Tilt turntable welding positioners provide motion in two axes. The
fixture work piece sits on a rotating turntable that rocks or tilts. The basic styles are
a flat-135[degrees] weld positioner that tilts the geared turntable from
horizontal through vertical to 45[degrees] past vertical and a welding
positioner that tilts from 90[degrees] forward through horizontal to 45[degrees]
backward. A flat-135[degrees] model is mounted on legs to provide enough
clearance above the floor for the load to travel through vertical.
Welding turning rolls, which support fabrications weighing from a few hundred
pounds to 1,000 tons, for rotating cylindrical objects such as tanks and
pressure vessels for making circumferential welds. To make longitudinal welds,
fabricators mount the welding head on a travel carriage. Some welding
fabricators equip pipe turning rolls with special fixturing to hold non-cylindrical
assemblies. Power turning rolls come in sets with a powered turning roll called
a driver and one or more idler rolls. Their simplistic design promotes a low
initial cost and keeps maintenance and repair costs low.
Headstock positioners have fixed vertical tables that rotate the assembly
between the head and tail stock tables, head-tailstocks handle enormous weights.
They rotate a vertical table about the horizontal axis, providing access to all
sides of large weldments. The headstock is powered the tailstock is not. Some
welding fabricators mount head- and tail-stocks on rails to allow an operator to
adjust the distance between the two units. They may also mount small items,
elbows or flanges in the headstock alone. Selecting the proper capacity of a
head stock, consider the torque needed for the largest rotating weldment to be
mounted, this dictates the required torque to accurately start and stop rotation
without speed variation. This is a concern particularly when positioning
weldments with components located far from the center of rotation, which creates
excessive torque that can strain the positioner drive and gearing.
A drop center welding
positioners, is a two-axis unit, comprises a beam
running across and below the center points of a positioner setup. The part to be
welded fits below the center of rotation of the positioner used to rotate the
part. These are used positioners for handling odd-shaped bulky parts such as
tank turrets and components of off-road vehicles.
The welder adjusts the welder positioner arm angles of balanced weld positioners, usually not
powered, so that weldment center-of-gravity lines up with its axis of rotation.
These rotating turntable positioners serve for fabrications that are too heavy to fixture
welds conveniently but not big enough to require a powered tilt
turntable positioner.
Welding positioners have charts giving weight load capacities at various
centers of gravity for distances (usually at 4, 6 and 12" intervals),
rotational speed
ranges, and constant operating speeds. Many suppliers offers positioners with
load capacities from 100 lbs. to 350 tons with tables from 12 to 60".
diameter; square or round tables run from 12 to 96 in. along sides. Capacities of its
tank turning rolls range from 2,000 lbs. to 350 tons. Head-and tailstock capacities run from 3 to
100 tons, and welding turntable capacities
from 3 to 200 tons.
Designing fixtures for weldment fit-up, follow these rules of thumb:
* Weld joints must be accessible. For robotic welding, use low-profile clamps
and locate them away from weld joints. Include at least two reference points in
the fixture, in a fixed relationship to the weld points, that the robot program
can use to locate the weld joints.
* The welding fixtures must be
rigid and rugged.
* Protect all weld fixture's components from weld spatter.
* Location of clamps must be easy for the operator to load parts and remove the
final welded assembly.
Ensuring precise, repeatable alignment of assemblies clamps exert pressure to
limit distortion due to robotic
welding. C-clamps come in several shapes and sizes for
quick fit-up Shops usually install racks of clamps of several throat
openings-the best clamp for any job has a frame and throat size no larger than
needed, reducing side forces on the screw and frame. Clamps forged or fabricated
of high-strength steel endure stresses better than those of cast iron.
Bar clamps, adjustable jaws on rigid steel backbones, work for a wide range
of applications. Some have a jaw and screw fixed to one end of the bar with one
jaw free to slide; others have two adjustable jaws and screws. A singe bar clamp
can replace many sizes of C-clamps tot a wide range of jobs.
Toggle clamps, jaw clamps with levers, speed up operations considerably; a
quick tug or push on the lever see or releases the clamp.
Grippers chucks, work
holding devices that attach to tables of the positioners, are self
centering chucks designed for a variety of work piece shapes, including
light-gage materials eccentric loads, and dish heads.
Pipe chuck clamps come hinged to fit over two pipe ends. standard style features a
lever and cam to tighten the clamp. For more clamping force, use a hydraulically
actuated model, generally available and used for large pipe, 16- to 60-in.
diameter. Manufacturers of pipe clamps warn welding fabricators to use the devices only
to align pipe ends for welding, and not to support the weight of a fabrication
while lifting or moving the parts to be weld.
Fabricating shops use platen tables and modular fixtures the clamping, aligning,
and fixturing parts to the welding table.
Platens are rigid cast-iron grids mounted on steel stands. Square holes,
typically 1 3/4 in. square on 3 1/2-in. centers, position tooling and
accessories. Tooling includes hold-down dogs for clamping and straightening;
tapered pins for posts; and bending posts for bending rod and tube. Other platen
tooling includes bolt-down arm clamp: for vises or C-clamps; nut-and-bolt
assemblies for holding parts; U-clamps for gripping pipe and tube; riser blocks
for holding parts at levels above the platen surface; magnetic clamps for
alignment; and vertical- and horizontal-slide damps for quick-action clamping.
Modular fixtures come with a three-dimensional work table that provides a
platform on which welders mount a variety of angles, blocks, and fixturing
accessories. The tables, made of high-tensile-strength steel and ribbed to
ensure stability and flatness typically to within 0.0004 in./ft, come in several
sizes to use alone or joined.
Fixture elements match a holes-and-grid pattern on the table. Positioning and
clamping bolts attach fixtures, work piece positioners, and other elements to
the work tables, or to each other. Clamps include toggle clamps and threaded
damps that feature a compensating mechanism that applies the clamping force
perpendicular to the thread.
Fabricators select among two methods to automatically load sheet and plate
onto fabricating equipment such as turret-punch presses and laser-cutting
machines. These two methods can be termed work-envelope loading, where the sheet
loads directly into the work envelope of the machine, and non-work-envelope
loading, where material is loaded away from the work envelope of the machine. An
example of work-envelope loading is a sheet loader used on a turret-punch
press---when removing and sorting parts coming off of the machine and loading a
new work piece onto the press, the machine is idle, so this type of
material-handling setup can limit production-run time. A non-work-envelope
loader such as a pallet changer or dual-table shuttle system enables a
laser-cutting machine to process one work piece while a second worktable is
unloaded of cut parts and loaded with a new work piece. The only machine downtime
with this type of material-handling arrangement is during pallet change, as
little as 20-30 seconds.
