Plasma CNC Torch Holder

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Last year I purchased a Grunblau CNC table.  I wanted to use it for a number of kinds of fabrication techniques, including milling, plasma 3d printing and laser cutting.  

I decided to start with plasma cutting. To adapt it for that I had to create a mounting mechanism for the plasma torch holder, as well as a water table insert.  

First I created a base to set the CNC table on that I could also use to store the electronics and water storage.  Since I wanted it fireproof, I fabricated the table out of metal tubing and covered it in 12 gauge sheets:

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For the torch holder, I investigated several design approaches.  The height of the plasma torch above the surface is managed by the Torch Height Control (THC) module based upon the plasma current.  Furthermore,  the Z home position is determined via a touch-off from the metal plate triggered by an ohmic sensor.  Theoretically this means the torch can be mounted statically to the Z axis.  However I decided on a spring-loaded coupling with a small amount of travel attached to a passive rail to maintain alignment.  This provides a margin of error in case the ohmic sensor failed.  And as a secondary backup, magnets mount the torch holder to the backing plate.  If the torch binds on something, the magnets disengage and the torch disconnects from the z axis.  Here is the initial design I came up with:

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The Z axis mounting plate has a grid of mounting holes on it.  I first designed a magnetic mounting plate to attach to this with four neodynium magnets.  This way the same plate can also be used for other attachments:

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Then I added alignment pins to keep the torch holder in place laterally.  To adjust the fit precisely I machined my own adjustment spacer:

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I simplified the design further by adding a second carriage to the rail and reversing it such that the rail attaches via rings to the torch and the rail itself floats via tension springs:

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Above the rail I mounted a proximity sensor so that if the torch extends too far into the plate the sensor is triggered, causing a software stop.  

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The other challenge was to figure out how to mount all of the connecting wires to the torch.  The THC controller includes a small wiring hub that goes near the torch to minimize noise between the sensors and the controller. I decided to mount this on the underside of the bracket behind the Z-axis.  Furthermore, I switched to a larger cable carrier with links that hinge open so that it could accomodate the torch cable, which is very thick.  I had to add a spacer between the cable tray and bracket since the new larger tray has a much larger bend radius than the original one:

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Finally the ohmic sensor wire runs down to the tip of the torch.  I used special wire that is chemically resistant and handles high heat to protect it better from the slag:

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Initial tests of the carriage under motion showed that the torch holder held firmly and there was no binding of the wires.  Moreover running with the electronics connected showed the sensor were properly functioning.  I tested the ohmic sensor by shorting between the ground of the frame and the torch head with a screw driver.

Part 2 will cover the water table insert and initial test of the full system.

 

 

 

 

Posted on March 8, 2014 .