Nut welding applications

Applications of weld nuts
 With the introduction of the motorcar and various other steel products manufactured on assembly lines, a fast effective method of drilling and tapping holes was needed. During the early stages the car body parts were of sufficient thickness to allow drilling and tapping. This was time consuming . It did not take long before someone camp up with the idea of drilling the body and welding a nut or bolt on with a gas torch or arc welder. With the discovery of resistance welding and its many applications i.e. Projection/spot welding, it was not long before this was the preferred method of welding nuts and bolts on to steel components.
 
 The idea behind projection nut or bolt welding is a standard type of nut or bolt can be manufactured with projections on it, located between the electrodes of a resistance welder, and with correct pressure, current and time welded to a metal body part. This reduces time and allows for the design of light i.e. thin metal parts.

Problems arising from Nut Welding.
There are basically three types of projection weld nuts.

  • The hexagon nut
  • The square nut
  • The extruded nut
  • The hexagon nut - is similar in shape to a standard nut. It is a six sided nut with three projections and a locater ring. The nut is in common use throughout the motor industry. It is used in all sizes from 4mm to 16mm.
  • The square nut - is a four sided nut with four projections i.e. one in each corner. This nut has no location ring and uses a special nut welding electrode which helps centre the nut to the body part. These nuts are very common in Europe and America.
  • The extruded or pressed nut - is only for projection welding. It is manufactured out of pressed sheet metal. The square nut and hex nuts can be used as standard but the extruded nut cannot. The nut is used for safety belt anchorage and in other critical areas but is not used as commonly as the other two nut types.

When projection nut welding , each type of three nuts gives rise to its own specific type of problem. Many of the plants we visit are resistance welding their nuts and then reassurance welding. This is a time consuming and costly if you take into consideration the factors of gas, labour and non-standard operation costs.

When welding the hexagon nut, we normally find three major problems. These are:-

·  Nut size

·  Projection accuracy

·  Locater ring

                                                                             Hexagon nut.
Nut size:
When welding hexagon nuts in sizes from M4 to M6 it is very often found that the nut welding machine has not been set correctly or that it is used for larger sizes as well. Bearing this in mind a large amount of heat and time passes through the nut, when with the correct setting, this causes the nut to collapse and compress the threads. A re-tapping operation is necessary and if torque tools are used, many nuts tend to come loose in the trim shop.
Projection accuracy:
The complex shape of the hexagon nut and the manufacturing processes used to make them give rise to inconsistent projections. As the tooling produces high quantities of nut sit wears, the quality level of the nuts drops. Take into consideration that there are only three projections on a hex nut and it is important that they are of high quality. If; by chance one projection is of bad quality, then the probability of nut welding coming loose is increased by 33% if not more.
Locater ring:
The locater ring, even though it helps with the location of the nut and stops the need for special location electrodes, it has a tendency to short circuit current needed on the projections through the locater ring itself and into the body part. This has the effect of creating a bad weld. in the majority of cases we have seen on our visits around the country, at least one if not all of the problems mentioned above with hex nuts are present.

                                                                              Square nut.
Due to the shape and manufacturing processes of the square nut, the square nut has been found to be a more effective type of nut to weld. Having four projections, the probability of a bad projection affecting the weld is lower. The square nut has the same problem as the hexagon nut when welding 4mm to 8mm weld nuts, the weld nuts where collapsing due to incorrect settings. Normally the square nut is bulkier then the hexagon nut so collapsing is not that serve. The square nut does not have a locater ring but has a special insulated electrode set to locate the shhet metal part to nut. This stops any current short circuiting and allows all current to pass through the projections.

                                                                             Extruded nut.
The extruded (projection) weld nuts are used in special operations such as safety critical areas. Due to this being a safety critical weld the setting of the machines and the condition of electrodes for welding are monitored and checked continually. During the manufacturing process the nut is pressed and not cold headed, as is the case with the hexagon and square nuts. It is seldom that we find problems with the extruded nut.

Solving the weld nut problem:
If we take into consideration points for and against each design, we end up with this situation:-

·  Both square and hexagon nuts collapse under excessive heat/pressure.

·  The square nut has more projections, thus improving reliability.

·  The square nut is a simpler design and easier to manufacture.

·  Hexagon nuts do not require special location electrodes.

·  Hexagon nuts lose weld current through the locater ring.

·  The square nut has a 25% higher weld strength. (extra projection available)

The extruded nut will be discussed as a projection body part in a later chapter.

As shown in the above, the square nut is superior to the hexagon nut, if a design modification is made to the square nut, this can be raised to four out of five points.

Square nut modification/improvement.
As discussed, the two worst problems found with weld nuts are:-

·  They come loose.

·  Thread stripping, due to thread compression.

If we take the square nut as standard and strengthen it, we can stop thread compression, increase weld reliability; reduce machine change-over time and lower piece part price. The method used to strengthen the square is to standardize on the blank size, i.e. for weld nut sizes 4mm to 8mm, the blank size is for 8mm. For 10mm up to 12mm, or 14mm the blank size is for 12mm or 14mm. With the introduction of two standard sizes covering six or seven thread sizes, projection nut welding machines can be set for high or low settings. If the machines is fitted with a dual stage timer and program 1 / program 2 switch, any size nut can be welded with one machine, resetting not necessary. When using a square weld nut, a special weld nut electrode must be used. The electrode is an air operated or spring operated unit with the electrode pin insulated from the copper die. The electrode is a five part assembly with interchangeable parts to allow it to weld nuts, small projection welding parts, weld studs and safety belt nuts. All this is achieved by changing one or two parts but leaving the main die set in the machine. Westken prefers to use to use air sponge die set, even though a spring version is available, because it is reliable, simple and cannot cause arcing and thread damage. The air helps to cool the electrode, the weld area on the nut as well as the area on the sheet metal. The electrode uses a tapered pin and not a stepped pin, this stops arcing on the threads of the nut and also makes the electrode easier to use in production. The tapered pin lasts longer than a stepped pin. A special water cooled jacket is also available for large nut welding.

The weld nut automated.
There are automatic nut feeders which enable a nut to be picked up from the hopper and placed onto the electrode at high speed. On some nut feeders each nut size must have its own machine, or have time consuming adjustments carried out each time a new size is used. The machines operate with the same basic concept:-

·  Nuts are placed in a vibrating hopper.

·  Air drives the hopper and feeds nuts to a feed corrector.

·  After feed correction, the nuts are fed to a magazine.

·  From the magazine the nut goes to a pick up point.

·  The pin retracts past the pick up point and on its return stroke picks up the nut and places it onto the electrode.

NOTE:

If a standard square nut is used with a standard blank size, the only change to the machine for different nut size is the pick up pin. Changing of the pin takes ten minutes as against thirty minutes and many more parts.

Settings for weld nuts.
Because of its size and projections, it is found that a short time and a high burst of current is used to weld nuts and the machinery carrying out the welding operation is not always in perfect condition, therefore setting up as per the book does not always work. If projection weld nuts are welded using sufficient pressure, low time cycle and sufficient heat and the weld does not hold, then a slight reduction in air / tip pressure, which increases the resistance may be all that is necessary. It is a fact that hexagon nuts are harder to weld than square nuts.

The purchase of projection welding equipment is not necessary for the welding of weld nuts. A standard 50KVA or 100KVA pedestal welder fitted with suitable electrodes does the job sufficiently. Pressure is not as important as weld current and time in the welding of nuts and as stated above the quality of weld nut is very important. If problems are being experienced with weld nuts, check the projections first; this is where 90% of problems are caused. If you are still experiencing weld nut, resistance or stud welding problems, please do not hesitate to contact us.

Nut Welding

Introduction to Nut Welding Applications by Andy Stephenson

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