Gear Cutting Tools
Hobs - Torion
Hobs - Torion
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Topping cutters<br />
The outside diameter of the gear<br />
is topped by the tooth root of the<br />
hob. Changes in the tooth thickness<br />
also result in changes of the<br />
outside diameter.<br />
Chamfer<br />
When hobbing helical spur gears<br />
with large diameters, the hobs<br />
cannot always be chosen long<br />
enough to cover the entire working<br />
area. To prevent excessive wear of<br />
the hob teeth in the approach<br />
area, the hob is provided with a tapered<br />
chamfer. For gears with<br />
double-helical teeth, two hobs<br />
with chamfer may be necessary, if<br />
the distance between the two<br />
tooth rows is relatively small.<br />
Depending on whether hobbing is<br />
by the climb or conventional method,<br />
the chamfer — generally 5 to 6<br />
x module long and 5° to 10° angle<br />
of inclination — is situated on the<br />
entering- or leaving end of the<br />
cutter.<br />
Gashes<br />
A high number of gashes increases<br />
the cutting capacity of the hobs<br />
and the density of the envelope<br />
network; they do however also reduce<br />
the useful tooth length,<br />
unless the cutter diameter is<br />
increased accordingly. For solid<br />
type hobs the gashes are up to a<br />
helix angle of 6° made axially parallel<br />
and over 6° with helix.<br />
DP and CP<br />
In English-speaking countries, diametral<br />
pitch and circular pitch are<br />
used instead of the module. lt is<br />
best to convert the above values<br />
into module and to proceed with<br />
the calculated module in the usual<br />
way.<br />
The equations for the conversion<br />
into module are:<br />
m = 25.4 / DP<br />
m = 25.4 · CP / 3.1416<br />
Rake<br />
Unless otherwise agreed, hobs<br />
have a rake of 0°. This does not<br />
apply to broaching tooth type<br />
roughing hobs, which have a rake<br />
of +8°, and indexable insert and<br />
skive hobs, which have a rake of<br />
-10° to -30°.<br />
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