1. Projection Design & Equipment

2. Projection Equipment Learning Activities View Slides;
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Lesson Objectives
When you finish this lesson you will understand:
Equipment requirements necessary for successful projection welding
Keywords
Projection Weld Electrodes, Low Inertia Electrode Holders, Projection Punch and Die Set

3. 5.0 Equipment Requirements
Electrodes and
Electrode Holders
Describe the design of electrodes and
electrode holders, and its significance
Describe the design of die assemblies
and its significance
Die Assemblies
The object of this section is to describe the mechanical and electrical characteristics of machines and equipment in projection welding processes.

4. Types of Electrodes Large flat electrodes, often made from bar stock
Not recommended on weldments which are distorted from the projection forming operation
Advisable when welding a few projections in a localized area
A series of local contact surfaces made by relieving a bar between projections or by brazing inserts of hard copper alloy to the bar
Tend to equalize the total current and force in spite of distortion or tolerance of the weldment
Recommended as best for all projection welding, and especially in those applications involving four or more projections.
Composite type electrodes
Using electrodes of proper design and alloy is a major factor in making projection welds. Flat electrodes are not recommended on weldments which are distorted from the projection forming operation. The distortion prevents the weld force and current from entering directly above the projections and results in unequal current and force division. Flat electrodes are advisable when welding a few projections in a localized area. Flat electrodes or local contact surface electrodes can be made from RWMA Class 2, Class 3 or Class 4 alloys. Class 3 is recommended.
If a hard electrode material is required, an economical solution is to use a composite electrode of copper backing with the appropriate hard facing. The facing suggested here is a copper-tungsten alloy brazed to the copper under controlled conditions. The ideal electrode alloy is one which is as hard as possible but does not crack or cause surface burning on the weldment. If cracking or surface burning is encountered, a softer alloy with higher conductivity and ductility is required.
While it is not the intent here to discuss the details of electrode design, two other factors must not be overlooked. These are the desirability of water cooling and the need for clean and firmly bolted contacting surfaces. Water cooling (either directly on the electrode or indirectly on the electrode holder) is desirable and may be required to avoid overheating and resulting oxidation. This is important in the case of precipitation-hardening type alloys which lose their hardness above a specific temperature. As with all contacting surfaces, the reason for cleaning and firmly bolting all together is to reduce the contact resistances which generate excessive heating.

5. Type of Electrodes Flat Type Electrode Composite Type Electrode
Localized Contact Type Electrode
Brazed
Joint
Electrode h
Copper W
Insert
Shear Burr
Inferior welds with varying degrees of expulsion and surface marking will result if the weldment is not clean of grease, rust, scale or dirt. A slightly oily surface is not objectionable.
If a number of projections are to be welded simultaneously, the variation in height between projections should be confined to 3 percent of the height of the projection. Excessive variation in height will cause expulsion and will exaggerate the formation of unequal weld diameters.
In preparing the weldment, it is imperative that shearing burrs (the above slide) be eliminated. As the projection collapses, burrs form shunting paths for both current and force. Uncontrolled shunting makes it impossible to produce consistent welds.
In any welding operation, the less variation in part tolerance before welding, the more consistent the weldments will be. The welding equipment will not successfully finish-form the weldment. This mechanical operation has a tremendous effect on the equalization of the electrode force over each projection in a multiple projection weld. Without proper equalization, the weld quality will vary widely.
Electrode h W
[Reference: Resistance Welding
Manual, p.3-12, RWMA]
Relief

6. Spring-Loaded Low Inertia Electrode Holders
Projection welding is done using conventional resistance welding equipment. Virtually all types of resistance welding power supplies (AC, half-wave DC, full-wave DC, frequency inverter type, capacitive discharge, etc.) and configuration (straight-acting, series, pinch, etc.) are used with projection welding. In selecting welding machines, care must be taken to accommodate both power demands and force requirements for the application. Projection welding, as described above, is considerably more flexible than other resistance welding processes, so the design of equipment must be application specific. This is particularly true in the area of electrode design. In projection welding, multiple welds are often made, so the welding electrode must be designed to cover a number of welds simultaneously, as well as maintain alignment of the parts. Heating during welding is localized, by the projection, at the point of contact of the part. As a result, the process is less sensitive to variation in the geometry of the electrodes, and allows considerable flexibility in electrode design. This gives projection welding an advantage over other welding terms.
The above slide shows spring-loaded low inertia electrode holders which provide an economical means of obtaining a uniform force pattern and aid in equal current division.
[Reference: Resistance Welding Manual, p.3-10, RWMA]

7. Grease-Equalized Electrode Holders
Grease-equalized electrode holders, which balance the force pattern and aid current distribution, may be used when the force magnitude exceeds the spring-loaded type in the previous slide. Ten projection welds are made simultaneously with this tooling.
[Reference: Resistance Welding Manual, p.3-10, RWMA]

8. Hydraulically Operated Press Welder
Hydraulically operated press welder with 18,000 lb force is presented in the above slide.
[Reference: Resistance Welding Manual, p.3-11, RWMA]

9. Low Inertia Welding Head
Relaxed Diaphragm
Compressed
Diaphragm
Partially Compressed
Diaphragm
Air Inlet
Welding-
Current
Switch
(Open)
Welding Head
Welding-
Current
Switch
(Closed)
Welding-
Current
Switch
(Closed)
Outer Shaft
Air Inlet
Piston
Cylinder
Spring (Under Initial
Compression)
Partially
Compressed Spring
Inner Shaft
Fully
Compressed Spring
A relatively unique concern of projection welding is follow-up of the welding head during projection collapse. Usually, for embossed projections, the projection collapses in a small fraction of the welding cycle. Slow follow-up implies a loss of welding force, and potential premature expulsion. To compensate for the collapse of the projection, special low inertia welding heads have been developed. An example of a design of a low inertia head is presented in the above slide. This system is a conventional cylinder arrangement augmented by a fast follow-up air driven diaphragm. The column supplying the welding force is actually two annular shafts coupled by a spring. The outer shaft is driven by conventional air system piston. The inner shaft is driven by the fast follow-up diaphragm. When the system is initially charged, there is full air pressure above both the piston and the diaphragm. During welding, as the projection collapses, the air volume in the cylinder (above the piston) will not increase in pressure fast enough to maintain welding force. The small volume of air above the diaphragm, however, can react quickly, and will maintain force till the main cylinder comes up to pressure.
Workpieces
Upper Electrode
Workpieces
Workpieces
Lower Electrode
(a) Welding head in open
position
(b) Welding head in position
for squeezing and heating
projection
(c) Welding head at instant of
projection collapse and
start of
nugget
formation
[Reference: Metals Handbook Volume 6, p , ASM]

10. Punch and Die Set Design for Mild Steel Sheet
Punch Insert A
Plug Fit
Punch
Die B C
1/4
1-3/8
1-3/8 A
1/4
A major consideration in projection welding is the forming of the projections themselves. Projection shape and size are rather critical issues, and projections are generally held to rather tight tolerances. Considerable empirical work has been done developing proper shapes and geometries for embossed projections. This work has resulted in a generally accepted set of designs for steel thicknesses ranging up to in. thick. To form these projections, stamping dies must also be designed to relatively tight tolerances. Typical punch and die set design data for embossed projections (taken form AWS-recommended practice) is presented in the above slide. In these designs, care is taken not only to maintain the shape of the projection surface, but the indentation on the back side of the projection.
Die Insert F B B H
Plug Fit
[Reference: Recommended practice for resistance welding, AWS
Document C1.1-66, AWS, 1966]

11. Punch Shapes Used for Various Thicknesses
(a) (c)
(b) (d)
In forming projections, a major consideration is the mechanical stability of the projection itself. Of particular concern is the wall thickness of the projection. If the projection wall is thin, the wall may shear under the application of force and current, resulting in inconsistent weld quality. Thickness of the projection wall is largely controlled by shape of the indentation. Some examples of various types of indentations are presented in the above slide. For thin sheets, either small-radiused or truncated-cone indentations are used. These are shown in Figure (a) and (d), respectively. These types of indentors are largely used to maintain a nominally constant wall thickness in and around the projection. For heavier section materials, wall thickness is less of a concern so there is more leeway in the types of indentors used. Typical indentors used include those used for lighter gauge materials, as well as pointed indentors in Figure (b) and small rounded indentors in Figure (c).
[Reference: Resistance Welding Manual, p.3-3, RWMA]