[Simonh1959]

It may seem a daft question but as the torch height is so critical to cut quality can you actually measure the physical height (and if so how) that it is cutting at or do you simmply monitor the voltage to calculate the height?

[Torchhead]

It may seem a daft question but as the torch height is so critical to cut quality can you actually measure the physical height (and if so how) that it is cutting at or do you simply monitor the voltage to calculate the height?

There are various methods of measuring actual physical height.(distance)

There is optical (laser, UV, etc) using sensors. The problem is that the flame itself precludes accuracy during a cut and those methods are expensive

There is capacitance sensors. I expect there will be a post shortly on where you can get that form of sensor. The down side is that the capacitance will vary quite a lot at the cut so you have to sense an area outside the cut; thus the metal rings you see hanging down around the torch tip. It's a pretty good method for IHS (intital height sensing) but not superior to ohmic touch. It has limitations for high speed Torch Height Control for plasma It works good for ozy-fuel cutting but then you don't need aggessive height control on 3/4" flat plate.

All if the methods of measuring distance must be precise (and it has to be precise because you are dealing with tiny distances: Cut heights of .060 that need a resolution of no more than .015. )

A funded University study of methods of measuring height for Torch Height Control was done (HONESTLY!) . They spared no expense and did everything from lasers to radar. After months and thousands of dollars the 40 page paper concluded that the best and most cost effective method was to measure the Arc voltage!

Maybe someday there will be a tiny radar nozzle you can just screw on the torch and it will be accurate to .005 and work in all conditions and it will cost $3.89 in quantity.....but until that time you are stuck with conventional Arc voltage method.

TOMcaudle
www.CandCNC.com

[plain ol Bill]

Torchead and Jim Colt would both agree that torch voltage is the best way to measure height and I agree w/ them. Now with that said -- Sometimes I will let the gcode start without the plasma turned on. When it starts to move after the false pierce hit the stop button. You can measure torch height using feeler gages under the tip (Ok Jim - nozzle:nono to double check. I do that occasionally just for kicks.

[Stout]

You could hit the E stop button and then measure the distance between the plate and the nozzle. That should give you the actual cut height for that arc voltage. The THC may need to the disabled before hitting the E stop.

On the T80M, the shield extends below the nozzle. The measurement to the torch would mean the shield? I have measured the distance from the tip of the nozzle to the tip of the shield at .055". At .0625" cut height, that doesn't leave much room to the plate so I'm going to assume the distance is to the shield. Meaning actual cut distance to the nozzle would be .1175" or so. But E stopping the machine and measuring the distance should be .0625" during a cut.

[Torchhead]

On the T80M, the shield extends below the nozzle. The measurement to the torch would mean the shield? I have measured the distance from the tip of the nozzle to the tip of the shield at .055".

Are you sure you have the right shield for mechanized cutting? Check your User Manual for part numbers. It sounds like you have a drag tip shield that has standoff for drag cutting. End of the nozzle should be flush with the end of the shield. Arc Gap is to the end of the nozzle.

[Stout]

I don't think they would make a drag tip for a mechanized torch. I just installed a new retaining cap, shield, and nozzle and measured .055" from the tip of the nozzle to the tip of the shield. I guess that's why they call it a shield.

[Stout]

Here's a shot of the assembled retaining cap, nozzle and shield. The nozzle is fully seated.

[jimcolt]

All Hypertherm mechanized cut charts (in the operators manuals) show the consumables with part numbers at the top of the page...and list both physical height and arc voltage on the charts for those consumables. The physical height suggestion is always a measurement from the closest assembled part of the torch to the plate. If you are looking at the unshielded cut chart...the closest part is the nozzle, if it is a shielded cut chart...the closest part is the shield. This distance will provide the best cut quality with the best combination of angularity, dross and minimal warpage.

Arc voltage listings...on the same cut chart were developed in Hypertherm's labs under ideal conditions cutting at the speed, physical cut height, amperage and consumables on each material thickness. Calibrated lab equipment was used with brand new consumables in the torch.

In a perfect world....doing everything as suggested on the cut charts....and assuming your height control installation is calibrated so that the reading at the height control electronics is exactly the same as at the raw voltage on the plasma cutter control board...the the physical height of the torches closest part will be correct when the voltage is set at the height control to the listing in the Hypertherm manual.

Here is what can cause the voltage/height ratio to be a little "off":

-Air pressure is not right at the torch....higher and lower air pressure changes the resistance of the arc, hence the voltage will be different.
-Moisture in the air changes arc resistance.
-Nozzle wear changes are resistance.
-Electrode wear changes the arc length....which increases the voltage reading at the THC.
-Using the incorrect mix of consumables will alter the voltage / height ratio.
-Poor electrical connection between plate and slats will change voltage.
-Changing cut speed will change arc voltage.
-Changing amperage will change arc voltage.
-Different installation (length of wiring, gauge of wiring) will change arc voltage.
-Calibration of voltage divider circuit, electronics comparator circuit will change voltage.

The point is....arc voltage feedback is a good relaible way for a cnc machine to detrmine if the torch is too high or too low......but there needs to be some human tweaking (on most machines) to compensate for the above issues that can cause the ratio of voltage to height to change. Some THC's actually take a reading of the arc voltage at the beginning of each cut cycle...if the torch is at a known height and calibrate the voltage/height ratio for every cut.

Still, and understanding of how the height control works in a plasma cnc system is the best way for a machine operator to get the most repeatable cutting processes!

Jim Colt Hypertherm

I don't think they would make a drag tip for a mechanized torch. I just installed a new retaining cap, shield, and nozzle and measured .055" from the tip of the nozzle to the tip of the shield. I guess that's why they call it a shield.