Spark erosion machines .

[Clive SteerParticipant @clivesteer55943]

Hi Mike

 

Although I’m an electronics engineer I’m trying to understand the basic action of the arc before applying any control or “feedback”.

I found that by just holding, in my fingers, the copper electrode near the workpiece I obtained sustained arcing and erosion but the electrode would occasionally weld. This is when I formed the theory about the natural vibration caused by the arc vaporising the fluid as I could feel these small vibrations. I also demonstrated this action by using the down feed of the BCA quill to initiate arcing without the solenoid being in the circuit. However the electrode needs to be fed towards the workpiece as both electrode and workpiece erode and extracted if the electrode welds. So only small adjustments to the gap need to be made to react to changes in the arc. However in any servoing system that needs to move fast inertia is the enemy and the heavy system arrangements built so far are literaly too clunky. Commercial EMDs either use hydraulics or stepper motor arrangements but these are needed to cater for what may be very heavy electrodes when making for instance moulds. A very early commercial EDM I saw back in the 70’s had a geared motor slowly driving the quill down and a clutch which disconnected the drive when a weld occurred allowing a powerfull spring to withdraw the electrode. This action was very inefficient and a later machine used hydaulics. The solenoid has an action similar to the motor and clutch system.

So an arrangement that provides a slow continuous downfeed and only activates the solenoid when a low gap voltage is detected may be more efficient. Alternatively a stepper motor can be used to produce a slow downfeed and rapid upfeed when needed but is this is more complicated from a control perspective. Anyway must get back to the experientation.

Clive

[Michael Cox 1Participant @michaelcox1]

Hi Clive,

Today I made a small experiment to try to sustain a spark without the solenoid action. I took the weight off of the rod for the test. I couldget not sustained arc or vibration, not even momentarily, with this method.

I too have noted the effect of finger pressure on the sparking. This was the main reason for adding the weight. The weight certainly increases the total current flow, frequency and erosion rate. If the armature is well outside the solenoid then welding is likely to occur and the force from the solenoid is insuffient to break the weld. However, with the armature well inside the coil then the force is large enough to break the weld and continuous operation is possible.

I was surprised when I added the weight that the frequency of operation went up since additional weight should reduce the natual frequency of oscillation of the spindle. However, your theory about spark shock oscillation may explain this. If this is the case then the main function of the solenoid is simply to act as a weld breaker. This would also explain the fact that the spindle can be oscillating at a steady high frequency of say 50 hertz with very little amplitude of oscillation but occasionally it will make a much larger oscilation and then revert back to its previous steady low amplitude mode. This occasional blip must be when a weld forms.

Mike

 

[Clive SteerParticipant @clivesteer55943]

Hi Mike

 

I reconfirm yesterday that I could get short periods of unstable arcing without the solenoid operating. In this mode the arcing sweet spot was only about 20 microns measured on my quill DRO. I’ve just bought a new 36V 3A bench PSU so I can arc at higher currents but also because I still need my bench PSU on my electronics workbench. The new PSU has digital displays of volts and current but the current indication isn’t as good as the analogue meter on the old PSU when the current is rapidly varying but I can live with it.

I tried a silver electrode and wear was no better than copper which much cheaper. I’m using old ring main cable for my source of copper electrodes and straighten them by rolling on the bench. I sunk a 2mm dia x 5mm deep hole in tungsten carbide which took about 40 minutes and lost 10mm of electrode so wear rate is, I believe, too high. I don’t know if this is to be expected or what other people experience.

Read the article by Mike Kapp and can’t agree that his is the smallest EDM as mine is much smaller. My solenoid is 14mm dia x 30mm long and is fitted into a cyclindrical adaptor attached to the top of the BCA spindle drawbar. The moving pole is connected to a short length of bicycle gear cable which in turn is connected to a 3mm dia x 60mm ground and polished rod which runs in a insulated precision bush that fits in a 3/8″ BCA spindle collet. I’ve attached a small chuck that came from an old Dremmel to the rod. The chuck has a range of collets that enables me to fit different sized electrodes. I used the bicycle gear cable as it only needs to pull and it won’t cause the rod to bind in the bush if there is any missalignment. However there needs to be some clearance between the rod and bush to allow it to move freely and this play can by the time you get to the electrode tip result it too much movement for the size holes I’m trying to sink. It is true that the electrode isn’t subjected to any major side forces so it may not be a issue. However I have another arrangement planned which will have no lateral play but still allow friction free movement although only over a small distance of about 3mm. This arrangement will also allow the electrode and workpiece to be mounted in any orientation rather than having to have the electrode vertical as is now the case. This will allow long objects to have hole sunk in their ends without the need to stand them vertical. In my case the objects are the vertical sections of a carriage clock case whose screws often rust up and shear off when being undone. The screws aren’t particularly hard but drilling forces and run out of thin drills can cause problems. EDM may be a gentler way of handling the problem.

 

Clive

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