Reverse Engineering a Helical Worm Gear


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Posted by L. Smith on October 16, 2006 at 20:01:23:

I'm at my wit's end over trying to reverse engineer a 32 tooth 12DP straight-cut helical worm gear. We are trying to have it made by a second supplier and each time, the sample comes back with thinner teeth than what supplier # 1 has been furnishing.
Drawing Specifications are:

LEFT HAND, 13 DEG. LEAD ANGLE
(MESHEDS W/3 THREAD WORM)
14-1/2 DEG. PRESSURE ANGLE
32 TEETH - STRAIGHT CUT
2.583" +.000/-.030 PD
DP = 12
MOP = 2.775 - 2.780 OVER 2 .144" PINS
NORMAL TOOTH THICKNESS AT .185" DEPTH IS .171" (REF)

Supplier #1 is cutting these teeth with a total depth of around .188" & tooth thickness is around .171 at .185" depth. The space between the teeth appear to be almost completely involute shaped with root radius of approximately .047". Top Land is approx. 051” wide.
Supplier # 2 is cutting samples with a total depth of around .205" and thickness is .133" at .185" depth. The “lower half” of the space between the teeth appears"U-shaped" & upper portion appears to be involute. This suggests severe undercutting of the tooth profile. Also Top Land is at least .080” wide.
My dilemma is both gears have essentially the same MOP (2.778")– so adding a MOP requirement to my drawing still didn’t allow Supplier # 2 to accurately duplicate the gear with thicker/stronger tooth profile made by supplier #1.
BTW - this is for an optional gear-set that fits in a gearbox that normally has a 31 tooth 12DP helical worm gear meshing with a 5 thread worm – so the 2.583” PD requirement actually comes from the 31 tooth gear. Both 3-Thread & 5-Thread Worms have a 1.083” PD.
I seldom ever deal with gear specifications in my design work, so if someone who works with gear dimensions – especially someone who adjusts key dimensions for fixed center distances, etc. could tell me what else I need to specify on my drawing, I would be most grateful.




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