Screw Machining: Swiss, Escomatics, and Multi-Spindle
Screw Machining -- In 1873 Christopher Miner Spencer developed an automatic lathe to manufacture screws in one operation. Today it is used for rapid and accurate production of countless varieties of duplicate parts from almost any material. The operations that can be performed on automatic screw machines are single-point turning, form turning, facing, chamfering, boring, drilling, reaming, shaving, tapping, threading, counterboring, countersinking, knurling, milling, trepanning, roller burnishing, broaching and cutoff.
In 1883, two mechanics, Edwin Henn and Reinhold Hakewessell shared an interest in developing a multiple-spindle automatic lathe. Their concept employed four spindles through which bars of metal could be fed and made to revolve while cutting tools were advanced to positions by cams. Henn and Hakewessell built their pioneer four-spindle multiple in 1893 in a one-room house with salvaged parts. The first successful four-spindle, 1" capacity machine was named ACME and the company was organized to build it in 1896.
In 1928 the completely redesigned its line of automatics and introduced a six-spindle model, and an eight-spindle version. They were then able to offer a range of machine sizes that would accommodate up to 8" diameter bars and a choice of four, six or eight-spindle models.
The multiple-spindle bar machine can perform side and end operations on each spindle simultaneously. The first spindle position feeds the bars of material out by an amount equal to the workpiece length plus the width of the cutoff tool. The last spindle position is the cutoff station. Independently operated attachments may be added to allow the operator to broach, burnish, hob, knurl, mill, slot, stencil, and thread.
Swiss Screw Machining -- The sliding headstock automatic machine was invented and built at Bienne in 1872 by Mr. Schweizer. It was developed for the production of collets for watch balance springs. A year later he added an attachment to the machine for the production of watch screws. In German Schweizer means Swiss, thus, the sliding headstock automatic became known as the "Swiss-Type Automatic".
The Swiss Automatics feature a sliding headstock, guide bushings and cross tools. The headstock moves lengthwise on the dovetail slide of the solid machine body. It gives the bar its rotation, and performs an independent function: it feeds the bar past the tools during the operation cycle. Cams control the longitudinal movements with near mathematical precision, transferring accuracy to the turned part. The guide bushing is located just behind the cross tools and is in alignment with the headstock. It cancels the effect of the pressure created by removal of material from the bar and prevents it from bending. The bar is held in two places--by the collet located in the headstock spindle nose and by the guide bushing. The latter is adjustable and can be set to eliminate play. The bar is presented to the tools free from bending and chatter. Cross tools are cam controlled and move only in a straight line to and from the center of the bar. Cutting tools are arranged fanwise around the guide bushing. By combining the radial movements of the cross tools with the longitudinal movements of the headstock the contours of the most intricate components can be generated. The use of simple inexpensive and easy-to-grind single-point tools, working very close to the guide bushing, offers an advantage. It allows the highest possible precision in the reproduction of contours, from the simplest to the most complex. The combination of the sliding headstock with guide bushing and cross tools makes it possible to obtain consistently excellent surface finishes.
Many turned parts are best suited to Swiss Automatics, such as parts that can only be seen with magnification to very large parts 32 mm/1.250" diameters up to 200 mm/8" of turned length. The machine can handle a workpiece to 660 mm/26" in overall length.
Escomatic Wire Machining -- It is unlike other automatic screw machining in that the material remains stationary and the tools rotate around it. This allows machining material from coil stock and eliminates bar material loading time. Also, one operator can tend several machines.
The spindle, supported in the headstock in ball bearings to ensure concentricity and alignment, carries the rotating toolhead, which is equipped with three tools. The tools are actuated independently and are controlled by separate cams. They are radially and axially adjustable.
The stock is supported and passed through a stationary guide bushing situated in the rotating toolhead. At the end of the feed tube, between the nonrotating guide bushing and feed collet, a holding collet is used to clamp the workpiece, eliminating axial movement of the part during straightening of the stock. Forward feeding of the stock is provided by a feed slide combined with the feed tube and collet, all controlled by a simple plate cam. Coil stock is straightened by means of a rotating device, an integral part of the machine. Straightening of the material is achieved by a combination of rotating jaws moving axially.
Use of a special holding collet next to the feed collet for forward feeding of material, eliminates for opening, return movements and clamping is necessary. This operation is performed at the same time as the recess or cutoff operation. The counter collet is located directly opposite the rotating toolhead. It has an axial adjustment of up to 20mm/0.75". The counter collet is set as close as possible to the tools and always holds the workpiece firmly when forming or cutoff work is done. This capability enables higher feed rates and burr-free cutoff. Maximum capacity is 6.350 mm/1/4" diameter and 62 mm/2.44" in length.
Runs of 1000 pieces or more make automatic screw machining an attractive alternative. The range of materials usable on automatic screw machines is broad. All grades of steel, copper alloys, brass, aluminum, precious metals, hard rubber, machinable plastics, phenolics and wood can be processed. Rounds, squares, hexagons, round-cornered hexagons, tubing, rectangles, ellipses, cogged or lobed shapes can be accomplished on automatic screw machines. Cold-rolled stock is preferred, but hot-rolled materials can be turned under certain conditions.
Machines are available that can handle bars or tubes up to 8" diameter. Most multiple-operation machining is done on bars that range from 1/4" to 2" round. The standard bar length is random 12 ft. but shorter lengths pose no special problems. Tolerances of +/- 0.002" on length and +/- 0.001" in diameter are considered the norm in screw machining.
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