GB2239202A

GB2239202A - Machining a blank to produce a toothed circular element

Info

Publication number: GB2239202A
Application number: GB9026939A
Authority: GB
Inventors: Terence North
Original assignee: David Brown Engineering Ltd
Current assignee: David Brown Engineering Ltd; Clyde Union DB Ltd
Priority date: 1989-12-22
Filing date: 1990-12-12
Publication date: 1991-06-26
Also published as: GB9026939D0; GB8928976D0

Abstract

A blank from small batches of toothed gears with fixed dimensions apart from their outside diameters is secured on the work-table of a hobbing machine, its periphery is milled to the required size (and shape in planes containing its axis) by a cutter 20 secured on the hob-spindle of the machine, and teeth are then cut in its periphery without removing it from the work-table by a hobbing cutter 22 secured on the hob-spindle. Separate milling and hobbing cutters 20 and 22 respectively are preferably secured contemporaneously on the hob-spindle and utilised successively. Alternatively, the milling operation is effected by the same hobbing cutter which subsequently cuts teeth in the blank. In either case, the relative rotational speeds of the work-table and the hob-spindle differ for milling and hobbing. Additionally if a shaft is integral with the blank, a keyway is cut by a further disc cutter on the hob-spindle. <IMAGE>

Description

"Producing a Toothed Circular Element" David Brawn Enqineering Lamited Agent's Reference 1756 This invention relates to a method of producing a toothed element of circular cross-section in planes perpendicular to its axis, that is to say a toothed element with rotational symmetry or a toothed solid of revolution. Such an element may for example comprise a cylindrical gear (including a wormwheel) for a speed reduction gearbox, a cylindrical gear with crowned tooth flanks, a gear having a curved periphery in planes including its axis for a toothed-gear-type coupling, a tapered gear, or a sprocket, and may be formed with a bore or integrally on a shaft.

Hitherto, such elements have been produced by hobbing teeth in blanks pre-itiadiinsd to final dimensions on all their surfaces including the periphery to be toothed. In the case of elements having relatively small dimensions, typically around the size of automotive or small industrial power transmission gears, for production in small numbers as needed in response to orders, individual batches of blanks of every size required have been made and held in stock prior to hobbing, sometimes for long periods depending on fluctuations in market demand. This is uneconcanical, and results in long procurement times whenever a need cannot be met from stock.Where the market demand is for immediate availability with highly unpredictable requirement levels, this stocking policy has hitherto been an expensive necessity.

The object of the present invention is to produce toothed circular elements quickly and economically.

According to the invention, a method of producing a toothed element of circular cross-section in planes perpendicular to its axis comprises the steps of securing a blank on the work-table of a hoboing machine; milling its periphery to the required size, and the required shape in planes including its axis, by means of a cutter secured on the hob-spindle of the machine; and then cutting teeth in the periphery of the blank without removing it fran the work-table by means of a hobbing cutter secured on the hob-spindle of the machine.

A preferred embodiment of the invention will now be described, by way of example, with reference to the acccmpanying drawings of which: Fig. 1 is a diagrammatic side elevation of a gearbox of a type to which the invention is applicable; Fig. 2 is a diagrammatic side elevation of an input pinion blank for said gearbox, about to be milled to the required size and cylindrical shape of periphery on a hobbing machine; Fig. 3 is a diagrammatic side elevation of said blank then about to be hobbed to form teeth in its periphery without removing it fran the hobbing machine; and Fig 4 is a diagrammatic side elevation of a countershaft for said gearbox, about to be milled to form a keyway in it whilst secured on a hcbbing machine for the milling and hooting of the periphery of a pinion formed integrally on it.

Referring now to Fig. 1, a fixed-ratio double-reduction gearbox 10 which is close-cospled to an electric motor 11 has an input shaft 12, an output shaft 13 coaxial with the shaft 12, and a countershaft 14 parallel to the shafts 12 and 13, and therefore has two pairs of constantly meshing cylindrical gears which have involute teeth. The input pinion 15, the intermediate wheel 16 meshing with it, and the output wheel 17 are all formed with bores for mounting on their respective shafts, but the intermediate pinion 18 meshing with the output wheel 17 is formed integrally on the countershaft 14.So that a range of speed reduction ratios can be obtained from the gearbox 10, the numbers of teeth on the gears 15, 16, 17 and 18, and hence their diameters, are variable. Therefore to caver a range of, say, 60 ratios a large number of blanks has hitherto been required in stock, even though some of the available ratios have seldom been ordered.

Taking as an example the input pinion 15, this can be designed with a bore and face-width of fixed dimensions, but a variable outside diameter. In accordance with the invention, there are made for stock a relatively small number of blanks 15B all having the largest required outside diameter. If then a single gearbox 10 having a reduction ratio requiring a smaller diameter input pinion 15 is ordered, such pinion is produced by securing a blank 15B on the work-table of a hobbing machine with its axis co-inciding with the axis 19 of said table, milling its periphery to the required size and cylindrical shape by means of a milling cutter 20 secured on the hobspindle of the machine with its axis co-inciding with the axis 21 of said spindle and then, without removing it from the work-table, cutting teeth in its periphery by means of a hobbing cutter 22 also secured co-axially on the same hob-spindie of the machine and spaced axially fram the milling cutter 20 by a collar 23 of suitable width. The milling cutter 20 is rotated with a suitable timed relationship between the hob-spiddle and the work-table to reduce the periphery of the blank 15B to the required size as shown in Fig. 2, and the hobbing cutter 22 is then brought into position relative to the blank 15B and rotated in conventional fashion with the appropriate timed relationship between the hob-spindle and the work-table to cut the requisite number of teeth in the periphery of the blank 15B as shown in Fig. 3. Both the milling cutter 20 and the hobbing cutter 22 are traversed in a direction parallel to the axis 19 of the work-table as indicated by the yarrow 24, or in a slightly inclined direction in a plane parallel to said axis to shift wear along the cutter in known manner.

Each of the three other gears 16, 17 and 18 employed in the gearbox 10 can likewise be produced d from a relatively small stock of blanks of the largest required outside diameter, the fact that the intermediate pinion 18 is formed integrally on the countershaft 14 presenting no problem.

Moreover, as shown in Fig. 4, one or more keyways of Woodruff type or of so-called "sled-runner" type can be milled in the countershaft 14, without reving it from the work-table, by means of a disc cutter 25 contemporaneously secured co-axially on the hobspindle together with the milling cutter 20 and the hobbing cutter 22, spaced axially from the nearer of said cutters by a collar 26 of suitable width, and utilised individually with the axis 21 set at right angles to the axis 19 in a plane parallel thereto. The e hob-spindle is rotated but the work-table is stationary. A keyway of Woodruff type is formed by simply using a disc cutter 25 of appropriate diameter and suitably closing the distance between the axes 19 and 21.A keyway of "sled-runner" type is formed by suitably closing the distance between the axes 19 and 21, traversing the disc cutter 25 in a direction parallel to the axis 19 of the work-table as indicated by the arrow 27, and then increasing said distance.

In a modification which is equally practicable but non-preferred because it reduces the hob life, the milling step is performed by the same hobbing cutter which subsequently cuts the teeth in the blank. The milling step is performed using a timed rotational relationship between the hob-spindle and the work-table which is theoretically suitable for cutting a large number of teeth in a blank of larger diameter, said number of teeth being sufficiently large to ensure that the cros5section of the actual blank in planes perpendicular to its axis is milled substantially circular. The hobbing cutter is then repositioned relative to the blank and is rotated in conventional fashion with the appropriate timed relationship between the hob-spindle and the work-table to cut the requisite number of teeth in the periphery of the blank. One or more keyways can be milled in the counteshaft as hereinbefore described.

Wormwheels, crowned cylindrical gears, gears having curved peripheries, tapered gears, sprockets, and like toothed circular elements can equally well be produced in accordance with the invention, with appreciable savings in procurement times and stock levels.

Claims

Claims:-

1. A method of producing a toothed element of circular cross-section in planes perpendicular to its axis comprising the steps of securing a blank on the work-table of a hobbing machine; milling its periphery to the requiered size, and the respired shape in planes including its axis, by means of a cutter secured on the hob--pindle of the machine; and then cutting teeth in the periphery of the blank without removing it from the work-table by means of a hobbing cutter secured on the hok-spindle of the machine.

2. A method according to claim 1, wherein a milling cutter and a hobbing cutter are contemporaneously secured on the hob--pindle and are utilised successively.

3. A method according to claim 1, wherein the milling step is performed by the same hobbing cutter which subsequently cuts the teeth in the blank, the milling step being performed using a timed rotational relationship between the hob-spindle and the work-table which is theoretically suitable for cutting a large number of teeth in a blank of larger diameter, said number of teeth being sufficiently large to ensure that the crcss-section of the actual blank in planes perpendicular to its axis is milled substantially circular.

4. A method according to claim 2, wherein the blank is formed integrally on a shaft, comprising the further step of forming a keyway in the shaft without removing it from the work-table by means of a disc cutter contemporanecusly secured on the hob-spiddle with the milling cutter and the hobbing cutter.

5. A method according to claim 3, wherein the blank is formed integrally on a shaft, comprising the further step of forming a keyway in the shaft without removing it from the work-table by means of a disc cutter contemporanecusly secured on the hcb-spindle with the hobbing cutter.

6. A method of producing a toothed element of circular cross-section in planes perpendicular to its axis substantially as hereinbefore described with reference to, and as illustrated by, Figs. 2 and 3 of the accxarrpanying drawings.