The effects and causes of hob flute lead error.
In previous issues, we looked at several types of hob errors: flute spacing error, rake error, and runout. Another important type of error can be found in the lead of the flute, also known as the gash. Flute lead error occurs when the flute deviates from the correct flute helix angle. For straight-gash hobs, this is when the flute is not consistently parallel with the axis of the hob. Besides incorrect original manufacturing, hob sharpening is typically the cause of this type of error. Ahead, this is explored in more detail as well as the consequences of cutting a gear with a hob with this error.
A hob with flute lead error will have one end that has had too much stock removed compared to the other end. This creates a hob with a tapered outside diameter. As this tool is used to cut gears and shifted through its useful length, size variation will occur in the produced parts. Extreme taper will generate gear teeth that are unsymmetrical. They will have involute profiles that are plus (+, positive) on one side of the tooth and minus (-, negative) on the other, which creates a leaning profile.
Because only a small portion of a hob’s length is used to generate the gear form, inspection of a single gear is unlikely to highlight the effects of flute lead error. Therefore, by comparing measurements of gears cut at the start and end of a complete hob shift, one should be able to detect any potential problems in the lead of the hob flute. In particular, a difference will be apparent in tooth thickness and depth.
What causes flute lead error? Assuming the original manufacturing of the hob was correct, flute lead error is caused by poor hob sharpening or a poor hob sharpening machine and/or setup. Hob teeth are form-relieved, so if the flutes are sharpened at an incorrect angle, all of the teeth of the hob will be adversely affected. Too much stock will be removed from one end of the hob, creating teeth that are thinner and extend to a smaller diameter than the other end. This creates the tapered outside diameter of the hob that was discussed above. Specifically, taper will be induced by sharpening if the headstock and tailstock are out of alignment. Also, an incorrect sine bar setting or incorrect lead change gears (if the machine is equipped with either) can also induce flute lead error.
The images below show a hob flute being inspected for taper along the faces of a single row of teeth.
The AGMA standard ANSI/AGMA 1102-B13, Tolerance Specification for Gear Hobs, sets forth criteria for flute lead error tolerances. For example, over 100 mm length, the deviation from the correct lead must be less than 40 µm (0.0016”) to achieve an AGMA “AAA” quality. Details on these qualifications can be found in Table 8 – Accuracy Requirements of the standard, specifically Test no. 7, shown below.
When gear tooth size variations occur over the complete shift of a hob, flute lead error is likely a cause. This can be confirmed by measuring the deviation of the flute from the correct lead, or by observing a tapered outside diameter of the hob. This error is typically caused by poor sharpening and can be corrected in two ways. First, ensure correct alignments in the sharpening machine, and second, confirm correct sine bar settings and lead change gears if used.