TIG brazing machine meets special requirements

TWI Bulletin, June 1987

by Michael Moore and Colin Watts

Michael Moore is Product Supervisor and Colin Watts, TEng, MIMGTechE, is CAD Systems Supervisor in the Equipment Engineering Department.

A precision brazing problem experienced by a Member company of The Welding Institute led to design and production of a purpose built TIG brazing machine. The methods by which the various problems posed were overcome to produce a machine meeting the member's requirements are described in this article.

Design and manufacture of a dedicated TIG brazing machine resulted from an approach by a Member company to the Arc Welding Department of the Institute with an arc starting problem. The task was to establish a low current (6A) TIG arc on to a 0.5mm diameter nickel plated molybdenum pin. The pin has a small shoulder on which a brazing washer and cap are located (Fig.1) and the objective was to braze cap to pin using a TIG arc as heat source.

To achieve acceptable results the brazing operation had to be completed without marking or discolouring the cap, which was manufactured from polished stainless steel with a ceramic sleeve. Workpiece temperature should not exceed 350°C. To compound the problem the joint to be brazed was located within a canned envelope (Fig.2) which completely obscured the arc because the torch nozzle almost completely filled the can. A further problem caused by inability to see the process was electrode alignment. This is critical as the arc has to be established accurately on a 0.5mm diameter pin.

The Member company was tackling these problems by using a system of mechanical jigging for electrode/pin alignment. It was assumed that the molybdenum pin always maintained the same attitude with respect to the can. As this was not always so in practice, correct electrode/pin alignment could not be reliably achieved. Also, the Member company was using HVHF for arc starting, but with a small target (0.5mm) and low current (6A) the arc, when searching for stable anode roots, often produced a damaged cap and therefore a rejected component. The failure rate with what was already a costly component at this stage of production was unacceptable.

Microplasma was considered but neither the transferred nor non-transferred mode of operation was satisfactory, causing damage and/or discolouration to the cap without effecting a good braze. Therefore it was decided to return to the TIG process but retain the microplasma torch with its pilot arc touch start facility, using it as an arc starting method for a normal TIG arc so eliminating the need for HVHF. Successfully brazed joints were achieved by using a cut down can envelope to reveal the joint for easy arc viewing and electrode/pin alignment (Fig.3). It was suggested to the Member that a dedicated machine for this process should be designed and built.

The Control Engineering Department (now Equipment Engineering Department) of the Institute was awarded the contract to design and build this special purpose machine, incorporating the following features:

• Accurate electrode/pin alignment;
• Stable low current arc starting and running control which, if not completing a successful braze joint, would leave the component undamaged and recoverable with a re-run;
• Single button control to suit semi-skilled non welding operators;
• The whole of the equipment to fit into the member's controlled environment facility.

From the outset it was obvious that some form of optical system would be required to achieve accurate electrode/pin alignment. This, with component jigging, traversing, and TIG torch mounting, will be described later.

Electrical control systems

The control system used divides into three areas:

1. Traverse speed and indexing;
2. Brazing process cycle and sequencing;
3. Arc initiation and parameter control.

Traverse speed and indexing

After considering various possibilities it was decided that a stepping motor with precise indexing ability would best suit the task of transporting and positioning the component accurately. A commercially available motor and controller were selected (Sigma motor 20-2235-D200-F37 and Digiplan SD3 Drive + control cards) which gave programmable indexing and speed control. The traversing sequence is as follows: with a component loaded and the traverse at the lower end of its travel, initiating the process sequence causes the traverse to transport the component to be brazed to within 20mm of the torch electrode. The motor reduces speed to allow component to approach electrode at a preset low speed until contact is made with the 0.5mm molybdenum pin. Then the motor is arrested abruptly. To achieve this a touch-down detection circuit is used, interfaced with the motor control system.

The next command to the traverse controller is to retract the component 0.5mm from the electrode to create a 0.5mm arc gap. The special ability of the programmable stepping motor to achieve this precise control as part of the process was used to advantage. The trigger for the retract command is derived from a current detection circuit. The final segment of the traverse sequence, on completion of the braze operation, returns the component to its original lower position where a limit switch arrests further downward progress and leaves the traverse ready for the next cycle.

Process sequence control

Touch-down detection

This was achieved by connecting a low voltage supply (24V DC), with suitable diode blocking from the main arc current, to the torch electrode and using electrical contact with the molybdenum pin as a simple switch closure to initiate the detection circuit. As well as halting the traverse this starts pre-purge gas shielding, at the end of which arcing begins. As the machine being designed was dedicated to a single task, the parameters once decided were unlikely to be changed. It was therefore decided to house the gas pre- and post-purge adjustable timers (0-5min and 0-30sec respectively) within the control cabinet instead of being a front panel operator facility.

Current detection

Figure 4 shows the control cabinet which houses an analog meter for arc current. The shunt for excitation of this meter provides a convenient proportional current signal source. The output from the shunt is fed into a signal conditioning circuit consisting of an integrator, comparator and output drive. The integrator provided a ramp-up signal (one second time constant) to be fed to the comparator which switches the output drive once the desired current had been reached. The output drive is interfaced with the programmable stepping motor controller which selects the appropriate programme sequence to retract the traverse by 0.5mm and so form the required arc gap. As arc current is a pre-determined parameter, the threshold switching level for this circuit is a preset value needing no operator adjustment.

Arc initiation and control

The power supply for the arc current is full wave rectified fully smoothed 90V DC. Arc initiation and control proved to be the most difficult parameters to regulate. Arcs could be struck which were not as repeatable as required and sometimes damaged the component. Simple remedies were used in an attempt to solve this problem; i.e. shielding gas flow rates were varied, different tungsten electrode angles and diameters were tried, but none of these solved the problem. The solution proved to be a mixture of remedies each in turn contributing some improvement, as described below.

1. Pre-arc electrode heating was used by monitoring the electrode/pin contact for one second after switching on the arc current, hence the ramped signal mentioned in the description of the current detection circuit. This electrode heating caused good thermionic emission and therefore created a more favourable environment for arc establishment.
2. Electrodes used up to this stage had always been 2% thoriated and so it was decided to try 4% thoriated. Electrode diameter had been fixed at 1.0mm. With the twin assets of electrode heating and 4% thoriation some success was experienced. In fact a 'good', well conditioned, electrode provided repeatable satisfactory arc strikes without damage to the component. Nevertheless with the replacement or regrinding of the 'good' electrode, failures were experienced.
3. Probably the most significant contribution to a solution was to increase arc current by 100% by adding a short pulse for the duration of electrode heating and arc initiation. This modification to arc current control produced 12A for arc initiation followed by a 6A arc (Fig.5). The duration of the arc could be selected using a thumbwheel switch control from 0-10sec in 0.1sec steps. As a result of these three actions repeatable arc strikes were produced with no damage to the component and at the same time good brazed joints were achieved.

Sequence cycle initiation

The brazing cycle is initiated from either the control cabinet or a remote control pendant which, when connected, inhibits the control cabinet initiation controls (Fig.6). Three push button controls are available to the operator:

1. Braze cycle initiate;
2. Reset;
3. Emergency stop.

Control 2 is used to reset the traverse after setting the procedures, which is accomplished using a 'set up' switch mounted on the control cabinet to inhibit arc current and gas flow circuits to allow dummy runs to take place. In addition to these controls, there are normal machine interlocks for the microscope and traverse movements.

Mechanics

The basic construction of the machine (Fig.7) consists of

1. Two guide bars and a ballscrew mounted into a base plate and a crosshead;
2. A moving workpiece platen fitted with linear recirculating ball bearings which run on the guide bars and a ballscrew nut running on the ballscrew;
3. X-Y microscope slides mounted on the moving platen, fitted with a slide block to receive the workpiece holding assembly;
4. Workpiece holding assembly which also provides workpiece location and an electrical connection;
5. Torch holding assembly which incorporates an X-Y microscope slide system;
6. Optical viewing system mounted on one of the guide bars.

The layout of the machine was extensively determined by workpiece and process requirements.

As the TIG arc is to be struck on a small target deep inside a can, an optical viewing system was chosen to align TIG electrode with workpiece. Microscope slides provide fine control of the small movements required to position the electrode over the target (Fig.8). The optical system is based on a mirror which is rotated to fixed positions to view and position the electrode and workpiece alternately. The viewing system is hinged from one of the guide bars. The hinge assembly has a spring loaded taper pin arrangement to lock the viewing system in position between workpiece and electrode.

A simple to use system was required for loading the workpiece on to the platen, holding it securely and providing an electrical connection for the brazing process. This was achieved by providing a shouldered close fitting location in the assembly for the can. As the can is pushed into the assembly the pin protruding from its base moves into a copper collet (Fig.2). The whole assembly is then pushed into the slide block on the moving platen which ramps part of the assembly, thereby compressing a disc spring stack which closes the collet. The electrical connection is provided by a ball ended pin, spring loaded on to the back of the copper collet (Fig.2).

The small margin of error involved in making a successful brazed joint requires a precision guide system for the moving platen (the electrode and workpiece when being aligned are 130mm further apart than when in position for brazing). A recirculating linear ball bearing system running on hardened and ground guide bars was chosen to maintain accuracy and minimise wear.

The drive system initially comprised a stepping motor driving the ballscrew directly through a coupling but intolerable vibration was experienced in the ballscrew/nut assembly, so to alter its frequency a timing belt pulley system, slightly gearing up the drive, was introduced. This solved the problem.

Access to the X-Y microscope slides and workpiece holding assembly required the guide bars not to straddle the working area without further separation. As loads on the system are small it was decided to keep appearance compact by positioning one of the guide bars further back.

The machine is now in satisfactory use at the Member company concerned.