helical gear question

[lfw618]

Hello,

Hoping someone could help clear up some confusion for me, or point me in the right direction. I'd been under the impression that for a helical gear, the true involute profile would be located in the transverse plane. However when specifying helical gears, pitch and tooth thickness are often given in the normal plane. Additionally I was under the impression helical gears could be cut with standard hobs, that are set angled from the gear axis, to create the helix angle. This would leave me to believe that the true involute form for helical gears is being made in the normal plane--the plane that is perpendicular to the action of the hob. Is someone able to help explain where I'm going wrong here? Should the true involute form be in the normal or transverse plane?

Thanks in advance.

 

[spigor]

The true involute form is created in the transverse plane.
To allow using the same hobs for different helix angles (including the 0 angle), gears are almost always specified in the normal plane. One exception is the TCP system, another would be the face milled spiral bevel gears.
In gear hobbing the helix angle is created by the machine kinematics. The hob is set angled to provide proper cutting conditions and correct pitch & rack angle in the transverse plane. The true involute form is created in the transverse plane with transverse pitch and transverse rack angle.

Hope that helps.

 

[lfw618]

I guess I may not be fully visualizing how hobs generate the involute form. I don't understand how gears are specified in the normal plane, to use the corresponding hobs, but then those hobs create a true involute form that is not in the normal plane.

 

[spigor]

As a hob travels through the entire face width of a helical gear, its cutting edges are enveloping a basic rack surface. This basic rack has the same pitch and rack angle as the hob- in the normal plane. In the transverse plane, the pitch and rack angle are slightly different, they depend on the helical angle, but still a basic rack with straight flanks is being formed. This basic rack in the transverse plane is generating a true involute form of the gear.

 

[mfgenggear]

Spigor is correct and a done well to explain.
I will add this.
The hob has a lead angle so in the hobbing machine the helix angle is obtained by adding the lead angle of the hob plus the lead angle of the helical gear.
the data input is normal dp & PA but because of the helical angle, is calculated to the transverse.
all measurements taken in transverse.

in other words if the helical gear is machined correctly, when inspected the involute, lead and tooth to tooth will be machined correctly.
thus a good gear.

so to reiterate
input is
#T, DP N PA N Helix Angle to obtain the proper gear attributes.

 

[mfgenggear]

A slight correction
Helix of gear - Helix of hob= x

X+ hob Helix = Helix setting of machine.
To obtain part Helix angle.