the X and Y-direction stepper motors, and the positive and negative rotation of the main shaft. The display system is used to control the H716501 LCD display. Both parts adopt a new type of single-chip GMS90C32 as the central processor. It can be compatible with MCS-51 series single-chip microcomputers. It has a fast pulse programming algorithm and adopts CMOS technology. The maximum operating frequency can reach 40MHz (this design adopts 18MHz). Good performance to price ratio.