A flat plate or straight bar is called a rack when it has a series of equidistant teeth. A rack whose tooth line is a straight line perpendicular to the direction of tooth movement is called a straight rack; A rack whose tooth line is a straight line inclined to the direction of tooth movement is called an inclined rack. There are many processing methods for rack, and milling is commonly used.
What are the methods of milling rack?
1、 Milling of straight rack
The straight rack is milled on a horizontal universal milling machine with a disc gear cutter.
1. Milling of short rack
(1) Selection of milling cutter: the milling cutter for milling rack generally selects a set of No. 8 disc straight tooth cylindrical gear milling cutter among 8. When the rack accuracy is required to be high, a special rack milling cutter can be used.
(2) Clamping of workpieces: workpieces are clamped with flat tongs or directly pressed on the worktable. When there are a large number of workpieces, special fixtures can be used for clamping. No matter which clamping method is adopted, the tooth top surface of the rack blank must be parallel to the workbench,
The positioning reference surface on one side of the blank must be parallel to the transverse feed direction of the workbench.
(3) Control of tooth pitch: when milling the rack, after each tooth is milled, the workbench moves one tooth pitch horizontally, which is called shift distance.
Common displacement methods are:
Dial method: use the horizontal feed handle of the workbench to turn the dial by a certain number of grids to realize the displacement. This method is only applicable to short racks with low accuracy requirements and a small number.
Dividing disc method: refit the dividing disc and dividing handle of the dividing head on the head of the transverse feed screw of the workbench.
2. Milling of long rack
(1) Clamping of workpiece: when machining long rack, because the horizontal movement distance of milling machine workbench is not enough, the longitudinal movement distance of workbench must be used to divide the teeth, that is, the positioning reference surface on one side of workpiece is required to be parallel to the longitudinal feed direction of workbench. The workpiece can be directly pressed on the worktable or clamped with a special fixture.
(2) Installation of milling cutter: when processing long racks, the tooth pitch is controlled by the longitudinal lead screw of the workbench, so the direction of the original milling cutter rod on the horizontal milling machine cannot meet the processing requirements. The direction of the milling cutter rod must be parallel to the longitudinal feed direction of the workbench. Therefore, the spindle of the milling machine must be refitted.
2、 Milling of inclined rack
The helical rack can be regarded as a part of a helical cylindrical gear with infinite base circle diameter. The helical rack is milled on a horizontal universal milling machine with a disk-shaped gear milling cutter. The milling method is basically the same as that of milling the straight rack, except that the workpiece rotates a spiral angle relative to the cutter during milling. There are two milling methods for inclined rack:
1. Workpiece tilting clamping method
When the workpiece is installed, make one side of the reference surface form a spiral angle with the direction of the dividing tooth displacement of the workbench. After milling one tooth, the shift distance shall be equal to the normal tooth distance of the helical rack. This method is limited by the horizontal stroke of the worktable, so it is only suitable for milling inclined rack with small helix angle.
2. Workbench rotation method
When installing the workpiece, the reference surface on one side of the workpiece is parallel to the longitudinal displacement direction of the workbench, and at the same time, turn the workbench by a spiral angle to make the inclined rack slot parallel to the milling cutter rotation plane. After milling one tooth, the workbench moves one end tooth pitch longitudinally. This method is suitable for milling long inclined rack on universal milling machine.
