Gashing is amachining process used to rough out coarse pitchedgears andsprockets. It is commonly used onworm wheels beforehobbing, but also used oninternal andexternalspur gears,bevel gears,helical gears, andgear racks. The process is performed ongashers oruniversal milling machines, especially in the case of worm wheels.[1] After gashing the gear or sprocket is finished via hobbing,shaping, orshaving.[2]
Gashers are large, heavy-duty machine tools. They have horizontal and verticalslideways, precise indexing, large diameterballscrews, and spindle drive motors up to 100 hp (75 kW). They are usually controlled viacomputer numerical control (CNC) or amicroprocessor. While they are usually used for rough cutting, they are also sometimes used for finishing. For example, gashers are used for the production of large roller-chain sprockets.[2]
Gashing was first used to rough out worm wheels using a universal milling machine, but then dedicated gashers were built to rough out other types of gears. Because the processes is carried out on two different machines the process differs.
The process uses amilling cutter with a cross-section that is slightly smaller than the final cross-section of the cut; it has the same diameter as theworm. The cutter is then angled to thegashing angle. The gashing angle is calculated from thelead and pitch diameter of the worm; often tables are available with the gashing angles for common diameters and leads.[3] The cutter is centered over the blank and then plunged into it to the proper depth. The cutter is finally withdrawn and the blankindexed to the next tooth space and cut. This is repeated until all of the tooth spaces have been cut.[1]
Note that a standardendmill with rounded corners can be used instead if a special milling cutter is unavailable. The endmill should be3⁄10 the pitch diameter.[4]
Note that is process cannot be used on a small worm wheel that mate with a multiple threaded worm.[1]
The process is similar to that outlined for the universal milling machine except a gashing angle is not used. When gashing spur gears, racks, or bevel gears the cutter is plunged into the workpiece and then moved linearly in the proper direction. If a helical gear is being roughed out, then the table rotates and moves along the vertical axis tointerpolate the helical path.[2]
The two most common cutting tools are formed milling cutters and cutters with indexablecarbide inserts. For very large gears a cutter might be replaced with two slitting cutters that cut triangular slugs from the blank.[2]