How to Buy a Automatic Screw Machines

The automatic screw machine is a fully-automated, high speed lathe. Automatic screw machines are built to produce turned parts like screws, nuts, washers, pins, and collars from round bar stock. The automatic screw machine was patented by Brown & Sharpe in 1865 and Christopher Spencer in 1873. It derived from the turret lathe because there was a need for more production efficiency of turned parts. The cam principle converted the lathe into a machine able to make the same piece configuration repetitively and automatically. Initially the automatic machine had only a single spindle. Multiple spindle automatics came into existence in the late 1800's and weren't used widely until World War I (WWI) when more productivity, more space, and better use of manpower were needed.

Today screw machines are manufactured with multi-spindles and attachments to perform operations like screw-slotting, index drilling, burring, threading, and counter-boring. Single or multiple spindle automatics utilize hydraulic, electrical, or computer numerical controls (CNC). Today automatics are capable of:

1. Loading the workpiece.
2. Starting the machine and coolant.
3. Initiate and complete the changing of speeds and feeds and tools.
4. Inspect or gauge the part.
5. Unload the finished part.
6. Begin the operation again.

The tools in an automatic screw machine are positioned on cross slides within the radius of the spindle axis and on a turret slide feeding longitudinal to the axis of the rotating work. Automatic screw machines find consecutive tools in the proper working positions to automatically change feeds and speeds and load new stock to tools for a duplicate operation.

1. The spindle grips the bar stock by a spring collet and rotates the stock. The roller chains drive the two sprockets on the spindle from drive shafts in the base. One of the sprockets drives the spindle by a friction clutch between the sprockets.
2. The turret is automatically indexed to bring each tool into position. It is driven by cams for the proper feed to each tool.
3. The cross slides are tool carrying slides at right angles to the spindle. They are driven by cams and operate through levers to give each tool the proper feed.
4. The main drive shaft provides power to the machine bed's lead work shaft through feed change gears. The feed change gears control the two front cam shafts with separate worms and adjustable worm wheels.
5. The machine motor is located in the base of the machine. The machine's horsepower will depend on the machine's size, speed, and stock to be machined. It is the central power source for all operations.
6. The mechancial safety clutch will disengage to stop the main drive shaft in case of overloading or jamming. It protects the operating mechanisms of the machine.
7. The cams control automatic movements, duration, and depth of all cuts. Cams move the slides and turret to feed cutting tools to the work. The cam principle is important to the screw machine's design for the cams control the automatic motion at selected speeds.

During setup, the tools are linked to each other and the feed regulating cams are adjusted to give the tools proper feeds. Before production, the number of spindle revolutions for each operation must be calculated. The operations and idle movements can take place at the same time if they're overlapped and the rest of the spindle revolutions are corresponding to the surface of the lead cam so the total of these spindle revolutions equal the full circumference of the cam. Spaces used for turret operations and lobes are produced to feed the tools to the work. The radial height or throw of the lobes will equal the length of a tool traveling to the work.

The rate of feed to the tool is controlled by the amount of covered cam surfaces. The lobes are connected by drops or rises during idle movements. The lead cam operates the turret tools. The cross slide cams rotate at the same rate as the lead cam and the work by the cross slide tools are placed out on these cams. An individual set of cams have to be produced for each job. Once the machine is cammed, parts can be made in two to 10 second increments at a rate of 360 to 1800 parts per hour.

Single spindle automatics have two types of cutting methods. The Swiss type automatics move perpendicularly to the work rotation's axis with the work being fed through the cutting cycle. They are called Swiss type because they were originally manufactured in Switzerland. Swiss type automatics are used primarily for precision and close tolerance work because the cutting action is close to the spindle eliminating any wobble. Swiss type machines are used for turning and forming and operate with five single point tools which are carried by toolholders mounted on a frame at the left hand of the headstock. The two lower tools are called rocker arm tools and are held on a rocker arm that swings them into their cutting positions. The three upper tools are known as the overhead tools and are held on independent dovetail slides which move them into cutting positions. The rocker arm tools produce accurate turning because of its rigid support. Moving the headstock in the hardened steelways will create a longitudinal feeding motion. The overhead tools will produce a traverse feeding motion.

The work rotates in a fixed position in the second variation of single spindle cutting action unlike the Swiss type automatic where the work is fed. The desired outline is produced by the cutting tools movement along or normal to the axis of spindle rotation with end cuts being executed by the turret tools. This is the standard operating procedure for a single spindle automatic screw machine. The material being generated on this type of machine is usually heavier than on the Swiss type automatic. A standard machine can handle product possibilities that are limitless with all its available attachments.

The multiple spindle automatics were introduced in the 1880's and 1890's but didn't come into prominence until World War I (WWI). The war dictated increased productivity needs, greater use of manpower, and more effective layout of floor space. The multiple spindle bar fed machine represents an expeditious way to produce parts from bar stock. The multi-spindle machine can machine 4, 5, 6, 8, or even 12 bars at the same time. With the stacked type of multiple spindle automatics, four bars can be fed through four chucks and machined in duplicate by tools slides operating at right angles to the axis of spindle rotation with additional tools positioned on a slide moving in from the end. The stacked type of multiple spindle machine can produce four complete pieces during each machining cycle. It's like four machines in one.

The indexing type multiple spindle automatic's cutting action is progressive on the bar stock. There are no identical cuts. The spindles are positioned in a cylindrical carrier. End working tools are mounted on a slide opposite each spindle position. Cross feed tools are on cross slides for each spindle position. The tools are fed into the work and retracted at the conclusion of the cut. When all tools slides are pulled back, the complete spindle carrier is rotated to bring each spindle and their revolving work pieces in front of the next set of tools. After the first rotation of the carrier cylinder the work piece is finished and cut off its bar. With every indexing cycle afterward another piece comes off. The order of operation creates a multi stage work piece at each indexing of the spindle bearer. The multiple spindle automatic will function unattended until all the loaded bars are processed.

The multiple spindle chucking machine is like the multiple spindle automatic except it is semi-automatic in production. The multiple spindle chucking machine is used for castings or forgings instead of bar stock. When the machine indexes, a finished part is released.

What are your production needs? You must decide on what machine will turn out the finished work piece at the lowest cost per piece. If production speed and the estimated duration of the run is important then a six or eight spindle automatic would be a good choice. If you process bars then a bar type automatic would be preferable. A chucking machine is to be used for castings and forgings. A stacked type multiple is used for rapid production of parts. An indexing type of multiple machine is equipped better for the fast production of a multi stage work piece. For close tolerance work, a Swiss type automatic should be used.

Horsepower? The greater the spindle speed the more horsepower needed. Of course an important consideration is the type of material to be machined. For example, steel requires more power at a given spindle speed than aluminum. Depending on your range of work in house, usually a machine with maximum flexibility will cover your different production requirements. If floor space is a consideration, the standard and special attachments are available for screw machines. An attachment or attachments can change a screw machine into a fully automatic multi phase machine. This will give you an opportunity to economize floor space as well as the reduction in the cost per unit of output.

Non-power examination
Inspect the bed and the sides of the machine. The structural support of the machine is important to the inside workings of the machine. Look for cracks or breaks. Examine the tops of visible nuts and bolts and check to see if the bolt and nut tops are still geometric and have not been rounded. A rounded nut or bolt is an indication of how well the machine has been treated. Check to see if the turret and cross slides are pitted and grooved which would indicate hard wear. Inspect the turret slide closely since it is always operating while clean cross slides may not be a true indicator since the cross slide tools may not be used all the time. It depends on the type of work being done. The turret slide is always in use.

Inspect inside the carrier gear box at the spindle gear and see if the gears are pitted or cracked. Turn the spindle by hand to check if there is excess play in the gearing. Do this for each spindle and compare on a multiple spindle machine. Examine the spindle gears to find out if they're worn or if the movement is inconsistent. Inspect the stock positioning mechanism within the spindle and see what condition the lock and pin blocks are in. Make sure they're not bent over since the bar stock must be firmly held to prevent any lurch or sway when the stock is spun. Also take a narrow bladed tool and put it into one of the drum screw holes and move the drum back radially to check for excessive play in the keys and taper pin. There should be very little looseness between the drum and drum shaft.

Examine the worm wheel for wear. The worm wheel drives the drum shaft. Turn a hand wheel on the machine to rotate the power feed cross shaft back and forth a half turn in each direction. Check the play on the hand wheel before the worm wheel begins to turn the drum shaft. The less play you find due to the hand wheel is good. If it takes at least a quarter of a turn on the hand wheel before the drum shaft begins to move, this could mean a badly worn worm wheel. Some slowness before the drum moves is normal.

Under power examination
Operate the machine through its speed range and cycle and listen closely for any adverse sounds. If you hear grinding type noises over the sounds of normal operation this could mean bad gears or bad bearings. To check for spindle concentricity use a dial indicator. A spindle which does not have a perfect circle around its axis is usually caused by worn bearings. A worn bearing are balls in the bearing which are out of round. When the bearing turns, it wobbles causing the spindle to wobble in rotation. This will cause inconsistent work. If the bearings are loose, tighten them. If the run-out is continually excessive then bad bearings could be the cause. When possible run bar stock and gauge the finished work piece. Know your tolerances and decide if the machine is for your production needs.

This is one article in a series of    How to Buy Metalworking Equipment.   Each article showcases and explains a particular type of metalworking machine. They were originally published in the Metalworking Machinery Mailer published by the Tade Publishing Group.

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