Gear Cutting - Pressure angle.

Hello Michael, an interesting citation (though the link now seems broken). If I recall correctly from my ebook of the FHJ text, Shortt showed a method of cutting a profile that would give a raised-cosine force profile, by using a cutter of the same diameter as the gravity roller. For my version of a synchronome I modified the method to use a smaller cutter because the roller diameter is not one that matches standard cutter dimensions. It's basically a method of cutter radius compensation. But I'm not sure how it relates to this problem?

I have produced a method to generate g code to allow one to turn the concave teeth on a "button method" cutter, and it could be modified to turn true involutes though the cutter compensation is trickier.

I've been looking at 30 degree pa gears.

Here is a drawing of a rack form and hob to cut it. I've followed the usual formula for the 20 degree rack but it just doesn't look right at 30. Not much in the way of crest and the root is almost non existent.

Is there a reduction in addendum and dedendum for increased pa?

I've trawled through all the excellent links you have recommended but I can't find anything. Maybe it is buried amongst some of the impenetrable (for me) maths.

Cheers

Steve

In a word - no. Addendum and dedendum are determined by DP or Mod with an arbitrary addition to the dedendum for clearance. Of course at some point each tooth clashes with the adjacent tooth as PA increases.

Andrew

Thank you Andrew that makes sense

So is the 30 degree rack profile in the drawing correct? I realise that I can fettle the dedendum ratio.

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

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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

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