【正文】 ＞40~50＞50~65＞65~80＞80~100＞100~120＞120~140＞140~160注：源自《實(shí)用注塑成型及模具設(shè)計(jì)》P41表215（+）號(hào)，用于基準(zhǔn)軸取負(fù)（－）號(hào)。表中公差數(shù)值用于非配合孔取正（+）號(hào)，用于非配合軸取負(fù)（－）號(hào)，用于非配合長(zhǎng)度取正負(fù)（177。）號(hào)。，如注塑件的高度尺寸，受水平分型面溢邊厚薄影響，其公差值取表中值再加上附加值。，3~，6~。 NC machinesMachine may be classified according to the number of axes of numerically controlled movements with respect to Cartesian XYZ coordinates. there may be other movement not numerically controlled .A twoaxis machines would have the table moved longwise and crosswise in a horizontal plane。 a threeaxis machines would have an additional vertical movement of the spindle, for example ,four,five or sixaxis machines provide additional liner or rotary movements. There classes of NC systems are monly recognized. One of these is called pointtopoint or positioning NC where a cutting tool and workplace are positioned with respect to each other before a cut is taken, as exemplified by an NC drill press. The path between points is of little concern and is not particularly controlled. Another class, known as a straightcut NC system, involves movements, between points like pointtopoint NC but along straight or curved paths determined by the machine ways or slides. An NC turret lathe may be of the second class. A third class is continuouspath NC, or more formally continuous tool path control, which does contouring or profiling or lines, curves, or surfaces, of all shapes. One way of achieving continuous path NC is to move the work piece or cutter from point to point along a straight line or curve with many points spaced closely enough together so that the position path of the cut approximates a desired curve within limits required. Another way is to drive machine members say as saddle and the table on it, along coordinate axes at varying velocities controlled so that the resultant motion is along a particular line or curve.An NC machine may be controlled through an openloop circuit. The openloop system is the simplest and cheapest but does not assure accuracy. A signal that is an order for a certain action is issued by the control unit. This travel through the travels through the drive mechanism which imposes the action upon the controlled member of the machine. The basic drive mechanism for a table or other machine member in an openloop system is a stepping motor, also called a digital or pulse motor. It has, for example,49poles around the armature inside of 50 poles around he stator. Two poles are always aligned. Poles are energized to align the next two poles and turn the armature through a definite small angle each time a pulse of electricity is received. The motor turns the lead screw, which moves the table a corresponding distance, If, for example, a pulse stands for a movement, and a movement of mm is desired, he control unit issues 50 pulses。 the motor turns through 50 steps and turns he lead screw, and the table is driven the mm required. The rate which the pulses are issued determines the rate of feed. The power of a stepping motor is limited, and if the resistance to movement is large, the motor may just stall and miss steps .there is no feedback to report the omission, and accuracy is lost. Direct drive with a stepping motor is confined to light service at fractional horsepower. For more power heavy service, a stepping motor is matched with a hydraulic motor through a servo valve for table drives up to KW and more.It is mon in closedloop systems for the pulsed mand signal for movement of the table of the machine to be converted to a steady analog signal. That signal turns on the power to the drive mechanism. Hydraulic and AC but mostly DC electric motors are all used for NC machine drives. The intensity of the signal is determined by the feed velocity ordered. For a higher feed, pulses are issued at a faster rate and the analog signal is larger, which turns on more power the to the motor to drive the table faster. A tachometer in the motor drive sends back a signal that is pared to the feed rate order to assure that the required feed rate is being delivered and to make correction if it is not. Commonly, a resolver is driven by the leads crew and returns a signal showing how far the table moves. More accuracy for a longer period of time is obtained with feedback device attached directly to the table, but such devices are more costly. The feedback signal is pared to the order, and where the feedback equals the order and signifies the required movement has been pleted, the power is shut off, and the movement stops. Since they are controlled through separate channels, all movement may act at the same time. Indeed, it is necessary that they act at simultaneously and be synchronized accurately for a contouring.