Here is a list of all the postings Clive Steer has made in our forums. Click on a thread name to jump to the thread.
|Thread: A delta motor running in star config.|
I don't know if I'm stating the obvious but a star connected (415v) will work on 240V but won't be capable of producing its rated power. This is why on VFDs motors cannot produce max power at lower frequencies as they reduce the voltage applied to the motor. An induction motor has common characteristics to other types in that it produces a back EMF related to speed so at higher speeds more volts is required to get the current that gives the power (W=V.I). A 2 HP motor is rated to produce 2HP safely and continuously and a overload trip system is there to stop the motor catching fire and not necessarily there to protect the machine it is driving. For instance when a motor starts up the current drawn (locked rotor) can be 10 times the allowable full load current but only for a short time as the motor/machine accelerates to design speed and the motor protection device (overload trip) allows for this.
If a 2hp motor is fitted to a machine designed for a 3/4 HP motor then the user must ensure he doesn't overload the machine. Most lathes with gearing rarely use the full power the motor can provide due to cutting tool and workpiece limitations.
Regarding fan cooling I tend to run all my motors on VFDs set with the max speed at 100Hz so a 1400rpm motor will be running at 2800 rpm. So when I slow it down the fan is still producing sufficient cooling even at 50% full speed.
|Thread: Dropping 12v dc to 6v dc|
The starting system described by Vintageengineer using a magneto and trembler coil was used for starting large capacity but slow speed petrol engines such as large aircraft radial types. The trembler coil is similar to an electric bell/buzzer with coil, contact and battery in series. One side of the coil is common to the ground of the magneto and the other to the "live" contact of the magneto. The engine is primed with fuel/air called sucking in by turning the propeller over several times and then one cylinder is brought just past full compression where the magneto contact will be open. The guy turning the prop will stand clear and say to the pilot "contact" whereby he connects the battery to the trembler system. As it buzzes the large voltage spikes produced by the trembler coil will get boosted by the magneto producing a stream of sparks at the plug and the engine will chuff into life.
The modern method for aircraft is to have an impulse mechanism on one magneto that flips the magneto as it gets to TDC to produce a good spark. The trembler system is in effect an inductive version of capacitor discharge ignition.
To use a 6V trembler coil system on a 12V battery may only need a larger dropper resistor as the trembler is only used for a short time.
|Thread: Claude Reeve Gravity Regulator|
Just read you thread on the gravity escapement and thought that you might like to know that there is a Gillett and Bland turret clock with a rare 15 leg gravity escapement, that can be viewed, in Tunbridge Wells, Kent. The clock, which a colleage and I recently refurbished and re-installed is in what used to be the Tunbridge Wells West station but has now been converted into a restaraunt called Smith and Western. The west station is near the Pantiles and next to the Sainbury supermarket that was built in the goods yard. For those with satnav the post code is TN2 5QL and best to call them on 01892 550 750 to check if it's OK to see the clock. Access to the clock room in the tower is easy by a spiral staircase. I'm sure they would appreciate the gesture if you had refreshments or a lunch. Close by is the Spa Valley railway for those interested in the heavier forms of engineering.
|Thread: Spark erosion machines .|
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.
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.
I read your article on your Mk2 and like you I gained my inspiraton to try this from Derek Lynas's articlebut the link to the Mk3 didn't seem to work.
I'm looking to make an EDM that can sink a hole 0.2mm diameter into a broken balance staff from a carriage clock platform escapement. To this end my first attempt, which I've only made in the last couple of days, is quite small and light. My first trial yesterday was with the solenoid working in what I call the relaxation mode. However by restraining the movement of the electrode with my finger I found I could get it to arc in a more continuous mode with only a slight vibration. I believe this natural oscillation of the arcing could be due to the dynamics of the arc and fluid whereby when the arc is struck the pressure generated by the fluid rapidly vaporising causes the electrode to move and break the arc. This is only a theory and may only be true for a light moving system. I see that on your arrangement you've added a weight which essentialy reduces the sensitivity of the solenoid but I believe it adds inertia and the force is still linear so does nothing to correct the almost square law non-linear force/position characteristic of the solenoid. Since I can move my eroder head up and down I decided to explored the change of oscillation characteristics with soleniod plunger position and then juggled with the shunt resistor to get more continuous arcing. The solenoid I have used has a cone shaped plunger and matching female pole which produces a more linear force/position characteristic than a simple cylindrical plunger and flat pole piece. I can send you a low resolution video, with sound, of my eroder in operation if you are interested. The sound is more important as you can't actually see anything happening visually. This afternoons trial was to sink a 2mm diameter solid copper electrode into a tungsten carbide milling cutter. I took 10 minutes to sink a 1.2mm deep hole at an average current of 0.8 amps with a gap voltage of 23 volts. I've yet to connect the scope up to see what the gap voltage trace is like. To get a finer surface I reduced the voltage but at the lower current I have to change the shunt resistor. However I have noticed that at the lower voltage the average current can be as high as when operating at higher volts. At the moment my gap cap is only 100uF and I will be trying different values to see what effect it has. My main concern is the rather high rate of electrode erosion which was some 3 mm when sinking the 1.2mm deep hole. At the moment the electrode is negative with respect to the workpiece and I'll try a positive electrode. So many parameters to juggle with.
I've just joined this forum having spotted this thread whilst searching for more info on spark eroders. I've made one which fits on my BCA mill which consists of a small comercially available 6 volt solenoid and uses the standard resistor/capacitor discharge circuit. I've made this oscillate naturaly, without the solenoid, because of the different strike and quench voltages of the arc provided the electrode is in very close proximity to the workpiece. However the arc is unstable and you can get a contact weld. The solenoid helps to break this weld but can over respond and withdraw the electrode much more than the few microns needed to quench the arc. This mode of operation, which operates like a door bell, is inefficient but simple.
However I've used this design but change a number of things to get virtually continuous arcing. Firsty solenoids have a very non-linear position/force characteristic which means that once there is sufficient current to make the plunger move it will rapidly accelerate to the closed position as the magnetic path shortens. This non-linearity can be counteracted by using spring which applies the inverse function. Now when this arrangemnt is used the solenoid plunger moves a much small distance, but sufficient to break the arc, and works more like a voice coil positioning servo. The action can also be improved by introducing viscous damping using grease.
Another change was to shunt the solenoid with resistors so that the solenoid only carries a proportion of the total average arc current. In my case the solenoid of 12 ohm resistance is shunted by a 6 ohm 40W resistor. The use of a light bulb is a good idea as these have a non-linear resistive characteristic because of the large difference between the hot and cold resistance. This helps divert more current to the solenoid to help break a weld condition. I also use a bench supply where I can set the voltage and the current limit whichgives a more linear charging of the capacitor. One thing to consider is that the capacitor used in the circuit should be one designed to have a low ESR and able to handle a high ripple current.
Although this circuit has a more continuous arc the position of the electrode does need to be adjusted more often to keep the erosion rate n the sweet spot. Again by juggling parameters mine has a sweet spot of about 3mm which is OK since I have the down feed on the BCA. This downfeed could be motorised by using the average voltage between the electrode and workpiece to drive a DC motor to wind the electrode down. This means that when the electrode is not arcing the voltage is high and will drive the electrode down and the down rate will reduces as arcing begins.
My arrangement arcs continuously with just a slight vibration of the solenoid.
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