Involute Gear

This page is currently under construction and might undergo drastic changes within a short span of time.

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)

Things Needed

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 (this can be machined from carbon steel and this process will be documented in the hob article
3. An indexing head attachment for the milling machine [1]
4. A mandrel matching the inside diameter of the gear (where the shaft will be mounted when finished; this too can be machined on the lathe)
5. Appropriately sized round stock of a material no harder than the material of the hob
6. An engine or benchtop lathe and appropriate attachments (3-jaw chuck, 4-jaw chuck, dial gauge, simple cutting tool, chuck attachment for tailstock, center drill, drill bit, reamer)

Blank Preparation

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 width of the gear to be cut will be cut from the end of the workpiece (twice that plus swarf for two identical gears). This should be done while still on the lathe to maintain precision thickness.

Prepare to Mill

Now the gear blank should be mounted on the mandrel and the mandrel in a chuck. The blank on the mandrel in the chuck should be checked for concentricity using a dial-gauge mounted on the tool-post on the lathe, hand turning the spindle. Once properly centered on the mandrel and in the chuck, the entire chuck should be transferred to the milling machine and mounted in the rotary table or indexing head. At this time, make sure that the rotary table or indexing head is at a 90 degree angle to the spindle (so that the piece is parallel to the mill table)

In this article, I will focus on using the so-called 'parallel hob' mentioned in hob.

The hob should be mounted in a collet and then on the mill spindle. The quill should then be lowered so that one of the inner teeth of the hob is roughly centered on the horizontal diameter. Lock the quill feed. the table should now be raised or lowered in .001" increments until centered within .001". Lock the knee.

Formulae and Cutting Details

The final depth of cut into the blank should be equal to the tooth depth which is determined by the formula 2.157/Dp for Dp>20 and 2.2/Dp for Dp<=20, where Dp is the diametral pitch, or number of teeth per inch of pitch diameter. Diametral pitch is determined by the ratio of the number of teeth to the pitch diameter. These three factors also determine the dimensions of the hob you'll require.

Just Start Cutting already -or- Hurry up and wait

Now the head (again, rotary table or indexing head) should be locked in a known position, and proceed to cut .005" into the blank. Now rotate the work before cutting again by 1/N rotations or 360/N degrees where N is the number of teeth on the gear. Cut .005" from this location. Repeat this process of cutting and turning until the first cut is reached again. You'll continue this process .005" at a pass until reaching the depth of cut (which is the tooth depth) which is determined by the formula above (2.157/Dp).

Post-cutting cleanup

After finishing the gear form, it is advisable to return it to the lathe and file away any burrs left behind from the process. After this, the gear form should be complete, and all that is required for service is to cut a keyway and provide a shaft on which to mount the gear. If a "double-wide" gear was cut for a hydraulic motor, now would be the time to return to the lathe to part the gear into two halves. Following this, It would be advisable to either end-mill .001-.002" from the cut sides (if so, be sure to include additional thickness in the blank before cutting) or very carefully face the cut ends using the lathe.

External Links

http://en.wikipedia.org/wiki/Involute_gear Helpful Videos [2]