email: tony@lathes.co.uk
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JACOBS Gear Hobbing Machine


Conceived originally as a means of demonstrating gear hobbing - and not intended as a working machine, for it was insufficiently rigid - the design and execution were by Mr. Tom Jacobs. Tom's interest in gear manufacture began when, as an engineering apprentice during the 1920s, he came across very large, complex hobbers producing industrial-sized examples. However, it took some fifty years before he had the time to develop his ideas, first for a demonstration model and then for an economical, small-scale version, centred around a machine built-up from fabricated parts and intended for completion in a workshop as owned by a typical model engineer. Capable of producing spur, helical, bevel and worm gears, the prototype was featured in a series of articles in Model Engineer between January and August, 1976.
Some years later a set of iron castings was made available by the Helix Company, their use enabling a slightly more robust machine to be built that, while modified in detail but to the same basic design, was intended for the making of spur gears only. Castings might still be available from College Engineering Supply - with completed machines, using these parts, frequently displayed at model engineering exhibitions. The gears fitted to the machine shown below are from a Series 7 Myford lathe (20 DP with a 14 ˝ degree pressure angle) - though any similar set, for example from a Boxford or South Bend, could be used instead.
The heart of the Jacobs is a cast-iron baseplate that carries a compound slide rest topped by a vertical slide. Carried on the latter is a housing - able to be adjusted precisely on all three axes - holding a spindle to the end of which is fastened the workpiece. Drive is normally arranged from a remote, fractional h.p. motor with the initial transmission by V-belt to a primary shaft and thence by reduction gearing to the hob spindle. The workpiece rotation is indexed to the hob spindle via a further gear train, a cardan shaft and a worm and wheel (photos 2 & 3).  The gear reduction from workpiece to hob for spur gears must equal the number of teeth of the gear being made - assuming a single start thread on the hob. A powered feed on the cross slide is produced by gearing the cross-feed screw to the workpiece spindle (photo 3) with the depth of cut put on with the vertical slide. To allow gears with a tooth count equal to prime numbers to be cut, a useful addition is a differential gear built, very cleverly, into one of the gears in the index train (photos 4 & 5).
The use of a cardan shaft with two universal joints to transmit the (indexing) drive is a compromise. Since the input and output shafts are not exactly parallel the workpiece movement is not quite at constant velocity, but inclined at an angle equal to the helix angle of the hob. However. when cutting spur gears, since the helix angle of the hobs used is generally no greater than 30, this error is small and acceptable for most model engineering work. Note: the compromise outlined in the last paragraph only applies to the CES design (as shown on this page) when cutting for spur gears - for all other modifications of the type it is eliminated. For helical gears the bevel drive is used and for nearly all worm gears the axis of the wormwheel blank is set 900 to the hob axis.
With straightforward modifications the machine can also generate helical gears (photos 6 & 7) - though the index ratio must be subtly altered according to the helix angle, the feed rate and PCD of the gear - an additional gear train being used in the example shown. In addition, a bevel drive or some other means is necessary to keep the input and output sections of the cardan shaft parallel (photo 8). Worm wheels (photos 9 & 10) can be produced using a powered vertical (plunge) feed - and, by driving the longitudinal slide through bevel gears and a further cardan shaft, ACME taps can be pressed (economically) into use as a hobs (photos 11 & 12) - though again, the index ratio must be modified to correct for the feed parallel to the hob axis. As a note of interest, selected ACME taps have proportions that are close to ideal for making worm gears, the OD/pitch ratio being about 5 (generally, this ratio is much higher). However, as the teeth are not form relieved like a hob, but only relieved at the lead-in, the feed needs to be longitudinal - a process termed
tangential feed
Limited by the rigidity of the machine, the largest gear that can be produced is to about 20DP or Mod 1.25 with a diameter of up to 6". Gears at the limit of this range in steel need up to 5 passes - so the process is best described as "steady". If patience is limited because other jobs call, limit switches can be fitted to the cross slide (photo 13) allowing the machine to run unattended.
Some examples of gears produced on this machine are shown at the bottom of the page in photo 14 (the worms were made on a lathe)..

Photo. 1
The original, unmodified Jacobs gear hobbing machine

Photo. 2 
Original CES design for spur gears only. Spindle drive by compounded gears and carden shaft

Photo. 3

Original CES design. A view showing the carden shaft and worm-and-wheel reduction gearing. The arrangement is such that, for spur gears, the gear reduction from workpiece to hob must equal the number of teeth of the gear being made - assuming a single start thread on the hob.

Photo. 4

To allow gears with numbers of teeth equal to prime numbers to be cut, a useful addition is a differential gear built, very cleverly, into one of the gears in the index train




Photo. 5
Clever - a differential gear in the index train that allows gears with prime numbers of teeth to be cut.

Photo. 6

With straightforward modifications the machine can also make helical gears - though the index ratio must be subtly altered according to the helix angle, the feed rate and PCD of the gear - an additional gear train being used in the example shown. In addition, a bevel drive or some other means is necessary to keep the input and output sections of the carden shaft parallel as seen in Photo. 8

Photo. 7
Another view of a helical gear being cut

Photo 8

In addition, a bevel drive (or some other means) is necessary to keep the input and output sections of the cardan shaft parallel. As shown the universal joints are set incorrectly, 90 deg out of phase (the photographer was insufficiently attentive when setting it up……..)

Photo. 9
Worm wheels can be produced using a powered vertical (plunge) feed.

Photo. 10
Another view of the power down-feed arrangement

Photo. 11

By driving the longitudinal slide through bevel gears and a further carden shaft ACME taps can be pressed into use as a hobs

Photo. 12
Another view of the power-driven longitudinal slide

Photo. 13

If patience is limited because other jobs call, limit switches can be fitted to the cross slide allowing the machine to run unattended.


email: tony@lathes.co.uk
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JACOBS Gear Hobbing Machine