Electrohydraulic Controls Boost Thermal Drilling

Thermal drilling relies on friction from a carbide bit to melt and extrude metal and form a hole. The process leaves a thicker lip around the edges of holes than with conventional drilling, providing more metal for stronger welds or threads. It also eliminates the waste metal shards generated by conventional drill bits.

Thermal drilling works well on thin materials that normally do not provide support for a threaded surface or sleeve bearing, and when there is a need to attach a welded or riveted nut or special insert. The process is particularly useful for making attachments to pipes which would otherwise require hardto- apply weld nuts or rivnuts, or have inadequate surface areas for soldering or welding.

According to information from Flowdrill Inc., St. Louis, the rotating tool used to thermally drill contacts the material with relatively high axial pressure. This generates heat and makes the material malleable enough to be formed and perforated. As the tool advances, some displaced material forms a collar around the upper surface of the workpiece. The rest of the material forms a bushing in the lower surface. Typical cycle time is 2 to 6 sec. The resulting collar and bushing can be up to three times the original material thickness, and the tool accurately controls hole diameter.

The degree of work hardening depends on the material, but it’s rarely a problem in most steel alloys. As a result, the formed bushing is remarkably strong and can be used for bearing sleeves or, when threaded in a separate process, provides high torque capacity and pull-out strength, according to Flowdrill.

Thermal drilling is suitable for a wide range of materials, including mild and stainless steels, copper, brass, aluminum, titanium, and most malleable materials. And it is typically less expensive than welded and riveted-nut processes.

Motion control
Standard drilling, NC, and CNC machines are all suitable for thermal drilling. But the process depends on the speed and force with which the specialized tooling engages the workpiece. For instance, Flowdrill tooling rotates at higher speeds than do conventional drills, typically running at 1,000 to 3,500 rpm and requiring motor capacities from 1.5 to 3.5 kW.

Hole size, material, and thickness all influence rotational speed, feed rate, and axial force. For example, thin materials can bend or collapse under excessive pressure, necessitating adequate support to prevent deformation. Predrilled holes can reduce the required axial force and also leave a smooth finish in the bushing’s lower edge.

As with any machine tool, good design is all about precisely and effectively applying power. A case in point is the thermal-drilling machine developed by Advanced Machine Automation (AMA) of Birmingham, Ala.

It uses two separate drilling stations to make varioussize holes in copper tubing. The first station drills holes up to 2.62 in. in diameter. It has two spindles, one for a conventional bit that predrills holes and a second for a Flowdrill thermal friction drill. To accurately position the thermal-drilling bit and apply varying forces during the process, this station uses hydraulics to press the bit into the tube.

The AMA machine relies on an RMC75 motion controller from Delta Computer Systems of Vancouver, Wash., to control a hydraulic actuator’s position and pressure. The two-axis controller also operates a second hydraulic actuator that positions the tube being drilled within a 0.003-in. tolerance. A two-position “bang-bang” valve, digitally controlled by the motion controller, operates a pneumatic actuator that drives the predrilling spindle into the workpiece.

The application demands an electronic motion controller to manage the hydraulics because proportional valves must be continuously adjusted as drilling forces change. This also minimizes vibrations. “A typical PLC would be too slow to do this well,” says Keith DeMonia, AMA’s general manager and chief engineer. “Normally, we only use PLCs to control functions like cycle counting, clamping, safety sensor checks, and lubrication cycles, but in some of our automated machinery, we use the Delta as both the motion controller and PLC.”

The biggest design challenge, says DeMonia, is controlling spindle position and pressure. The drill bit approaches rapidly, then slows as it applies force to the workpiece. Precise control is critical, as it’s possible for the hydraulics to generate too much force.

The Delta controller receives absolute positioning data from a magnetostrictive lineardisplacement transducer (MLDT) inside the cylinder. As drilling progresses, the controller monitors outputs of two pressure transducers (one at each end of the cylinder), calculates the difference between the two, and adjusts force via a servovalve.

To tune the system and avoid vibration and chatter, technicians use Delta’s software to plot differences between target and actual parameters such as axis position and velocity. The plotting software quickly shows if changes in control-loop gain reduce the error between target and actual values. Once properly programmed and tuned, the automated process shortened cycle times by 75 to 80% compared to manually sequenced machining systems.

The second station drills smaller holes with bits up to 0.62-in. diameter. This station uses a second Delta RMC75 motion controller and a hydraulic actuator to position copper tubing laterally in front of the spindle. In this case, drilling only requires 250 to 300 lb of force, so a pneumatic actuator presses the spindle into the tube. When pressures are relatively low and precise control is not critical, pneumatics is a good choice. One just calculates the maximum force that an air cylinder must generate and uses that to control the spindle feedrate.

Hydraulics proved more economical than electric motors in this application because a single pump supplies several hydraulic axes. Had fewer axes and less force been needed, electric servomotors may have been the costeffective choice.

A laptop PC serves as a human-machine interface (HMI) and master controller for both drilling stations. A Visual Basic program controls the machine, and an Excel spreadsheet maintains recipes containing drilling parameters (feedrates, drilling pressures, spindle speeds, hole location, diameter, and depth) corresponding to different parts. To interface the PC and motion controller, AMA uses an Ethernet connection and RMCLink software, also from Delta.

OEMs are using AMA’s thermal-drilling machinery to make stronger solder joints in HVAC headers and to improve the strength of aluminum handrails, frame members for the auto industry, and metal furniture structures.