Ohmic Sensor vs. Floating Head

[shiner2001]

Since I've had my table, I've had a floating torch head and magnetic breakaway. I've become curious if it's worth it to upgrade to an ohmic sensing setup. The big advantages I can see is that with thin material, I wouldn't get any deflection of the material, and time. On projects with lots of pierces, it seems like the job would go quite a bit quicker with the ohmic sensor, just based on watching videos of how fast that sensing process is. Thoughts from anyone that's used both setups or have a recommendation one way or the other?

[SeanP]

Good question that Derek, I'm interested in this as well, also does having a water table cause more problems than it's worth?
Pity you couldn't switch between both, not sure if that's possible.

[grindergary]

I have tried both. Ohmic worked well for downdraft situations but with a water table, if there is any water when it goes to sense top of material it can give a false reading causing torch to fire with too high a pierce.

I have tried using air to blow away water but I like to work on other things around the table and not have to babysit the torch all the time. With a touch off, I can walk away for a few minutes and not worry it will have any problems.

HTH

Gary

[jimcolt]

Ohmic sensing was first developed about 25 tears ago on high end industrial Plasma cnc machines. On all of these machines today it is the standard way of doing IHS (Initial Height Sensing), or finding the surface of the plate. Accurately locating the surface is key to correct pierce height, and on many cnc plasma height controls that use auto voltage sensing (also known as "voltage sampling) the ohmic sense is used to recalibrate the zero point before every cut. This recalibration ensures that the correct torch to work distance is always maintained regardless of consumable wear (without recalibrating the torch will get closer to the material as the electrode and nozzle wears) and speed changes made by the operator (if the operator reduces cut speed between cuts...expect the torch to move closer to the material, with voltage sensing the torch will recalibrate and maintain the correct physical cut height).

So, to answer your question...Yes, Ohmic sensing is the best way to set an accurate pierce height, and with voltage sampling it is the best way to set the real time cut height (no need with these systems to set an "arc voltage"). The drawbacks with ohmic sense are with its sensitivity to highly conductive water from the water table....it will sense the water if it is over the top of the material (or even a droplet sitting on top of the material), think that is the surface.

One fix I have done in the past that only works if can manipulate machine code....is to issue a momentary (Maybe 1/2 second pulse) on signal to the plasma cutter when the torch is moving towards the material, this momentary start pulse will start the air flow (post flow) on the torch...which will blow water away from the sensing area, allowing the ohmic sense to sense the plate, not the water. The torch (Hypertherm anyway) will refire during this postflow airflow.

Jim Colt Hypertherm

Since I've had my table, I've had a floating torch head and magnetic breakaway. I've become curious if it's worth it to upgrade to an ohmic sensing setup. The big advantages I can see is that with thin material, I wouldn't get any deflection of the material, and time. On projects with lots of pierces, it seems like the job would go quite a bit quicker with the ohmic sensor, just based on watching videos of how fast that sensing process is. Thoughts from anyone that's used both setups or have a recommendation one way or the other?

[WyoGreen]

I just yesterday installed the Ohmic sensor on my table. I have not yet cut with it, but will shortly. My reason for the change from the floating head was because I am cutting more thinner gauge metal now than I used to. As far as water on the plate (I use a water table), my reasoning is that the plate is usually dry on the first pierce, and the air rarely shuts off after that, so any water on the plate should be blown off on subsequent pierces. Seeing as you get deflection on the thiner gauges, I can't see where the thickness of water film would be much different from the deflection. If anything, you'd be piercing and starting the cut a bit higher with the water, rather than a bit lower on the deflection issue, which should be a little easier on the consumables.
Like Jim mentioned, it shouldn't be hard to change the post to start the air before the pierce if the need arises to blow water off the plate. Perhaps HT could have an option to just let the air run all the time, or make the post flow adjustable to fit the needs of a particular table.

At any rate, only time will tell which I like better. Steve

[acourtjester]

Wyo you are going to retain the floating head switch as a safety in the cases that the ohmic sensor may not detect the metal surface.
I moved mine to the limit switch input from the home position for that reason.
I also have a water table and following air stream from the torch between cut locations usually blows any water away from the next pierce location.

[Brand X]

Most floating heads are not really designed all the well for thin metal. If you take the time to get it right, Like zero drag, and carefully matching the spring torch weight,it's excellent. Long lightweight springs, and very high quality low drag rails, are the key success.You can't lump them all in the same category, because there is a huge difference in getting it correct or not..Got to love it on rusty old saw blades, when setup correctly.. Zero issues down to the .035 I cut..

[East German]

Hallo

I have both systems and can turn off the sensor with a switch.

My water bath is 160 mm below the sheet metal. So no problems with the water.

When painted surfaces are cut is the floating head the number one!

The floating head is a backup system if the other is not working when this happens by slag or dirt.
The way of my switch ist 0.5 mm the height is then still ok.

I find you need both!

Regards Peter

[jimcolt]

Most high end industrial plasma systems use Ohmic as the primary surface sensing, and automatic switch to torque sensing or stall sensing (sensing a slight increase in z axis motor torque caused by contact with the material....stall is used with steppers, torque with servos). It is nice to have both.

Jim Colt Hypertherm

[shiner2001]

I run a downdraft system, so it sounds like the ohmic setup might work well in my application, especially since I cut mostly 16 gauge material.

[tcaudle]

A lot of this depends on which system you have. The MACH based system uses one input (Z home) for the touch off and the ohmic sensor becomes the Touch off sensor. It has code (if you are using the POST from SheetCAM) to detect a tripped sensor BEFORE it starts the touch off and it also has a "slow down" zone (Plunge Safety Clearance" set in SheetCAM where that slow down starts. The Default (set by SheetCAM) is tiny.....019" That needs to be about .375 to .5"

The MUCH faster touch offs you are seeing is probably the newer LINUX based controls that do everything a lot faster and

The CommandCNC Linux systems have the option to use two inputs for the touch off (probe function) so if the ohmic misses, the mechanical switch is used instead and the stored offset for the switch is used. You can also hook it up so the mechanical switch is E-Stop (limits do not work well because Limits are ignored on a ref move) The dual touch system still checks the ohmic before it starts the probe move to make sure it's not tripped which would give a false touch-off

While auto set (sample and hold) of the Preset volts is interesting for flat cuts, it has to be turned off for doing a cut on sloping material , diamond plate and corrugated.
Cutting thin material can be a challenge. Sometimes you do not have the time to even attempt to do the sample and hold time at the beginning of a cut. The method of auto setting the cut voltage , pierce delay, DTHC delay , Initial cut height from the G-code as defined by a tool set, allows the cut to start and progress instantly. In fact, on cutting corrugated, it does not need a beginning flat spot to do a touch off to get a valid sample.

The ohmic touch off has become the most popular Option on the systems; so much so that we are making it part of our Special bundles.
Ohmic does tend to speed up on even the faster system because the mechanical switch needs to back off the switch first to detect the untrigger event before it rapids up to pierce height where as the Ohmic untriggers a lot quicker.

The method of using a counter balance (spring, weight , etc) to reduce the Z downforce on the floting head is also good idea but a porrly desinged Z can cause problems by sticking and not letting the torch drop back down. You have to keep it clean and lubed.

[SeanP]

The method of using a counter balance (spring, weight , etc) to reduce the Z downforce on the floting head is also good idea but a porrly desinged Z can cause problems by sticking and not letting the torch drop back down. You have to keep it clean and lubed.

Yes I get that with the linear bearings on the velox, you need to keep on top of them with lube.

Thanks for the detailed explanation.