Gashing
Encyclopedia
Gashing is a machining
process used to rough out coarse pitched gear
s and sprocket
s. It is commonly used on worm wheels before hobbing
, but also used on internal and external spur gears, bevel gear
s, helical gears, and gear racks. The process is performed on gashers or universal milling machines, especially in the case of worm wheels. After gashing the gear or sprocket is finished via hobbing, shaping, or shaving.
. 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.
with a cross-section that is slightly smaller than the final cross-section of the cut; it has the same diameter as the worm. The cutter is then angled to the gashing angle. The gashing angle is calculated from the lead
and pitch diameter of the worm; often tables are available with the gashing angles for common diameters and leads. The cutter is centered over the blank and then plunged into it to the proper depth. The cutter is finally withdrawn and the blank indexed
to the next tooth space and cut. This is repeated until all of the tooth spaces have been cut.
Note that a standard endmill
with rounded corners can be used instead of a special milling cutter is unavailable. The endmill should be the pitch diameter.
Note that is process cannot be used on a small worm wheel that mate with a multiple threaded worm.
The two most common cutting tools are formed milling cutters and cutters with indexable carbide inserts. For very large gears a cutter might be replaced with two slitting cutters that cut triangular slugs from the blank.
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
process used to rough out coarse pitched gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s and sprocket
Sprocket
A sprocket or sprocket-wheel is a profiled wheel with teeth, cogs, or even sprockets that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it...
s. It is commonly used on worm wheels before hobbing
Hobbing
Hobbing is a machining process for making gears, splines, and sprockets on a hobbing machine, which is a special type of milling machine. The teeth or splines are progressively cut into the workpiece by a series of cuts made by a cutting tool called a hob...
, but also used on internal and external spur gears, bevel gear
Bevel gear
Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped.Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well...
s, helical gears, and gear racks. The process is performed on gashers or universal milling machines, especially in the case of worm wheels. After gashing the gear or sprocket is finished via hobbing, shaping, or shaving.
Equipment
Gashers are large, heavy-duty machine tools. They have horizontal and vertical slideways, precise indexing, large diameter ballscrews, and spindle drive motors up to 100 hp. They are usually controlled via computer numerical control (CNC) or a microprocessorMicroprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
. 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.
Process
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.Universal milling machine
The process uses a milling cutterMilling cutter
Milling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
with a cross-section that is slightly smaller than the final cross-section of the cut; it has the same diameter as the worm. The cutter is then angled to the gashing angle. The gashing angle is calculated from the lead
Lead (engineering)
Lead is the axial advance of a helix or screw during one complete turn The lead for a screw thread is the axial travel for a single revolution....
and pitch diameter of the worm; often tables are available with the gashing angles for common diameters and leads. The cutter is centered over the blank and then plunged into it to the proper depth. The cutter is finally withdrawn and the blank indexed
Indexing (motion)
Indexing in reference to motion is moving into a new position or location quickly and easily but also precisely. After a machine part has been indexed, its location is known to within a few hundredths of a millimeter , or often even to within a few thousandths of a millimeter , despite the fact...
to the next tooth space and cut. This is repeated until all of the tooth spaces have been cut.
Note that a standard endmill
Endmill
An endmill is a type of milling cutter, a cutting tool used in industrial milling applications. It is distinguished from the drill bit, in its application, geometry, and manufacture...
with rounded corners can be used instead of a special milling cutter is unavailable. The endmill should be the pitch diameter.
Note that is process cannot be used on a small worm wheel that mate with a multiple threaded worm.
Gasher
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 to interpolate the helical path.The two most common cutting tools are formed milling cutters and cutters with indexable carbide inserts. For very large gears a cutter might be replaced with two slitting cutters that cut triangular slugs from the blank.