DIY Hobbed Drive Gear
Release status: experimental
| Description | A mounting jig to make stainless steel hobbed drive gears.
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Introduction
I was wondering if it was possible to make a stainless steel hobbed gear drive using a M3 turning tap. Some techniques described in RepRap are based on pressing taps mounted in drilling machines against bolts, using bearings mounted in a vise to let the bolt spin. This method works reasonably well for soft iron bolts, but when I tried to replicate the method in hard stainless steel rods, the only result I got were lots of broken taps.
In this page I describe a simple mounting jig that was used successfully to make MK8 stainless steel hobbed drive gears. Besides the jig, the technique requires a lathe and a milling machine to make precision machined gears.
Overview
The standard MK8 Drive Gear have an hob diameter of 7 mm. This is the diameter of MK8 Drive Gears listed in Drive-gear and manufactured by Aus3D, RepRap.me, TaoTac.com and Twelvepro.com. Others manufactures offer a diameter of 7.25 mm (Garage Days 3D and Alfa-Tech3D) or 7.1 mm (TriDPrinting.com).
Perhaps the last option has an actual hobbed diameter slightly greater than 7 mm to consider the bite of the teeth, but 7.25 mm is a diameter to large to be explained by the bite. To make things worse, the manufactures does not specify if this diameter is the major or minor diameter of the gear thread.
There is some uncertainty about the optimal number of cuts on the gear's circumference and due to the reason explained before, I preferred to calculate the diameter gear from scratch. For this, lets consider that a M3 tap has an major diameter of almost 3 mm and a pitch equal to 0.5 mm. The number of cuts on the gear internal circumference must be a multiple N of 0.5 mm. If the internal diameter of the gear’s thread is d, we have:
So, for a diameter d = 6.7 mm, we obtain an N equal to 42 cuts as shown in figure 1. The outer diameter of the gear’s thread will be 7.2 and considering that the hole required to tap an M3 thread is 2.5 mm, the total diameter of the gear will be 7.2 + 2.5 = 9.7 mm.
Figure 2 gives the gear dimensions that will be made in next sections.
