A longitudinal groove along the axis of the hob that forms the face of one row of hob teeth and the back of the preceding row. Although technically called 'flutes' they are more commonly known as 'gashes' and it is with this latter name that I shall refer.
The gashes, as well as providing a space for the chips (swarf) to form, also allows the re-sharpening of the hob by grinding the teeth faces. Care must be taken when re-sharpening a hob as the accuracy and alignment of the gashes has a large influence on the quality of the gear produced.

There are three main areas where errors can occur:-
1. Gash Lead
The gash's can be straight whereby they are parallel to the axis and commonly called 'straight gash' or with a lead and called 'spiral gash'.
The hob must be sharpened at the gash lead specified on the end of the hob, if there are errors, the cam relief will result in a taper on diameter.
With an incorrect gash lead ground in the hob the cam relief will cause a taper on diameter also the hob lead being increased on one side of the tooth and decreased on the other.
This will result in producing a hob with a pitch that varies from one end of the hob to the other (Fig.1) and with a reduced pressure angle on one side of the tooth and an increased pressure angle on the other side. This will cause variations in tooth form, tooth thickness and tooth depth as the hob progresses through its hob shift.

2. Gash Radiality
Hobs can be designed under certain circumstances with a negative or positive rake but most are produced with a radial face rake, (Fig.2). The rake of the hob teeth will be marked on the end of the hob and must be maintained throughout its life. Any deviation outside of the permitted sharpening tolerance will result in generating errors.
Fig.3a - A correctly sharpened hob presents a true basic rack generating a true involute profile.
Fig.3b - An incorrectly sharpened hob showing a positive rake. The cam relief on the hob causes the constant profile to drop from front to back on the hob tooth. Whilst the tip of the hob tooth is correct, an error has been created at the root of the hob. The cam effect is such that the tooth depth is enlarged and the pressure angle thus reduced. A hob sharpened like this will generate an involute profile error with plus metal at the tip of the gear.
Fig.3c - An incorrectly sharpened hob showing a negative rake. The effect will be the reverse of Fig.3b.
Fig.3d - An incorrectly sharpened hob showing a convex rake. This is one of the most common errors in hob sharpening and is normally the result of grinding a spiral gash hob with a grinding wheel that has been dressed for a straight gash hob. The helical interference as the hob spirals past the wheel will give a convex cutting face resulting in hollow involute profiles.

3. Gash to Gash
This is eccentricity in the hob tooth spacing from one gash to the next. This eccentricity will produce profile errors due to uneven stock removal, Fig. 4.