C3 Mini-Lathe Metalworking Beginners
Making a telescope worm wheel and gear set on the mini-lathe. Part 11, Preparing the worm gear hob
Finally we have the threaded worm gear finished. You have a steel bar with two threaded sections, one for the hob and one for the actual worm. At the moment the gear hob is a normal thread just like the worm gear. We need to cut some teeth.
Using a dremel with a fine cutting disc you need to cut lengthwise spiral grooves in the hob section. You are trying to make the hob section into a tap. However, do not make the grooves too wide, otherwise the worm wheel blank will not be drawn around. For similar reasons, make sure you spiral the grooves around the worm gear hob.
I've put up a picture of a spiral cut tap - although this show the general idea - these flutes or too wide for our purpose.
It is also advisable to give about a third of the teeth some back rake. This is a very fiddly job with a grinding stone in the dremel - grind up to the cutting edge, but do not cut any further - make sure you leave the tooth intact.
Because we are grinding the flutes, we should not be kicking up a burr into the thread itself - however, check very carefully for burrs and remove with a small file before hardening the hob - once it is hardened you can only use a dremel cutting tool, and those are not easy in a confined space.
Next we have to harden the hob. The requires a blow torch - a hot one. They can be purchased from most hardware stores, but it needs to be hot enough to temper steel.
Wrap a wet cloth around the worm section to keep to cool, and heat the hob section until it glows cherry red, and then quench in oil or water (depending on the steel you have). Try not to heat up the worm section.
Now you need to temper the hob. Whilst it is proper to clean the oxide off the hob and then use your blow torch to re heat the bar until it is a light straw colour, this is a fiddly and difficult job. I take the whole bar and put it in my domestic oven set a maximum, which is about 230C, for an hour or so. After some time in the oven, the clean parts of the bar will all be a nice uniform straw colour - roughly the correct hardness for the worm hob. Leave the bar to cool naturally.
In the photo you can see the nice straw colour on the unhardened part of the worm.
Do not dodge the step of hardening the hob. Left unhardened the hobbing tool will become blunt and its outer diameter will be reduced before you get half way though cutting the gear in the worm wheel blank.
Once the bar has cooled, you need to remount it on the lathe with the steady and dead centre in the tailstock. Indicate on the turned section of the bar to ensure that the hob is turning exactly concentric to the lathe.
Make a final visual inspection of the hob to be sure that there are no rogue burrs and errors that you ought to be cleaning up with a fine file.
All images and articles copyright www.mini-lathe.org.uk
Hints and tips