A spark erosion apparatus Mk 3
I have started a new page to describe the latest version of the spark erosion apparatus since it includes some major changes. Most of the spark erosion head (coil, oscillating spindle etc.) is unchanged from the previous version. The biggest change is that a motor has been added (see top of header photo) which drives a dovetail slide. This motor is activated when the tool starts eroding the workpiece so that the back plate moves down as the material is eroded. This keeps the stroke of the oscillating rod constant throughout the erosion process. Without this enhancement the stroke increases as the work is eroded and the erosion rate decreases. An additional benefit is that there is a graduated collar on the dovetail drive pulley so that the rate of erosion can be monitored during the erosion process.
The motor is activated by an additional contact such that as the oscillating spindle penetrates the work it starts to touch the contact. This output is fed to the motor, via a transistor, which then lowers the back plate.
The motor is activated by an additional contact such that as the oscillating spindle penetrates the work it starts to touch the contact. This output is fed to the motor, via a transistor, which then lowers the back plate.
The making of the dovetail has been described previously, see here.
This photo shows it installed on the spark eroder. The central sliding part of the dovetail is bolted to the back plate of the spark eroder head. The fixed part of the dovetail is bolted to the black delrin insulator that slides on the bar of the stand.
This photo shows it installed on the spark eroder. The central sliding part of the dovetail is bolted to the back plate of the spark eroder head. The fixed part of the dovetail is bolted to the black delrin insulator that slides on the bar of the stand.
The dovetail can be seen attached to the delrin insulator with the the gib screws visible near the wiring. The M6 screwed rod is attached to the moving slide of the dovetail. The motor is mounted on a plate and this plate is attached by two pillars to the fixed part of the dovetail. The motor is a twelve volt 30 rpm unit from China. The drive from the motor is transmitted via an O ring drive belt to the feed nut via a graduated collar.
This shows the motor mounting plate in more detail. Note the large 14 mm diameter hole visible below the motor pulley. This is for the rod on the base stand to pass through. There must be plenty of clearance here because the rod is at the ground potential and the plate is at erosion potential.
This view, looking down on the dovetail assembly, shows the feednut on the end of the feedscrew. Note the groove turned in the feednut. The bar attached to the fixed part of the slide is the mounting bar that supports the pillars for the motor plate using the outermost holes. The two inner holes in the bar are threaded M4 and they, and the two M6 tapped holes in the fixed part of the slide, are mounting holes for the plate that holds the nut captive.
The nut capture plate is milled from a piece of 6 mm plate. Note the slot and the milled recess. This fits into the groove on the feednut. The central hole just above the slot, tapped M3, is for the feednut bearing.
Here the nut capture plate has been installed and bolted into position.
The feed nut bearing block has been added. I drilled this for three screws but in fact only one screw hole was used since the other two were too close to the slot in the nut capture plate. Note that the nut protrudes from the bearing block. The graduated collar fits onto this and is secured to it by a grubscrew
The graduated collar and drive pulley. This was machined from aluminium. There are 10 graduations and this with the 1 mm pitch M6 feedscrew means 1 graduation corresponds to 0.1 mm.
The additional contact to detect the position of the oscillating rod is a 6 BA brass screw (with the red wire attached). This is mounted on a piece of perspex . The perspex is slotted so that it can be slid up and down to adjust the position of the contact. The perspex is clamped in the required position by the M4 socket head button screw behind the contact screw. When the brass anti rotation collar touches the contact screw it sends a pulse of current down the wire to the motor.
Two aluminium angles are bolted to the top of the coil. One side has a choccy block connector strip for the wiring. On the other side is mounted TIP 32 transistor and a reversing switch for the motor.
The motor current is too high (circa 0.5A) to be directly operated from the new contact so the transistor is used to reduce the contact current to a few milliamps. The yellow lead is actually a 3.3K resistor encapsulated in heat shrink tubing that connects the new contact to the transistor base.
The reversing switch is necessary so that the motor can be reversed at the end of an erosion run to drive the dovetail slide back to the start point.
The motor current is too high (circa 0.5A) to be directly operated from the new contact so the transistor is used to reduce the contact current to a few milliamps. The yellow lead is actually a 3.3K resistor encapsulated in heat shrink tubing that connects the new contact to the transistor base.
The reversing switch is necessary so that the motor can be reversed at the end of an erosion run to drive the dovetail slide back to the start point.
The schematic diagram for the motor control circuit is shown above.
The main advantages of the Mk 3 version are that the spark conditions remain constant during erosion, and that the speed of erosion can be monitored via the graduated collar.