|
At the turn of the 19th and 20th centuries Edwin Harrington, Son & Co. were based at Seventeenth and Callowhill Streets, Philadelphia, USA, in an area famous for its many machine-tool builders.
Their range of "Extension" or sliding-bed lathes were not unique, many makers produced them either as part of their regular line, or to special order; however, Harrington and Son - besides manufacturing Radial and Multiple Spindle Drills and Chain Hoists - appeared to specialise in them, and these pages illustrate ones produced by the company in the early 1900s.
All the lathes had a two-part bed, with the upper arranged to slide on the lower so that the distance between centres, and the width of the gap, could be increased or decreased as a particular job demanded. The idea was to combine several machines in one: with the gap closed the machine became a stout, backgeared, screwcutting engine lathe (or 'centre' lathe to British readers) with the capacity for regular work. When the bed was extended, long, thin or irregularly shaped work, perhaps with large diameters at inconveniently placed intervals, could be also be accommodated. This flexibility in sizing and set up allowed smaller engineering shops to take in work which should, nominally, have been beyond their capacity - or in the case of a larger enterprise, would have ensured that the biggest lathes were left free to be used for the heaviest jobs.
The machines were built in four sizes, named according to the swing above the upper and lower beds, and all were backgeared and screwcutting - the leadscrew being carried by the upper bed and connected to the changewheel drive by a a pair of gears, the lower of which was connected to a splined 'power shaft'.
The makers specified a countershaft speed of between 45 and 85 rpm, and this, combined with a "triple-reduction" backgear - with an enormous 54 : 1 ratio on the largest lathe - resulted in a bottom speed slow enough to allow successful machining on the outer edges of work up to 5 feet in diameter. In addition to backgear, and the four or five-step headstock cone pulleys, the two smaller lathes had a two-speed gearchange on the headstock and the larger ones three - giving, in the best cases, a selection of 45 different spindle speeds. The drive could be by either a traditional wall or ceiling-mounted flat-belt countershaft system (or overhead line shafting) or from a built-on variable-speed motor driving direct though exposed spur gears at the rear of the headstock. Quite how the latter system sounded when well worn, and being worked hard, is best left to the imagination.
The lower bed was of deep section and braced internally by ribs; the top bed was stiffened by cross ties at "frequent intervals" and slid in deep guides on the lower part. When the appropriate length of top bed had been wound out the upper and lower sections were locked together by T bolts and the outer, overhanging part of the upper bed supported on two loose screw jacks. On the smallest machine the top section was extended under the direct influence of a screw, with its operating handle positioned at the end of the bed; on all larger versions this screw drive was augmented by reduction bevel box to ease the load of moving what was a considerable lump of cast iron.
When the top bed was extended, and the gap opened, an extra carriage support, fitted as standard, came in to play; positioned under the front edge of the apron, it reached down to floor level where a machined 'way' was formed for it along the lower edge of the bottom bed.
The lathes all had reversing power sliding and surfacing from a separate power shaft, the leadscrew only being used when generating threads; the power to the feeds was taken through a clutch and gear drive with all the controls grouped exclusively on the apron. As an option, power feed could also be fitted to the top (tool) slide - and made to operate at any angle.
|
|
