Hob Runout

Hob Runout

Hob Runout

One of the biggest causes of gear manufacturing problems is hob runout, yet this is often overlooked by the gear industry.
It occurs in two areas but can be the result of numerous problems.
Area 1: In normal use on the hobbing machine.
Area 2: When resharpening the hob.

Area 1 - Runout on the hobbing machine:
These runout errors are made up from:
1. Problems within the machine itself
2. Result of poor setting of the hob.
These errors may not necessarily be encountered singly, it is not uncommon for more than one to be encountered at the same time.

Machine Errors:
In Fig.1, you can see an extreme case of hob runout. These errors are seen both radially (^) and laterally (>).
Problem 1: On the left of Fig.1 is the hob arbor tailsteady, problems occurring here will result in radial runout of the hob and may be caused by:
a) tailsteady not being clamped securely,
b) worn or loose bearings in the tailsteady,
c) worn or undersized location diameter on the hob arbor.
Problem 2: On the right of Fig.1 is the drive head, problems occurring here will show both radial and lateral runout of the hob and may be caused by:
a) worn or loose bearings in the hob spindle,
b) hob arbor mis-located due to swarf or damage.
Each of these problems will be responsible for poor tooth surface finish and profile irregularities in the component.

Fig.1 Runout in hobbing m/c

Fig.2 Hob reference diameters

Hob Setting Errors:
If your hob manufacturers work is not to be undone, each hob MUST be checked for runout after installing on the hobbing machine, but how do we do this?
During manufacture two reference diameters are ground that are true and concentric to all diameters of the hob. These are used to check radial hob runout on the machine, see Fig.2.
When clocking (indicating) these diameters, a hob must have a maximum runout of:
0.012mm (0.0005") TIR for rough hobbing
0.005mm (0.0002") TIR for finish hobbing
Any runout MUST be kept 'in phase' between the two ends of the hob.

Hob runout here can be the result of:
a) hob arbor not running true,
b) bent or damaged hob arbor,
c) undersize hob arbor,
d) oversize hob bore,
e) non-parallel hob arbor spacers,
f) hob arbor clamping nut face not square with the pitch diameter of the thread,
g) foreign material on the mounting surfaces.

In the following diagrams (Fig.3 through to Fig.6 inclusive) we can see the result on the profile that various types of hob runout have on the component with the help of simulated gear graphs. The graphs depict the left and right hand tooth flanks.

Fig.3	Fig.4	Fig.5	Fig.6


Hob running true	Both sides out evenly	One side out	Both out unevenly

Fig.3 - the hob is running true, the graph shows slight undulations due to feed.
Fig.4 - the hob is running out both ends evenly, the graph shows an 'in phase' condition appearing as increased undulations.
Fig.5 - the hob is running out on one end, the graph shows an 'out of phase' condition with increased undulations on the right flank but still slightly affecting the left flank.
Fig.6 - the hob is running out both ends unevenly, the graph shows an 'out of phase' condition with greatly increased undulations.

All of these hob runout errors will result in a difference in profile from one tooth flank to the other in the component. This will also vary from one end of the hob to the other with the use of the hobbing machine's hobshift facility.

Area 2 - Runout during resharpening:
Hobs are precision tools manufactured to very strict limits. When you purchase a new hob you expect it to produce good gears throughout its life, unfortunately, subsequent resharpening often affects the results.
Despite all efforts to eliminate runout on the hobbing machine, if it is not running true when resharpened, runout will be ground into the hob!
See also Hob Flutes - Gashes for Gash Lead, Gash Radiality and Gash to Gash Errors during resharpening.

Addendum:
Often during lectures, with delegates numbering approximately 35 all from different companies, I will ask for a show of hands to the following questions:

Q: How many clock their hobs on the hobbing machine?
A: On a good day I may get ONE or TWO put their hands up! (5%)

Q: How many clock their hobs on the resharpening machine?
A: NO hands!
Q: How many don't know if their hobs are clocked at resharpening?
A: All hands up!

Q: What would be your reply to the same questions dear reader?
Note: If I had a perfect response to these questions, people would be either fibbing or too good to need my lectures! I do tend to preach to the unconverted, but where are the converted?
I wonder what an internet poll would reveal?

webmaster@drgears.com