Gear Cutting – Pressure angle.

[Bill Davies 2Participant @billdavies2]

Steve, regarding the ratios 1.00 x module for addendum (in mm) and 1.25 x mod for dedendum are the correct ratios for metric gears.

The 3.142 pitch indicates it is a 1 mod gear/rack (conventional rounding: 5 and above round up, less than 5 round down). The pitch line (PCD in DP terminology) should have 1/2 the pitch in each tooth and space segment. This will determine the width of the tip of the rack. I haven't calculated the overall height to sharp corners (2.72), so you may be correct.

Hobs are not produced to a sharp angle, but obviously allow some clearance above the outside diameter of the gear.

Bill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 15/04/2020 09:48:50:

Hello Michael, an interesting citation (though the link now seems broken).

[…]

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I’ve just checked the link, John

… Works fine for me.

275 page download appears in seconds.

MichaelG.

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P.S. ___ forgot to add that my response was specific to Bazyle’s challenge.

Edited By Michael Gilligan on 15/04/2020 20:46:57

[Anonymous]

Posted by Steve Crow on 15/04/2020 17:32:37:

So is the 30 degree rack profile in the drawing correct?

It looks a bit odd, but I think it is correct. Of course higher pressure angles are rarely used for spur gears as they result in flattened teeth which have a significant radial component to the contact force. Generally one only finds high pressure angles in multistart worm drives. Using high pressure angles in this case avoids serious undercutting of the worm wheel by the hob.

Andrew

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