Resistance Welding

   Companies utilizing pre-formed tips or rods in their tipping process use resistance welding. Its main attraction to the machine designer is its low initial cost. However, it has inherent disadvantages that can cause problems in a filing room environment.

   Resistance weld electrodes contact the material to be welded. They conduct welding current, apply force and dissipate some of the welding heat. they can carry currents of 5000 amps/sq. inch to 70000 amps/sq. inch; they must conduct heat fast enough to prevent welding themselves to the work piece.

   In order to obtain high quality welds, the electrodes and work pieces must be free from rust, scale, lubricants, dust and dirt, components which are all present to some degree in the filing room. Resistance welding creates unique problems when used in the tipping operation. Because the tip is applied under pressure and effectively intrudes into the saw plate when current is applied, is displaces saw steel or a combination of saw steel and old tip material equal to the degree of intrusion. This excess material or "slag" is pushed out of the sides of the plate behind the new tip. Murphy's Law assures us that this displaced material is never equal about the plate center line. Since this slag is extremely hard and abrasive, and in most cases uneven, it imparts an extremely detrimental effect on side grinding wheels, making it almost impossible to maintain side grinding accuracy.

  In the event of a tip loss or breakage with or without minimum loss of shoulder material, the original saw tooth profile must be restored in order to resistance weld a new tip into position, if tooth pitch is to be maintained. Clearly a new tip cannot be resistance welded into a steel saw tooth that no longer exists, so repairs to saws tipped by this method of welding involves much preparation.

   The limited sizes available in either preformed rods or tips, and the lack of range of tipping materials serve to restrict the customer's choice and dictate the amount of side grinding necessary in many cases. Often tip size also restricts the potential number of resharpenings possible.

   Another important point to consider when tipping with expensive alloys is that inevitably some material is going to be wasted prior to re-tipping by the resistance weld method. This is due to the requirement that the operator either "grinds in" the saw profile in order to remove the old tip, or live with its conversion to slag as it is displaced by the new tip.

   Finally, on most resistance welders, the annealing process is achieved by contacting the fresh tip with another electrode which is designed to pass a reduced current through the tooth welding in order to heat the tooth. The tooth effectively becomes a heat sink which, if adjustments are made correctly, allows just enough heat to flow into the saw plate at the weld line in order to anneal or normalize the weld zone. Unfortunately, the tip has to become quite hot in order to achieve this result. While this has no impact on a stellite tip, there is evidence to suggest that these temperatures can "draw down" the hardness of a high speed steel tooth, negating some of its advantages.

   Jacobsen Manufacturing Company's JM-2000 tipping system utilizes the plasma arc  method of welding. This system uses a nonconsumable tungsten electrode. A torch nozzle or orifice surround the electrode. The arc ionizes the argon gas fed into the torch to form plasma. Passing the ionized gas through the narrow torch orifice increases the plasma flow and  can raise temperatures at the tip up to 50,000 degrees F. The torch uses a transferred arc technology: i.e., the arc forms between the torch and the saw plate.

   A saw is loaded into the machine, the saw tooth is automatically indexed into the correct position between the chill blocks or clamping jaws, in which the two halves of the chill molds are mounted.

   The mold halves come together, clamping the saw tooth between them to form the cavity shaped to the size and specification chosen by the customer.

   The plasma arc which is positioned over the cavity, is then activated. The original steel tooth, or in the case of a re-tipping operation, the old remaining tip, is melted almost instantaneously and flows to the bottom of the chill mold cavity. This part of the cycle is called the Preheat Cycle.

   Next, while the torch remains activated, welding wire is fed into the plasma are above the mold cavity. The amount of wire and the speed at which it is fed are preset by the operator. The wire becomes molten and flows smoothly into the mold cavity where it forms a homogeneous deposit, mixing with the saw steel only at the base of the cavity to become an integral part of the saw plate. The main are then shuts down and the semi-molten tip is allowed to completely solidify within the water cooled chill molds prior to the mold opening and the subsequent start of another tipping cycle.

   It should be noted that inert argon gas envelopes and shields the entire area during the preheat, tipping and cooling operations.

   Plasma weld technology is recognized as having the following advantages over resistance welding. It is capable of welding almost all materials, offers better control of arc energy, and produces a smaller heat affected zone.

   When utilized in the JM-2000 system, it offers fast processing times, coupled with unsurpassed flexibility. No saw preparation is necessary prior to tipping. Existing tips are simply remelted in the mold and new molten alloy is added to form the new tooth.

   Tip shape is only dependent on the mold cavity shape and can be specified by the customer. Broken or missing teeth, including those with minimal shoulder loss, can be quickly and easily repaired by reloading the saw onto the tipper and performing a simple semiautomatic repair.

   The JM-2000 tipper forms a wear resistant tip cleanly and accurately. By utilizing a chill mold which securely clamps the saw tooth during the tipping operation, tip symmetry and rough kerf sizing can be guaranteed within +/- .002", and because the mold sizes are available in increments of .005", the tip is sized to the customer's specifications, not those of the welding rod or preformed tip. Much less grinding is required after retipping. Many materials may be experimented with, since the machine is compatible with all spool-wound welding wire.

   Because the JM-2000 does not displace saw steel or slag in the tipping operation, there is no uneven, hardened material behind the tip to wear out expensive grinding wheels.