Thread: MY 4`x 4` table
well has it been more than a year since i touched plasma table, well now warm weather is hear and im on mend perhaps this year will see it finished, if i can remember what im doing.
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For your Initial Height Sensing (IHS) you can use a simple touch and retract system. This consists of a small linear bearing inside the torch holder bracket. The rail will be about 15mm longer than the bearing. The drive system connected to the rail and the torch holder connected to the bearing. Mount a small limit switch above the bearing. When the torch goes down to hit the plate the height motor continues to drive down until the bearing "lifts" and activates the switch. This is your bottom height sense. You can then use a simple adjustable timer to time the retract lift to get the correct height. The 20mm round type timers that adjust via the front dial are good for this.
Normal arc loss in detected by usng a hall sensor which is basically a current clamp. This is normally locked out for a small time to allow the arc to be established.
Normal arc start is as follows:
Gas flow on.
High frequency on.
Arc current flow is detected by the controller then it ramps up the current to the set value.
Arc height control is normally done via a voltage divider which measures the arc voltage. As the torch height increase from the plate the arc voltage rises and so does the output from the divider. The Arc Voltage Control (AVC) then sees the rise in voltage then signals the head drive to lower the torch towards the plate till the set voltage is reached. Same idea if the torch goes down towards the plate. The AVC normally locks the height control at a set distance before and after corners normally around 25mm. This prevents the torch movement as the drive slows down to take the corner then accelerates away from the corner.
As for piercing plate a normal lead in value is around 1.5 to 2 times the thickness of the plate so for 10mm thick the leadin would be 15 to 20mm.
Fine workmanship, i'm thinking along the same lines myself, although i really need a 8 x 4 table, can you tell me where you got the electronic gubins from. Sorry the engineering bit comes easy, it's the electronics that get my knickers in a twist.
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