Fabricating Worm Gears Using the Taig Lathe and CNC Mill

Why?

If you are looking for a way to make worm gears that requires little or no work, don’t read any further. The primary reason I took on this project was the challenge. I, like many desktop home machinists, build tools so that we can build more tools. The fun is the process. The “nominal” reason I needed a worm gear was for a rotary stage that I am building for the Taig mill.

How

Several web sources (as well as a wonderful magazine article from the 1940s) describe how to make worm gears using a threading tap as a hob. Several sources claim great results; others claim frustration and likely failure. Sounds like a challenge to me!

The basic idea is to mount the gear blank on a standoff on the cross slide of the lathe so that it can rotate freely; then to cut the teeth using a threading tap held in the chuck of the lathe. The cross slide is adjusted to pull the blank against the rotating tap and voila, teeth are slowly cut in the blank. As the teeth are cut, the blank rotates, resulting in a self-paced rotation. A little math to calculate the diameter of the blank and that’s about it.

Gashing

On the techniques I had encountered in reading about gear hobbing is called “gashing”. As I understand it, it is the concept of rough cutting or “gashing” a set of slots around the blank (the same number of slots as the desired number of gear teeth). These gashes are basically crude teeth that engage the hob and get the blank turning in synchrony with the tooth pitch of the hob.

I came up with a possibly novel way of performing the gashing. Ideally, I would use a rotary stage with a dividing head to index the gear blank and cut the gashes. Problem was, I was making the gear for the rotary stage I needed!

The brainstorm was that, while I didn’t have a rotary table, I do have a CNC Taig Mill.

Boring 100 holes around gear blank to serve as “gashing”

Sort of Gashing Using the CNC Mill

What I did was to start with a blank that was a larger diameter than the gear I needed. I then programmed the mill to bore a series of 100 holes at a diameter slightly larger than the finished gear diameter (I was shooting for a 100 tooth worm gear with a pitch corresponding to a ½-13 tap). The diameter of the bored holes was .050”; I bored them to a depth of about ¼ the thickness of the blank. I then flipped the blank over and rotated the blank a few degrees relative to the original set of holes in a direction corresponding the inclination of the pitch of the thread. I then bored another 100 holes to a depth of about ¼ the thickness of the blank.

What am I trying to do? Given this set of holes coming into the blank from both sides, if I turned a trough into the blank slightly deeper than the holes and of diameter of the minor diameter of the tap, the holes would serve as a series of “side” teeth that would serve as gashes to synchronize the tap as it engaged the blank.

I am finding this particularly difficult to explain: hopefully, referring to the accompanying pictures will help.

Gear blank after boring gashes (Before turning center trough)

Milling center “trough” (Note how blank is mounted to cross feed).

Crank Power

I don’t have a variable speed motor on my lathe so I decided to build a hand crank that could also be driven by my battery powered electric drill. The hand crank approach had appeal it that I could go very slowly at first to make sure I was synchronized.

I build the hand crank using a unbored collet blank. I bored and threaded it and screwed it onto a ¼ inch diameter rod with a tightening nut on the pulley end that references the internal hollow spindle shaft. I added a simple crank handle. The handle nut serves also as the coupling to the electric drill. I figure the crank will also come in handy in the future for tapping, threading, etc.

Hand crank

Driving spindle with variable speed electric drill.

Blank set up, ready to cut with tap.

Early in the teeth cutting process.

It Actually Works (Who, me surprised?)

It all came together surprisingly well. The “gashing” worked as planned; the blank was fully synced to the tap from the moment I started turning the tap (I used a fast spiral tap to avoid possible “stuttering” with a conventional straight fluted tap). I am very pleased with the resulting gear (all 100 teeth look very uniform and cleanly cut).

Once the teeth were cut to depth, I took the gear off of the cross feed, chucked the gear into the lathe and removed (by turning) the original gashing holes.

When I finish building the rotary stage, I will lap in the worm and gear with some valve lapping compound to smooth out any minor rough spots (none are visually apparent).

Finished gear mounted on current state of rotary stage. (Note: gashing holes cleaned off by turning).

Close up (at least as close as I can get my camera to focus!)