C3 Mini-Lathe Metalworking Beginners
Making a telescope worm wheel and gear set on the mini-lathe. Part 9, Setup to thread the worm gear
Before we can actually start cutting the thread on the worm gear, you need to spend time carefully setting up the lathe for the operation. You are only going to get one chance at this, so I suggest a lot of practice runs before going to the effort of making a nice worm gear blank. We need to setup the mini-lathe for cutting the worm gear thread on the mini-lathe. To do this we need to make a threading tool.
Commercial threading tools are normally 60 degrees at the tip. We need to grind our own from HSS with a 40 degree tip. The tip of the tool itself needs to be a tiny flat about 0.3mm wide. Because we are going to be advancing the tool at an angle, most of the cutting will be done by the left hand edge of the cutting tool, so the back rake on the tool needs to be inclined slightly down from the left to the right.
The tool needs to be very strong - do not make a long slender tool, make it as short as possible. The limiting factor is usually measuring the angle and getting it close to 40 degrees - make it slightly more the 40 degrees rather than less.
Mount the tool in the toolpost, and then adjust the compound slide to an angle of just less than 20 degrees - about 18 degrees is good. The in and out movement of the tool during threading must be accomplished only be the action of the angled compound - do not touch the main cross slide adjustment after you start threading the worm gear. See this article about threading tool advance angles
Make sure that the threading tool can reach all of the worm and hob regions in one pass. This might entail moving the bar very slightly or adjusting the steady rest. This is the last chance for adjusting the part in the lathe - after we start threading, and until we have finished turning the bearing surfaces, it is vital that the part does not shift.
It is also important to accurately measure the depth of the cut. To accomplish this, we need to rig up a dial indicator bearing on the toolpost as shown in the photographs. This will be an accurate measure of the depth of the cut.
Setup the change gears and thread chase dial for threading the correct pitch, either 1.5mm for nominal 7 inch wheels, or 1.25mm for nominal 6 inch wheels. Checkout the worm gear calculations page for further details.
With the lathe setup for threading, make a couple of threading passes along the worm and hob section of the worm gear blank.
I normally do one cut which just scratches the surface of the worm gear, and then do another pass, making sure that the tool matches up properly with the first cut.
Once you are confident that everything is ready, we can do the fun bit and actually cut the thread!
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