When the need for better gears than could be made by casting became apparent, both the milling machine and the planer or shaper were utilized in machining the teeth. The accuracy of the teeth depended upon the proper shaping of the teeth of the cutting tool, and the spacing on the accuracy of the indexing mechanism used. Both of these conditions still affect the accuracies of gears, more in some types of machines than in others, but with improved methods and materials, extremely accurate work is now produced by these methods. The designing of machines that could generate the teeth shape of the desired curve was first accomplished on the planer type of machine and later by the hobbing or continuous milling process. Both of these have grown to be standard methods and several types of machines are now in daily use. Machines of this kind are completely auto matic after the gear blank is put in place. Two tools are used, one cutting on each side of the tooth. Both the cutting, feeding and indexing are automatic, the machine stopping when the gear is completed and ringing a bell to call the operator. Here the shape of the tooth is governed by a hardened steel form that guides the tools as they feed into the work and shape the tooth.
An entirely different type of planing gear machine is known as the gear shaper. This uses a cutter in the form of a gear. This cutter is very carefully made, the shape being finally formed of ter hardening, by a fine grinding wheel that generates the shape of the tooth. In this machine one of the gears is the cutter and both cutter and gear block roll slightly between each stroke of the ram.
Another type of gear cutting machine employs a similar prin ciple. This machine in which the cutter bars are horizontal in stead of vertical has two cutters at work, one on each side of the gear blank. By this method it is possible to cut the double helix or herring-bone gears, with no clearance space in the centre. Both the cutter bars and the gear blank move in unison through the proper number of degrees to give the desired angle to the helix of the teeth in the gears. The cutter bar moves from side to side, planing first one side and then the other, and leaving a per fect joint or mating point, at the centre. The proper twist is given the cutter bar by a cam on the cutter bar. Gear teeth with a single helix or straight across the gear blank, commonly known as spur gears, can be easily cut on this machine.
Still another method of cutting gears by the planer-generating process is known as the Maag system. Instead of using a cutter that is practically a hardened gear, this system uses a hardened rack which moves past the gear being cut at the same rate the gear blank is revolved. Many gears are still cut by the milling process, which includes both the formed milling cutter that is fed straight across the face of the gear (and is also used for heli cal gears to a limited extent), and also the hobbing, or continuous generating process. Here the milling cutter is replaced by what
is called a "hob" or continuous milling cutter in the shape of a screw. By setting the cutter at the angle of the thread, and re volving the gear blank at the proper speed, the hob can be fed across the face of the gear blank and generate teeth in accordance with the form of the teeth of the hob, which conform closely to that of a rack.
The development of centreless grinding machines and of the use of wide grinding wheels, together with the growth of the ring wheel type and automatic sizing grinders, are perhaps most noticeable improvements in the field of grinding machines since 1918. The whole machine field has been affected by the growth of grinding. Much flat work that was originally done on the planer was first taken over by the milling machine and later by the grinding machine. Such work as flat irons, mating sur faces of motor blocks, transmission cases, and the like, are usually surfaced on grinding machines of disk type, or with ring wheels. The disk machines generally use disks made of heavy paper and coated with abrasive. These formerly had plain surfaces but are now frequently embossed to give a cushioned effect and to pre seri, a series of small grinding surfaces instead of a large solid surface. The depressions serve to hold and carry away the metal particles removed by the abrasive. Various types of fixtures are used to hold the work frequently passing it between two disks and so finishing two sides at once. These disks are on both vertical and horizontal spindles, the latter utilizing the weight of the work to aid in securing the necessary pressure against the abrasive. Grinders using the ring type of wheel have been so developed that they rival the milling-machine and planer as to the amount of metal removed per hour. While much of the work done in this way is a roughing operation, some types of machines are also used for final finishing operations. This type of machine, using magnetic or other chucks, frequently holds many pieces of work at once. In some cases the work is placed in holders as the table revolves, the magnets becoming active before the work reaches the grinding wheel. After passing out from under the wheel, the magnets become inactive and the work can be removed by hand or is forced out of the chuck by what is known as a plough. The magnetic chuck has aided in the development of plane surface grinding. It provides a method of holding work that is either too thin to be held by clamping, or of such shape as readily to be sprung out of shape either by clamps or by the wheel pressure.