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How to Buy 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:
Loading the workpiece.
Starting the machine and coolant.
Initiate and complete the changing of speeds and feeds and tools.
Inspect or gauge the part.
Unload the finished part.
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.
PRIMARY PARTS
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.
The turret is automatically indexed to bring each tool into position. It
is driven by cams for the proper feed to each tool.
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.
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.
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.
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.
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 AND MULTIPLE SPINDLE
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.
CHOOSING THE RIGHT MACHINE
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.
WHAT TO LOOK FOR
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.
