Does anyone know if it's ok to cut digital scale units that are too long? Mark K.
19 years ago
Does anyone know if it's ok to cut digital scale units that are too long? Mark K.
Yes, have done so. We wanted a couple of 3"ers.
Great, thanks. I'll give it a try. Mark K.
I described the method I used to cut a few scales a few months ago. Here's the Google archive of the thread...
Thanks! Actually I have two scale units that are too long, one on my mill, the other is about to go on my homebuilt benchtop spark eroder. Both of the cheapie (but perfectly useful) type, not glass ones. I always wondered how they work, now I will be able to dissect the offcut. Regards, Mark
Could you describe which type of erosion machine you have, or built? Any more info would be appreciated.
It's totally home built (and works fine, automatically). The mechanics were the easy bit. Mechanics: The "frame" is a cheapo hobbyists benchtop drill stand, the tank is a cheap automotive parts washer. The tank sits on the drill base and the column is bolted through the bottom of the tank and sealed. The "drill" table has a small 4"x6" magnetic table bolted to it. The tank is split into two parts, front and back, with a wall separating it just in front of the column. The front is the work area, the back is the storage/drain part which contains a filter and the parts washer pump. The front half can be drained by the pump into the back half in about a minute, and vice versa. The same pump also does the flushing during sparking. I removed the quill (and motor and pulleys, obviously) and replaced it with a slideway which has a stepper motor working the leadscrew (just a bit of good quality M6 studding actually). This works the head which takes the electrodes in various ways. The whole unit is the same size as a hobby drill.
The electronics are somewhat more complicated and I won't go into them unless anyone is interested. The average spark current is up to 4 amps and the spark gap is adjusted by the voltage. Pulse width, duty cycle, voltage and amps of the spark are all adjustable. If anyone wants to build their own, it is essential that they have some knowledge of electronics.
I for one, am interested, and electrically/electronically ept.
The spark: Both "on" and "off" times of the spark are adjustable between 25 - 300 us. A trusty 555 drives a hefty mosfet (when enabled by the processor). The stepper motor is driven by a ULN 2803 octal darlington driver and controlled by a PIC 16F84 processor, as are the sensing electronics (see below).
The power supply: A rectified smoothed 50V toroid provides the amps and the current is controlled through switched banks of 200W aluminium resistors. min setting is 0.5A and there are 3 switches to add another 1 + 1 + 2.5 Amps. A small O-300V (rectified, smoothed) transformer provides 20ma of initial strike voltage to control the gap (The two supplies are simply joined via diodes, the mosfet sinks both) and make the dielectric conductive to the main supply. Since much of the damage to the electrode is done by the initial strike (the leading edge of the spark) you want very small amperage at the beginning of the spark (avoid capacitance), it is the amperage that does most of the work later. A long high amp pulse does less electrode damage than many short pulses (however if it is too long, arcing occours). (Many of the old spark eroders were capacitative discharge and used to eat electrodes).
Sensing: Since obviously you don't want the spark to be running when there is a short circuit, a logic gate disables the spark when there is a short circuit between sparks and tells the motor to go up. If there is no spark detected (by other logic)then the processor knows it has to go down a step instead. If there is a spark and no short circuit, then the electrode stays stationary and keeps sparking.
A 3 position switch controls Off-Manual-Auto. An up and a down button moves the electrode in the manual setting by single steps if pressed briefly or continuously if held. During Auto mode, the electrode is moved down until the first spark (or short circuit) is sensed (the processor also compensates for play in the leadscrew) and then continuously adjusts the gap as described before.
Good flushing is essential for smooth operation in edm.
That's the basics, if you have any questions, fire away.
Mark K. wrote: (Snip)16F84 source code?
It sounds like you have something really slick here, Mark. I'd really love to see that power-supply circuit. Do you have a schematic?
Thanks. I never really drew a proper schematic and the sketches I made are long gone. The only bit that wasn't straightforward lectronics was the sensor to see wether a spark had occoured (as opposed to a short circuit). I seem to remember scaling down the voltage with a resistor divider and some zeners and then having to use an optocoupler for some reason. The circuit "evolved" which is why I never got around to putting it on paper. Regards, Mark K.
I use picbasic pro, I have the sourcecode somewhere but it's mostly just a half-step stepper motor driver.
Btw. the ULN2803 driver chips are very cheap and useful octal driver chips. Since they can be paralleled they are ideal for stepper motors. I just solder them on top of each other for increased current.
Ah, found something! I think that is an earlier healfstep stepper motor routine which I used for testing the quill drive. The final version is pretty much the same but also scans for whether to go up or down or do nothing depending on what the sensors say. (hope it shows ok on your screen)
'STEPPER MOTOR DRIVER FOR PIC 16F84 WITH UP/DOWN BUTTONS AND SINGLE STEP
TRISA = 0 'SET PORT A TO OUTPUTS PORTA = 0 'SET TO 0 TRISB = %11111111 'SET PORT B TO INPUTS VMOTORPOSITION VAR BYTE 'WHICH STEP THE MOTOR IS ON FIRSTEPU VAR BIT 'INDICATES FIRST UP PRESS FIRSTEPD VAR BIT 'INDICATES FIRST DOWN PRESS '!!!!!!!!!!!!!!!!! PROGRAM STARTS HERE !!!!!!!!!!!!!!!!!! STARTHERE: PORTA = 0 SCANUPBUTTON: IF PORTB.0 = 1 THEN GUPSTEP 'CHECK PRESS OR CONTINUE FIRSTEPU = 0 'IF BUTTON RELEASED RESET TO 0 SCANDOWNBUTTON: IF PORTB.1 = 1 THEN GDOWNSTEP 'CHECK PRESS OR CONTINUE FIRSTEPD = 0 'IF BUTTON RELEASED RESET TO 0 '_________________________________________________________________ 'xxxxxxxx PUT OTHER SCANNING ROUTINES HERE xxxxxxxx '_________________________________________________________________ OTHERBUTTONS: GOTO STARTHERE 'START AGAIN or put other buttons above
'!!!!!!!!!!!!!!!!!!!MAIN PROGRAM ENDS HERE !!!!!!!!!!!!!!!!!
'-----------------SUBROUTINES AND GOTOS START HERE----------
GUPSTEP: GOSUB UPSTEP 'DO A STEP UP AND IF FIRSTEPU = 0 THEN 'IF ITS FIRST STEP THEN PAUSE pause 150 endif UPNOWAIT: FIRSTEPU = 1 'SET THIS BIT TO INDICATE FAST STEPPING GOTO SCANUPBUTTON 'SCAN THE BUTTONS AGAIN '--------------------------------------------------- GDOWNSTEP: GOSUB DOWNSTEP 'DO A STEP DOWN AND IF FIRSTEPD = 0 THEN 'IF ITS FIRST STEP THEN PAUSE pause 150 endif DOWNOWAIT: FIRSTEPD = 1 'SET THIS BIT FOR FAST STEPPING GOTO SCANDOWNBUTTON 'SCAN THE BUTTONS AGAIN '-------UP STEPPING SUBROUTINE-------- UPSTEP: VMOTORPOSITION = VMOTORPOSITION+1 'INCREMENT STEP COUNTER IF VMOTORPOSITION = 9 THEN 'RESET COUNTER IF OVERRUN vmotorposition = 1 endif SENDSTEPUP: BRANCH VMOTORPOSITION, [UNOTHING,UP1,UP2,UP3,UP4,UP5,UP6,UP7,UP8] MOTUPSPEED: PAUSE 1 'MOTOR SPEED RETURN 'BACK TO BUTTONS UNOTHING: 'NOTHING UP1: PORTA = 1 'UP STEP SIGNALS GOTO MOTUPSPEED UP2: PORTA = 3 GOTO MOTUPSPEED UP3: PORTA = 2 GOTO MOTUPSPEED UP4: PORTA = 6 GOTO MOTUPSPEED UP5: PORTA = 4 GOTO MOTUPSPEED UP6: PORTA = 12 GOTO MOTUPSPEED UP7: PORTA = 8 GOTO MOTUPSPEED UP8: PORTA = 9 '----------------DOWN STEPPING ROUTINE DOWNSTEP: VMOTORPOSITION = VMOTORPOSITION-1 'DECREMENT STEP COUNTER IF VMOTORPOSITION = 0 THEN vmotorposition = 8 endif SENDSTEPDOWN: BRANCH VMOTORPOSITION, [DNOTHING,DN1,DN2,DN3,DN4,DN5,DN6,DN7,DN8] MOTDOWNSPEED: PAUSE 1 RETURN DNOTHING: 'NOTHING DN1: PORTA = 1 'DOWN STEP SIGNALS... GOTO MOTDOWNSPEED DN2: PORTA = 3 GOTO MOTDOWNSPEED DN3: PORTA = 2 GOTO MOTDOWNSPEED DN4: PORTA = 6 GOTO MOTDOWNSPEED DN5: PORTA = 4 GOTO MOTDOWNSPEED DN6: PORTA = 12 GOTO MOTDOWNSPEED DN7: PORTA = 8 GOTO MOTDOWNSPEED DN8: PORTA = 9 GOTO MOTDOWNSPEED
PS. to other reply: The power supply is very simple. The big 50V toroidal transformer is just rectified and then smoothed with an inductor and capacitor before being routed to the resistor banks that control the amps. You don't really need a voltage regulator because the voltage from it is not that crucial. It's the more accurate voltage from the small amperage, higher voltage transformer that actually sets the spark gap. The two are simply joined by diodes.
The whole thing doesn't take up much room and fits on top of the drill stand where the motor and pulleys used to be. I don't understand why commercial spark eroders have such a massive separate cabinet, it's completely unnecessary. (btw, the biggest advances in edm power supplies have been made by the chinese, they now have small,efficient switch mode power supplies and produce next to zero electrode wear). Right now I'm in the middle of building a pair of injection moulding machines to replace my old Austin Allen moulders. Regards, Mark
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