Single Position Hob Design

[030276win]

Hello All,
Firstly what does a 8/10 DP Cutter mean, is it a 9.97458 mm Diametrical Pitch, (8*pi*25.40) and wat does the 10 stand for.
Have a sample cutter and when i view it I get a pitch of ony 9.312 over all 12 teeth (it has only 11 teeth), the lead angle is 12.25Deg.

Is there some correction to be given!!!

 

[gearcutter]

8/10 DP is 8 DP stub pitch. The 10 represents the whole depth of 10 DP so, being a smaller pitch than 8 DP, the depth ends up being shallower than for 8 DP causing the stubbed teeth. Whole depth = 2.157/DP, so for 8/10 the whole depth would work out to 6.40mm. The stub system uses the old depth standard of 2.157/DP which differs from the current standard of 2.25/DP.

 

[030276win]

ok gr8...and do u have any idea about the pitch of the cutter....all different teeth show different pitch i mean ranging from9.285mm to 9.39mm (for 12 teeth)....

shouldn it be 9.975mm (8*25.4*pi)..??

 

[gearcutter]

If it's a hob then the pitch isn't determined by the number of teeth (I'm not sure what you mean by counting 12 teeth). For a single start hob the pitch is calculated by the axial distance from a point on one tooth to the same point on another tooth measured on a straight line in the transverse or axial plane.

 

[030276win]

exactly, the measurement from one point to the next tooth is 9.39 instead of the required 9.475

is it a correction?

 

[dimjim]

The pitch should be
(Pi x 25.4)/Dp

Are you measuring chordal
pitch instead of circular
pitch?

 

[030276win]

ya tats how i am calculating it...but the sample doesnt show tat pitch when measured!!

it shows less, is it cos of the 12.25 deg lead angle??

 

[dimjim]

If you are trying to layout a hob drawing to
imitate the cutting action, ignore the lead
angle.

The 4/5 and the 8/10 Fellows Stubs are the same
as the 4 and 8Dp Stub System for calculation
purposes. The hob is the reverse of the tooth.

So the Tooth Addendum is .80/8 = .1000
Tooth Dedendum is 1.0000/8 = .1250
Circular Tooth Thickness = (Pi/2)/8 = .1963495
So the hob or rack detail is
Addendum .1250
Dedendum .1000
Width at pitchline .1963495

Another way to look at the Fellows Stub is to use the
smallest number to determine the circular tooth thickness
as above and use the ratio as follows:

8/10 times 1.0000 for the Addendum = .1000
8/10 times 1.2500 for the Dedendum = .1250