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Helical gear cutting

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b.lindsey:
George, thanks for collecting all of this information in one thread. I am making the topic "sticky" for the time being so it won't fall down the list until everyone that has an interest in the subject has a chance to read it

Bill

gbritnell:
Andrew,
The reason the crests look so wide is because of the angle of the helix. You'll have to take my word that the gears mesh nicely.
gbritnell

nj111:
Andrew, I was typing this and George beat me to it with a very concise answer! -
I'm not an expert  but I do have a couple of old Sunderland gear planers in the shop at home and sometimes I need to cut a helical gear on them.
I was thinking maybe the answer to your question is because the helix angle has an effect on the pitch required to generate the tooth.  I'll try to explain as best I can:
If you view a helical tooth looking square onto on either side face of the gear rather than considering it's cross section you will see a wider tooth than normal for any given helix angle. The tooth is the same height but it's wider / fatter to look at.  The greater the helix angle, the wider / more distorted the tooth appears to be - from that viewpoint.
On a gear planer there are two gearboxes to set up to produce the part , one for the number of teeth and the other for the tooth size i.e. DP or Module.
When you set up the pitch for a helical gear  you have to allow for this extra width. In other words you have to set the machine as if you were cutting a larger tooth pitch - but you are not.
For example I had to make a MOD 2.5 gear with what appears to be the most odd helix angle of 36.869 degrees. At first I thought - why ever did they choose such an odd helix angle? - but the answer was actually straightforward because cos 36.89 is a dead 0.8  Following the calculations through it meant that to cut those MOD 2.5 teeth at that particular helix angle I had to use a MOD 2.5 cutter but set the machine to index as if it were a MOD 3.125 tooth.  (quite simply 3.125 x 0.8 = 2.5) All nice and easy round numbers to work with. The gear came out perfectly - but it's kind of hard to get your head around.
I assume that is the reason some odd looking helix angles are encountered , as actually during the calculations they will in fact relate to dead sizes of pitch available on traditional gear cutting machinery, and perhaps this is also a factor in what you observe above. - I'm not 100% sure but it could be a factor? Nick

tangler:
Chuck's fixture is a brilliantly simple way to set up the long leads necessary for cutting helical gears.  There is a refinement for achieving a more accurate tooth form for the helical gear.  Most treatises on cutting helical gears with Brown and Sharpe type milling cutters say that the selection of the tooth number for the cutter should be modified by dividing the tooth number required by the cube of the cosine of the helix angle - so for a 20 tooth 32DP 45 degree helical gear you would use a cutter meant for 57 teeth.  Elsewhere, Andrew (jadge) has gone through the math(s) to show why this works.  We model engine makers mostly use helical gears for cam shaft timing which is a pretty undemanding low power application so extreme accuracy of the tooth form is not required.

Cheers,

Rod

mcostello:
Brilliant!

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