weldment design 03 - welding jigs

When you're talking about welding, the use of jigs, fixtures and positioners is usually desirable, for at least four reasons:
1. To minimize distortion caused by heat of welding.
2. To permit welding in a more convenient position.
3., To increase welding efficiency and productivity.
4. To minimize fit-up problems. With a welding jig or fixture, the components of a weldment can be assembled into accurate alignment and held securely in proper relationship and with correct fit-up during positioning and during welding so that all finished parts coming from the jig or fixture will be uniform.
For high quantity production, it is very economical to design and construct an accurate, durable jigs and fixtures.


WELDING JIGS
These are specialized devices which enable the components being welded to be easily and rapidly set up and held.  A welding jig should be :
(i) Rigid and strong since it has to stand contractional stresses without deforming.
(ii) Simple to operate, yet it must be accurate.
(iii) Designed such that it is not possible to put the work in it the wrong way.
(iv) Faced with wear-resistant material to stand continual wear.
In certain cases, as in inert gas welding, the jigs is used as a means of directing the inert gas on to the underside of the weld (back purge). Jigs may also incorporate a backing strip.
In welding parlance, jigs are considered as stationary, while fixtures rotate, usually on trunnions, about a vertical or horizontal axis, either by hand operation or through motor and reduction gear drive.

WELDING FIXTURES
They serves the same purpose as a welding jig; but, in addition, it permits the changing of the position of the work during the actual welding, so as to place the welds in a plane convenient to the operator at all times. This increases welding speed.
Fixtures are considered to be of a more general character and not so specialized as jigs. Fixtures may include rollers, clamps, and wedges etc., used for convenience in positioning of the work.

The actions provided by welding fixtures could be
> Supporting
> Clamping
> Grounding and
> Imparting movement (usually referred to as travel or traveling) to name but a few.
Welding fixtures may also perform other functions that are not directly related to the actual welding operation. For example, fixturing may be an integral part of a conveyor system. Or it may position the work for machining operations such as drilling, milling and grinding.



JIGS & FIXTURES SELECTION
Fixture Selection - In selecting fixtures for mechanized welding, the engineer must evaluate considerations such as
1. Production volume
Elaborate welding equipment or work fixtures cannot be justified unless the part volume is substantial. If part volume does not exist, the next thing to consider is substantial footage of welding that lends itself to mechanization. In this case fixture adaptability, simplicity and cost often are the important factors for the selection of fixtures. This may mean combining a variety of work supporting or holding devices with a simple beam-with-carriage fixture.

2. Joint Geometry
Joint geometry plays an important role in the selection of a welding fixture. For example,
(a) Circular welds are usually made with the welding head stationary and the work rotating.
(b) Welds in a straight line usually require a welding equipment fixture that travels the (welding) head along the joint with a stationary work fixture providing clamping to maintain part alignment and fit up.

3. Stationary Welding Head
Because a stationary welding head requires work movement, it invariably calls for complex fixturing. In fact, the work fixturing necessary for stationary head applications can be highly automated.
Some fixtures are designed so that the weight of the incoming work activates the positioning, and clamping devices, and then initiates the welding cycle. As a result, even 1he simple act of pushing the start button is eliminated.

4. Movable Welding Head
In this case, the work remains stationary and the welding head travels along the joint. Head movement may result from
- Travel of a carriage on a beam or rail
- A traveling boom mounted on a stationary column,
- The travel of the entire manipulator riding on a track in the floor,
- Travel of a tractor riding the workpiece surface. In this case both the welding equipment and workpiece are traveling but in opposite directions to keep the weld puddle in the flat position.
The beam-with-carriage type welding equipment fixture is usually limited to head travel in straight lines parallel to and on one side of beam support.
A boom type welding manipulator equipped for both boom and column travel can swing the welding head through a 360" arc. It is capable of welding in a straight line in any direction away from the column to a distance somewhat less than the length of the boom and can service welding stations located anywhere within this area. It duplicates the performance of a beam-with carriage type fixture when the boom is held stationary and the entire manipulator is traveled on the floor tracks.
Addtionally, it can weld on either side of the column supporting the boom. Both the beam-with-carriage and the boom type welding manipulator can serve as welding-head-stationary fixtures.



FIXTURE DESIGN BASIC
> The fixture should be strong and light but rigid enough to ensure accurate alignment.
> Whenever possible, a fixture should be positioners, enabling all welds to be brought to a convenient welding position.
> The fixture should permit quick and easy positioning (by one hand, if possible). Thus, balancing of the fixture may be necessary.
> Design should be as simple and inexpensive as possible; accuracy and elaboration should be no greater than required. Only essential dimensions should be controlled in a fixture. Appearance should be disregarded. Welded construction is best, tool room work should be avoided and machining should be kept to a minimum.
> The use of light alloys for moving parts reduces weight. Air or electric motors should be used for revolving, and air or hydraulic rams for tilting the fixture assembly.
> A fixture should be built around the work and should locate and clamp components in position so that assembly, tacking, and welding may be carried out in one fixture. The fixture should ensure one way correct' assembly only.
> The fixture should permit freedom of movement in one plane to avoid residual stress in the completed weldment. Design should permit heat dissipation to release, rather than bind, the assembly being welded. Rams or bumpers may be used to dislodge heat bound parts.
> Joints must be readily accessible for welding. By slots or other means, the fixture should readily present seams on the reverse side of the object.
> Fixtures should be kept cool enough to handle air, water, fins or insulated handles can be used.
> Parts should be prebent in the fixture, if necessary, for final accuracy.
> To facilitate shop flow, jigs or fixtures can be mounted on wheels or used in conjunction with floor mounted or overhead conveyors.
> Indexing arrangements are helpful in providing accurate, quick positioning of work.
> Positioning of the operator by ladders, trestles, cradles or trolleys is sometimes advisable.
> Clamps must operate quickly. Screws and moving parts should be protected against weld spatter. Fusion to a fixture or to clamps can be avoided by the use of slots or copper backing. If possible, clamps and locating devices should be integral with, and hinged to, the fixture.
> Either integral or separate copper backing bars should be used in cases of poor fit or for light plate.
> Integral gas backing is highly important for some alloys.
> Use of nuts and bolts, wrenches, C-clamps, wedges and hammers and hand screws should be minimum.
Use of eccentrics and cams, cranks, pinions and racks, air or hydraulic rams, solenoids or magnetic clamps is preferred.
> Methods of grounding the work are important considerations in fixture design, since they affect arc action, quality of weld, and speed at which welds are produced.The ground must make good contact with the work through the medium of copper, copper graphite brushes, sliding shoes etc.
> A good fixture design avoids the problem of arc blow by taking following measures:
      ~All steel other than being welded should be kept .at least 25 mm from the arc.
      ~Clapping fingers may be made from a non-magnetic material.
      ~The ground connection should be made directly with the work and not with the backing bar.The point of connection should be as far from the arc as possible. The ground can be split to offset arc blow.
      ~When possible, the fixture should be built from low carbon steel; large masses of steel should be normalized to minimize residual magnetism.
      ~When horn type fixtures are used, the welding should be toward the closed end.
> The fixture should be designed for ease of accessibility to the welding head when procedure adjustments and servicing are necessary. Accessibility is also necessary in the positioning of welding controls.
> The design of fixture should be such that the weldment can be easily and quickly removed from it

Don't try to design a welding fixture for your boss's mouth..it's highly dangerous..