AGMA 16FTM03

This paper shows how to produce worm screws when your actual gear hobbing machine configuration has the hob axis perpendicular to the workpiece axis with a swiveling of ± 30 [deg] for the hob head to produce helical gears. As we know, from the perpendicular axis of the work piece, the helix angle of a worm screw is of 90 [deg] ± 30 [deg]. With this type of hobbing machine, it is not possible to produce a worm screw by the two conventional ways: with a worm milling cutter cutting one thread after the other, or using a worm screw hob to produce the multiple threads screw in only one cut pass. To solve the problem, let us think about how a hob meshes with a straight gear. Conceptually, it is like a worm screw meshing with a gear. Now the idea is to think the opposite: the tool is the straight gear and looks like a shaping cutter. The workpiece is the worm screw. Using this concept, all the axis between the work piece and the tool are reversed. The swivel axis of the tool has now to rotate from 90 [deg] minus the worm screw helix angle. That means it has to swivel ± 30 [deg] from the perpendicular between the tool axis and the work piece. Theoretically, with this process your machine can now produce any type of worm screws. The rotation speeds are also reversed. In conventional gear hobbing, the work piece rotation speed is given by the hob rotation divided by the gear tooth number, multiply by the gear’s threads number. Now, the cutting speed is given by the rotation of the workpiece. So if a cutting speed of 8.66 [ipm] needs to be reached on a pitch diameter of 0.275 [in], the workpiece rotation will reach 9100 [rpm]! The tool speed will be calculated by dividing the speed of the workpiece by its number of teeth and multiply by the screw number of threads. Everything is reversed. But what is the big advantage? If an axial feed rate of 0.002 [in/piece_revolution] is given, the cycle time is going to be extraordinary fast. A brass part about 0.472 [in] long and turning at 9100 [rpm] will be machined in 1.6 seconds, around 8 times faster than a conventional worm cutter! Now your initial problem has become a super-productive process! The only obstacle is to find a hobbing machine with high speed synchronization for the workpiece spindles. This machine needs spindles that can reach 16,000 [rpm] to cover all the range of small and micro size worm screws. The tool tooth penetrates in the material like a turning tool cutter but the difference is that the tool is turning and all its teeth are going progressively in the material to cut the worm screw profile. Each tooth takes a little amount of material which makes the process quite soft and constant. The incredible cycle time and the smooth surface finish quality makes this process very interesting for high productivity precision worm screws.

Edition:

16#

Published:

09/01/2016

Number of Pages:

15

File Size:

1 file , 1.6 MB