Worm gear

[GordonL]

I am trying to make a worm and worm gear on my lathe using a hob. I can determine the dimensions such as OD, PD etc from the W.M. Berg site or several other sites but I am still confused as to how I determine how deep to cut the "threads" on the hob or rather how to determine when I am deep enough. Basically depth of tooth is addendum plus dedendum. The compound is set at 19° for a 20° pressure angle so other than calculating using trig which seems rather imprecise how do I know when I am deep enough?

I am making a 4" turn table and using 1/10 DP with 72 tooth worm for rotating the turn table. Theoretically the depth should be .072 on the worm and .069 on the worm gear.

In looking  at some other videos etc I see some folks start out with a concave surface on the worm gear. Is this necessary or it is just saving time on the cutting? Everything I look at seems to show the finished product but not how we got there.

[Lew_Merrick_PE]

Gordon,

A Worm/Worm-Gear set is, essentially, a Trapezoid Thread screw driving (or, occasionally, being driven by) a Spiral Spur Gear.  In a Spur Gear the OD of the gear is the Pitch Diameter + 2/Diametral Pitch of the gear.    Thus, the driven area of a Spur Gear is 4/Diametral Pitch.  The Root Diameter of said Spur Gear has an additional clearance of .3125/[Diametral Pitch[/i] below that (Root Diameter = Pitch Diameter - 2.3125/Diametral Pitch).  The Pressure Angle of the Involute Tooth you are using will be half the included angle of your mating Trapezoid Thread.

The Pitch Diameter of the Trapezoid Thread should be located 2/Diametral Pitch below the Major Diameter of the Worm.  The Minor Diameter of the Worm should (in classical definition) be located 2.25/Diametral Pitch below the Pitch Diameter giving you a thread depth of 4.25/Diametral Pitch for your Trapezoidal Thread.  [You really want to use wires to measure the Pitch Diameter and Pitch of your Trapezoidal Thread.]

The Tooth Width on your Spur Gear is Pitch Diameter*sin(180/Number of Teeth (+0/-Clearance) on the Pitch Diameter.  This is also the Thread Width and Thread Spacing (though the Spacing tolerance will be +Clearance/-0) to define the per Revolution Pitch from which the Spiral Angle for the mating Spur Gear may be calculated.

Does this help?

[Lew_Merrick_PE]

Gordon,

The reason to make the Worm Gear mate more closely to the Worm is to increase the contact area (also called enveloping) to increase the overall strength of the system.

[GordonL]

Thanks Lew. I have been through all of the basic formulas and theory and I mostly understand that. I am trying to figure out how to translate tooth depth to actual feed on a non CNC lathe. I will have to look more closely at measuring it with wires. At this point I have made the worm and the spiral hob for cutting the the gear by cutting a shoulder on each end of the actual thread (gear teeth) at the bottom depth and feeding in until the tool touches that shoulder. So far it looks good but I have not yet cut the gear or tried to mate the gear and the worm. In this application wear is not a factor. On this mini turn table if the gear makes 500 complete revolutions in my life time it would be a lot.

Lew. I have not heard too much from you since the old Generic Cadd days. Did you ever get your CAD program completed?

[Lew_Merrick_PE]

Gordon,

If your OD's are dead nuts on, then the DoC will be 4.3125/Diametral Pitch for both the Worm Gear and the Worm (though the Worm is often cut to "only" 4.25/Diametral Pitch).