Gear Cutting

Project Designing the Involute Gear Profiling Machine

Designing an Automatic Involute Gear Profiling Machine Part 3 Initial Design Considerations One of the major considerations when designing a miniature version of any metal cutting machine is the inevitable problem a lack of body-mass to cope with the inevitable severe cutting forces. There is no real substitute for mass in the basic machine structure to effectively absorb the effect of cutting forces and vibrations. Unfortunately, as machines are miniaturized the body mass drops off rapidly and so it is very difficult to compensate for these things and still retain appropriate proportions and visual appeal. The problem is particularly exacerbated in the case of the model Sunderland Gear Cutter machine as this uses a planing cutting action which sometimes involves large and interrupted cutting forces. In the case of the Involute Gear Profiling Machine all of these problems exist and are by definition difficult to avoid in any design. It generally comes down to having to make some sort of compromise for the inadequacies of body mass in the miniature version, however, this is does not necessarily detract from the success of the intended machine so long as it operates within its design parameters. There are several areas needing special consideration in the miniature machine when compared with a full size version. " Size " Set Up " Flexibility " Accuracy SIZE - the intention is to design and make a small working machine that is functional and useful. In the case of the Involute Gear Profiling Machine one that is not too big or obtrusive in the home workshop. Ideally, this machine will easily fit on a work bench-top and require minimum power to operate. SET UP - whether miniature or full size there is a preliminary requirement to set up the machine for cutting/profiling gear teeth. This normally comprises of mounting a suitable cutter and gear-blank on the machine. It is then necessary to initially machine or cut several 'starter' teeth in the gear blank tot the required depth. Thereafter, the machine can automatically shape all the gear teeth to generate the correct involute profile. The initial set up process to cut several 'starter' teeth is by far the most punishing process since the rack-cutter has to cut several teeth all at the same time to a chosen depth which involves removing a lot of material. Unfortunately, using a rack-cutter, the rack teeth shape have long cutting sides and produces a 'wedging' cut which presents considerable demands on the machine. In the case of gears with large teeth sizes it is thus necessary to introduce a compromise approach and pre-cut all the gear teeth with a simple straight sided fly-cutter using the milling machine and some sort of dividing arrangement. These teeth will, of course, still require further machining to produce the correct involute profile provided by the Gear Cutting machine. FLEXIBILITY - on the full size Sunderland Gear Cutting Machine there is flexibility to cut both spur and single or double helical gears and probably many other variations, such as bevel gears. To provide this sort of flexibility in the miniature version the machine would become over complex and require a lot more space to accommodate the extra mechanisms. For this reason, the miniature version is be restricted to cutting spur gears only, however, these are the most common gears used in the home work shop. ACCURACY - the Sunderland process by definition generates gear teeth with the involute profile. As long as the miniature version minimises any 'lost motion' or backlash within the mechanisms and makes small incremental cuts the resultant gears will be accurate. KEY UNIT MECHANISMS There are several key unit mechanisms used in this machine and in order that the builder is clear as to their relevance these are described in some detail. Quick Operational Overview The purpose of this model machine is to generate an involute profile applied to teeth on a gear blank held on an arbor. The mechanisms required to achieve this are contained within or attached to a box frame and taken together work in accordance to the basic process described for the Sunderland Gear Planing machine. An electric motor supplies rotational power via a drive unit which considerably reduces the rotational speed to operate a drive-shaft, crank and connecting rod. The connecting rod operates a horizontal slide called the rack-cutter body in which is fitted the rack-cutter and this cuts or planes metal in a gear blank. The rack-cutter body is part of a composite unit which can also move vertically to provide 'lift'. Each time the rack-cutter body completes its forward cutting stroke it then, on the reverse stroke, operates a push-rod which in turn operates a ratchet wheel fixed to a shaft. This shaft, the cam shaft, also accommodates two main cams, one to 'lift' the rack-cutter unit vertically and the other to disengage the arbor and its support infrastructure from the rack-cutter path. The arbor has fitted to it the gear blank and also a 'copy' gear, the latter having a copy-rack which is also fitted to the rack-cutter unit. This means that when the appropriate cam disengages the arbor from the path of the rack-cutter it also disengages the copy-rack from the copy-gear. The purpose of the copy-rack and copy gear is to provide rotation to the arbor and crucially, it does this relative to the amount of lift of the rack-cutter unit. The ratchet wheel used to rotate the cam-shaft has 50 teeth and so each stroke of the rack-cutter causes the cam-shaft to rotate a single increment and after a total of 50 increments one complete turn or processing cycle of the cam-shaft has occurred. During this cycle the action of the rack-cutter will remove material from the gear blank and together with the rotation of the arbor generate an involute profile to one or more teeth. To ensure all the individual operations happen at the right time the cams are shaped appropriately as specified in a timing diagram.