C3 Mini-Lathe Metalworking Beginners
Making a telescope worm wheel and gear set on the mini-lathe. Part 10, Threading the worm gear
Now for the good bit. Threading the worm gear - finally we get to cut some threads! It is important to know the correct depth for the threading tool cut which for my nominal 7 inch wheel is going to be 1.03mm. To measure this accurately, carefully adjust the tool so it is only just touching the surface of the part, and set the dial indicator to zero. Do not touch it again. The dial indicator will now show the depth of the cut, regardless of how many times you wind the threading tool in and out - and you do not have to rely on the divisions on the compound slide handle.
You need a lot of experience to cut a worm gear on a mini-lathe. Practice several times before you do the real one. It is critical to only advance the tool a small amount on each cut. No more than 0.003 inches. Keep cleaning off the chips and use large amounts of cutting oil. Do the operation in several visits to the lathe - keep the concentration high! Making a mistake in a worm gear threading operation is extremely easy.
This is not a threading tutorial so you will have to trust to your abilities, and not my detailed description! IF you don't know how to thread on a mini-lathe, go away and learn. Make something simple like pill box with a screw lid! This is serious threading. The only thing harder is a multi start thread!
As the work progresses, the part will heat up and the friction from the fixed steady and the dead center will increase. Although it is ok to tweak the fixed steady or the dead centre during the first half of the operation - as you get nearer the finished shape you have no option except to let the part cool before continuing. Adjustments will move the part slightly and cause problems.
Once you have cut the thread to the depth required, all that remains is to turn down the sections on which you will mount the bearings for the worm gear housing. Again, do frequent rests to let the part cool. Turn each section down to a about 0.1mm of the required size, and then finish both with tiny cuts, trying to get it is smooth as possible.
You might find my tips and turning operations handy when trying to reach the correct diameter for a bushing slip fit. I use a piece of scrap and turn it down so the bushing slips on nicely, and then use that to calibrate my micrometer for the real worm gear. Getting the outer diameter of the sections wrong is not going to make you happy. Using bearings for mounting a worm gear is unlikely to work because they are simply too big.
Before you declare yourself finished make double sure that the bearing sections are the right diameter, and the shoulders next to the worm are nicely smooth before you are finally allowed to remove the bar from the lathe and go and find the dremel and the cutting disks to prepare the hob section.
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