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Ubatoid
01-04-2011, 11:57 AM
Let me just throw this out there for consideration or comment. From what i can see the router and milling boys boys use an electrical contact to get a material hight where us plasma boys it is generally accepted that a floating torch is used. My question is that surely adding an electrical contact to the shroud of the plasma torch this method could be used to acheive the same when plasma cutting ?

Just a thought, would save a lot of work on a floating head...

Teknition
01-04-2011, 12:27 PM
Yes, it can and is done. commonly referd to as "touch n go", "touch and go", or ohmic contact. Touch and go works with a microswitch in conjunction with a floating head on the Z axis. I beleive ohmic contact would utilize a resistance value between the torch shield and the workpiece.

Brad

Ubatoid
01-04-2011, 01:02 PM
Ahhh, my next question is going to be, is there any pro and cons to either method ?

CPierce18
01-04-2011, 01:54 PM
I'd like to add a question to that if I may. If I were to hookup a wire to my plasma torch shielding cup, and one to my material grounding wire, so when the machine's torch moves down to find the surface before the cut it completes the circuit and zero's out… If I were to do that, would I fry my breakout board when my plasma fires? I'm confused with that, and i ReAlLy don't want to try it and chance it.

Ubatoid
01-04-2011, 04:07 PM
Good question. A simple test with a meter will tell you if you are getting any voltage at the shield when the arc is on. I would suspect your shield is ceramic though ? so you shouldnt. I enviseged using simple probe attached to the torch or similar. Just a thought.....

Teknition
01-04-2011, 05:24 PM
Ahhh, my next question is going to be, is there any pro and cons to either method ?

Im no expert at it by any means, but from what I've gathered, when using the ohmic method, the sheet has to be very clean or it doesnt work well. So if you are cutting steel that is rusty, ohmic would be a second choice unless you really enjoy grinding each sheet before cutting.

As far as I can see, the only real issue to touch n go would be if you were cutting very thin sheets. When using touch n go, the torch comes down and touches the sheet, causing the torch slide plate to be lifted and therefore closing the microswitch. You can counter load the slide plate with spring/s to take care of some of the torch weight but there must always be some weight present to cause the torch to fall and open the switch. So, if you are cutting very thin sheet, you could run into problems with the sheet bending a bit when the torch travels down.

As for the wiring question, I will have to refer that one to someone else. From what I have read, the table itself may be in excess of 100 volts when the plasma is running. That is why a proper earth ground rod should be used on the table. My gut feeling is that wont work, otherwise no one would be going thru the trouble of a slide plate and micro switch ;) .Maybe Tom from CandCNC will stop by and answer this one.

Brad

Ubatoid
01-05-2011, 08:22 AM
You raise a good point there. Whilst the bulk of the thinner sheet we use is spotless, the thicker material does tend to be scaley / rusty. Debate closed I think.
Thanks for the input.

jjellingson
01-05-2011, 10:09 AM
I notice on youtube plasma videos, many people tend to touch off on the begining of all cuts and some don't. Advantages? and are they adding these extra moves into the code for each reference? Thanks in advance.

jimcolt
01-05-2011, 11:24 AM
OK.....I'll clarify a few things!

Ohmic contact, or the use of electrical contact between the front of the plasma torch and the plate....to accurately locate the surface of the plate has been used on almost all industrial plasma systems since about 1986.

Most industrial height controls have a two phase IHS (initial height sensing) system that at the beginning of every cut cycle indexes the torch towards the plate until the shield (or on some older technology torches, the nozzle aka tip) contacts the plate, (sensed electrically) then the torch retracts to the recommended pierce height and fires. If the plate is not sensed electrically due to a painted, masked, or other non conductive surface, then a backup IHS using torque sensing (an increase in z axis motor torque is sensed when contact with the plate occurs) occurs.

Torque sensing works well on plate that does not deflect, ohmic sensing works well on plate that does deflect (thin plate) as it will sense the surface without any moving.

I will attach a picture of a Hyperthem Powermax45 torch with the ohmic retaing cap and shield, and a wire attached to complete the circuit.

So, Ohmic sensing is the most accurate method of finding the surface of the plate, assuming the surface is electrically conductive.

It should be relatively easy to make the ohmic process work on a z axis height control, as long as you have the correct torch with the ohmic capability. On entry level cnc machines (low cost) I know of at least two that use the ohmic IHS system on their torch height controls, they are PlasmaCam and Dynatorch.

Jim Colt Hyperthem

stirling
01-05-2011, 12:48 PM
Hi Jim - just to echo CPierce18 above: A thing I've yet to figure out is what is the voltage between the shield/nozzle/tip that your wire is attached to and the table/work when your CUTTING? i.e. is it torch volts?

Thanks

Ian

jimcolt
01-05-2011, 01:01 PM
During cutting....the shield and the nozzle are floating....in other words there are no hardwired connections to either. However, if you connected either to an oscilloscope....you would see all kinds of wild voltages during cutting as the arc and the plate is very close to each of these, and there will be transient voltages present. The nozzle in a plasma torch is at the same potential as the plate during the pilot arc only.....as soon as the arc transfers to the plate the plasma system electronically disconnects the nozzle from the positive side of the circuit (older plasma systems actually used a relay called the pilot arc relay).

The height control system, when it uses the shield to sense contact with the plate to locate the surface....does this before the arc fires, then the connection is usually (depending on the height control manufacturer) disconnected via a high voltage relay while the torch is running.

Some height controls do monitor the shield while the torch is cutting to detect collisions with the plate....and they retract to the proper physical cut height when a collision occurs (Hypertherm industrial height controls have this ability).

Hopefully that answers your question.

Jim Colt

stirling
01-05-2011, 01:38 PM
Hopefully that answers your question.


Certainly does Jim

Thanks

Ian

Teknition
01-06-2011, 12:50 AM
If the plate is not sensed electrically due to a painted, masked, or other non conductive surface, then a backup IHS using torque sensing (an increase in z axis motor torque is sensed when contact with the plate occurs) occurs.
Jim Colt Hyperthem

Jim,
Thank you for the education on ohmic contact, I knew someone much more versed in it thank I would come along. Since the original poster appears to be building or thinking of building his own table, could you clarify a few points.

Is it correct to say that for ohmic contact to work reliably on clean and not so clean steel, you need the 2 phase system?

Do you know of anyone offering entry level electronics for a DIY build that would support torque sensing?

Do Dynatorch and Plasmacam units have torque sensing?

Regards,
Brad

jimcolt
01-06-2011, 08:41 AM
1. I own a PlasmaCam......and it is very rare that I ever have a "failure to sense" the surface with the Ohmic contact. With their system a failure to sense will simply pause the part program....the operator can reset it in seconds.

On industrial systems which are expected to run 3 shifts a day without a hitch...it is necessary to have a redundant system. If for instance there was a piece of tape on the plate somewhere...the ohmic would not function. Some shipyards also prime their plate before cutting...this would not work with ohmic. Many industrial systems cut underwater an inch or two to protect factory workers from fumes and ultraviolet as well as to cut down on audible noise (slight expense of cut quality in terms of edge roughness)...ohmic will sense the surface of the water.

2. I do not know of anyone currently offering Ohmic contact for a do it yourself torch height control....you may want to speak with Tom at www.candcnc t ask this question though....I know he has some new developments on the way.

3.I believe the Dynatorch has torque sensing as a backup, the PlasmaCam does not.

It is kind of funny....you hear from many suppliers that don't have the ohmic contact function on their products knock the functionality of this method of finding the plate surface. It is a very simple way to sense the surface, and very accurate, and it works (for me) about 99.9% of the time. Having a 2 phase version with torque sense backup....or you could use "floating head" backup (real simple). Seems like a no brainer to me! If it didn't work well....would the height controls used on high dollar industrial machines use it?

On the Hypertherm height controls the ohmic function is also used as a collision sensor after the cut has started. If the torch is cutting along at .062" off the plate and it hits a piece of slag....the ohmic senses it and the electronics rapid moves the torch up 1/2 of the programmed cut height distance, then delays for a few milliseconds before monitoring arc voltage again. This helps to avoid obstacles or thermal movement of the plate.

Another function that the Hypertherm height controls use the ohmic contact for is....on really thin plate, when the ohmic function senses a millisecond late (dirty surface) and deflects the plate maybe 1/4"...the torch then reverses but does not start counting distance (encoder feedback) until the shield loses contact with the plate. This ensures that pierce height is correct even on thin plate that can easily deflect.

Jim Colt





Jim,
Thank you for the education on ohmic contact, I knew someone much more versed in it thank I would come along. Since the original poster appears to be building or thinking of building his own table, could you clarify a few points.

Is it correct to say that for ohmic contact to work reliably on clean and not so clean steel, you need the 2 phase system?

Do you know of anyone offering entry level electronics for a DIY build that would support torque sensing?

Do Dynatorch and Plasmacam units have torque sensing?

Regards,
Brad

Ubatoid
01-06-2011, 02:09 PM
Jim, Legend !
There is a lot to consider there for my new build, I will certainly add ohmic to the list of stuff for consideration. Thanks for the input.

Teknition
01-07-2011, 12:38 AM
Thanks for the well explained answers Jim. I can see where ohmic contact could be handy especially when monitoring for tip ups and slag. Im still unsure about the practicality for a home build until there was alot more support from the electronics guys.

Brad