A helical broach which eliminates the need for a fixing means for fixing a workpiece during gear machining is provided. The helical broach includes: a plurality of cutting edges; a spiral flute formed between adjacent cutting edges to extend in a spiral manner with a predetermined helix angle with respect to an axial direction of the helical broach; and a gullet formed between adjacent cutting edges to extend with a predetermined helix angle with respect to a circumferential direction of the helical broach. In the helical broach, the expression: 0.5tan /tan(+)0.7 is satisfied, wherein is the helix angle of the spiral flute, and (90) is the helix angle of the gullet with reference to a central axis of the helical broach. An internal gear machining method using the helical broach is also provided.

The shell of this helical broach (1) is formed by stacking a plurality of wafer shells (20.sub.W(1)-20.sub.W(N)) in the axial direction, and is obtained by forming on the wafer shells (20.sub.W(1)-20.sub.W(N)) finishing blades (30.sub.W(1)-30.sub.W(N)) corresponding to teeth grooves on a piece to be cut (W) and forming the finishing blades (30.sub.W(1)-30.sub.W(N)) such that the blade width gradually increases with each of the aforementioned wafer shells (20.sub.W(1)-20.sub.W(N)) from the leading end of the cutting direction toward the trailing end of the cutting direction.

A cutting element for a tool to create a helically extending, trapezoidally undercut groove in a cylindrical surface of a bore. The cutting tool, preferably configured as a cutting insert, has groove-cutting teeth to create a symmetrically cross-sectioned groove which are arranged in series in a division harmonized with the pitch of the groove to be created. These groove-cutting teeth comprise at least one pre-machining tooth to create and machine a base groove and several trapezoidal teeth following the at least one pre-machining tooth, which have a tooth head profile which expands trapezoidally in cross-section in the vertical direction of the tooth with two flanks delimiting a flank angle for further machining the base groove to a trapezoidally undercut final cross-section. The flank angle delimited by the two flanks increases from at least one trapezoidal tooth to a following trapezoidal tooth to a defined final dimension.

The finishing part (4) of this helical broach (1) is formed by a first shell (20) and a second shell (30) which are divided in the axial direction, and is obtained by forming a first finishing blade (50), which comprises a prescribed gear tooth helix angle () end a first blade groove helix angle (.sub.1), on the aforementioned first shell (20) and forming a second finishing blade (60), which comprises the aforementioned prescribed gear tooth helix angle () and a second blade groove helix angle (.sub.2) which differs from the aforementioned first blade groove helix angle (.sub.1), on the aforementioned second shell (30).

For producing a cylindrical surface that has a surface structure of predetermined geometry suitable for application of material by thermal spraying, a geometrically predetermined groove structure of minimal depth and width is introduced into the surface by a tool embodied as a follow-on tool in that a groove cross-section is processed successively to a final size. In order for the surface to be producible in mass production with constant quality, the groove structure is worked in such that first a base groove is introduced with a groove bottom width that is smaller than the groove bottom width of the finished groove. Subsequently, at least one flank of the base groove is processed for producing an undercut groove profile by a non-cutting action or cutting action wherein the introduced groove structure is deformed in such a way that the groove openings are constricted by upsetting deformations of material.

An internal helical broaching tool has a pre-broaching tool portion ex-tending in one broaching direction and a subsequent precise sizing tool portion. The latter, as a whole, is made of a hard metal with a density of greater than 10 g/cm3. This results in an internal helical broaching tool with increased service life at a defined machining performance.