Involute Gear
This page is very much a work in progress-improvements are forthcoming.
There are several methods to cut an involute gear, but I will focus on hobbing techniques, as a simple hob can be made with the most basic lathe. (see Hob)
The tools necessary for this process include:
1. A vertical milling machine (horizontal will probably work as well, but I will focus on vertical here)
2. The appropriate gear-cutting Hob for the dimensions of the gear desired
3. An indexing head attachment for the milling machine [1]
4. A tailstock attachment for the same
5. A live center to mount in the tailstock and a dead center to mount in the indexing head
6. Appropriately sized round stock of a material no harder than the material of the hob
7. An engine or benchtop lathe and appropriate attachments
First, the gear blank needs to be cut. In the case of making a set of identical gears for a hydraulic gear pump, a single double-wide gear can be made and cut in half when finished. If the thickness of the gear is very important, then the swarf of the blade that will be used to cut the gear in half must be considered when cutting the length of stock needed.
Before mounting in the lathe, a length of roundstock should be cut. This step does not require precision, so a hacksaw will do in a pinch. (A chop saw is preferable) The piece should then be mounted in a 3-jaw chuck on the lathe if possible (that is, if the piece will fit in the 3-jaw chuck) A four jaw chuck may be used, but extra care must be taken to ensure that the part is centered. now the end of the piece should be machined to a smooth finish and center-drilled to roughly 3/16" after removing the piece from the chuck, turn it around and repeat the process used on the first end. (If the stock is short enough, it can be cut down to the outer diameter of the gear desired while still mounted in the chuck the first time, but it is advisable to center-drill the piece to mark the center of the hole where the shaft will be mounted.)
When ready to trim down to diameter, ensure that the lathe RPM and feed rate are set properly for the diameter you are cutting from (before the cut) and the material being used. The rough formula for RPM is (CS x 4)/D where CS is the cutting speed which is specified for the work material and D is the diameter of the workpiece. The feed rate used will depend on the material being used, shop practice, and the desired finish of the cut. Typical lathe feedrates used for carbon steel are .002-.005"/revolution.
Now the hole by which the gear will be mounted should be machined. This step will involve first drilling then reaming the hole to produce a smooth, precision finish on the inside of the gear. First, the appropriate drill bit for the reamer to be used will be mounted in the chuck on the tailstock. Now the hole will be drilled past the width of the gear, leaving deadspace so that the reamer can ream the entire width of the gear. Next, the drill bit will be replaced with the reamer, and the hole will be reamed to size.
Now the chuck will be transferred to the milling machine and mounted in the rotary table or indexing head.