CNC Plasma cutters, general idea and a lots of info for building them.

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22 Dec 2019 12:43 #153025 by tommylight
If you plan on cutting thin material up to 3 or 4mm, Ohmic sensing is better, anything thicker and floating head is the way to go.

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22 Dec 2019 18:37 #153055 by rodw
Take care with your torch selection. The plasma cutter you shared does not have a central connector as shown on your ebay torch so the two are not compatiible out of the box.

And yes, you might be able to do ohmic sensing using the hypersensing circuit. It will at least be be electrically safe.
forum.linuxcnc.org/plasma-laser/37311-hy...-to-do-ohmic-sensing

But the blowback mechanism will probably short the tip to the material while not cutting becasue the arc is established this way before the mechanism :"Blows back" to separate the dead short.

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02 Jan 2020 10:56 - 02 Jan 2020 10:56 #153784 by The_wolf_of_walmart

If you plan on cutting thin material up to 3 or 4mm, Ohmic sensing is better, anything thicker and floating head is the way to go.


I would imagine I'd be cutting thicker than that MOST of the time, but I'd hate to sacrifice the ability to cut one over the other. How difficult is it to cut <4mm with floating head and vice versa, >4mm with ohmic?


Take care with your torch selection. The plasma cutter you shared does not have a central connector as shown on your ebay torch so the two are not compatiible out of the box.

And yes, you might be able to do ohmic sensing using the hypersensing circuit. It will at least be be electrically safe.
forum.linuxcnc.org/plasma-laser/37311-hy...-to-do-ohmic-sensing

But the blowback mechanism will probably short the tip to the material while not cutting becasue the arc is established this way before the mechanism :"Blows back" to separate the dead short.


Ah my mistake- Wrong link; They do make that torch with the correct connection as well.

I'm finding a lot of general description on blowback start but I can't find anything that illustrates what is actually happening in the torch. Under what circumstances would the piston be energized, the torch not cutting, and low enough to short to my material? I would assume the arc starting height is taller than the piston before it blows back?
Last edit: 02 Jan 2020 10:56 by The_wolf_of_walmart.

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02 Jan 2020 11:09 #153785 by thefabricator03

The_wolf_of_walmart wrote: How difficult is it to cut <4mm with floating head and vice versa, >4mm with ohmic?


It can be a big pain the the ass.

I have done a bit recently with 1.6mm black hot rolled sheet and if the material is warped the torch will come down for a touch off - push the sheet down until the limit switch trips - moves up to pierce height then lowers to cut height but the material will push back up and cut height will be lower than the top of material. Then the torch drags and destroys the part.

Only way around this is to baby sit the machine and keep the material flat by pushing it down with a length of rod or RHS until the cut has started and the THC will take care of the warped sections and maintain the correct arc length.

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03 Jan 2020 04:42 #153860 by The_wolf_of_walmart
Ok I can definitely see why floating head is bad for thinner materials. Why is ohmic bad for thicker materials?

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03 Jan 2020 05:15 #153864 by thefabricator03
Its not bad at all,

The problem I have been having with the ohmic sensor is false readings triggering a stop.

I cut all sorts of parts and one of them has a large slot in the center, When the air from the torch blows water from the center of the slot into the torch is causes a fault reading and stops the machine.

I have not had time yet to play with RodW's new hypersensing design but it should have some ability to tune the reading voltage to overcome some of the false readings.
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03 Jan 2020 07:50 #153868 by rodw

thefabricator03 wrote: I have not had time yet to play with RodW's new hypersensing design but it should have some ability to tune the reading voltage to overcome some of the false readings.


I'm not sure if it will! I never had any probe errors until I added a water table. Sometimes I get a false trigger via a short on a water splash much higher than the material. But what I've found is that Plasmac recovers so perfectly I don't stress about it. I just hit resume and keep going. Pulsing the torch to get post flow going before the first pierce helps I think. And also pulsing the torch is helpful clearing water out of the torch before resuming. I also now park the torch in the middle of the table at the end of a sheetcam job so water does not go everywhere.

I actually tried cutting some 4mm at 40 amps for the first time last week. The first part was a failure as it was not cutting right through so I reduced the torch volts by 3 volts and nudged the amps up a little bit on the fly and by the time I was ready for the next cut, It was pretty much perfect. So I just need to reduce the cut height by 0.3 mm and see if the on the fly changes can be translated to final settings. The next cut was perfect and the dross just knocked off pretty easily.

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03 Jan 2020 08:10 #153871 by tommylight

The_wolf_of_walmart wrote: Ok I can definitely see why floating head is bad for thinner materials. Why is ohmic bad for thicker materials?

Floating head will work if you have finer slats under it, it will work anyway you look at it, but it will need some help from you with thin material.
Ohmic is better at sensing thin materials but it is less reliable than floating switch/head.
Personally i never use ohmic, usually we cut 3 to 6mm, and rarely 1.5mm and 8 and 10mm, so floating works perfectly.
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03 Jan 2020 10:17 #153880 by rodw

tommylight wrote: Ohmic is better at sensing thin materials but it is less reliable than floating switch/head.


Having started with just a floating head, I have to say that the benefits of ohmic sensing is vastly superior to a floating head. Some say its not suitable on rusty steel or painted surfaces but its fairly unusual to cut this. Most of us are cutting new sheets of metal. IIts not tested but I think the hypersensing should allow probing on rusty material and possibly painted surfaces by setting a reduced threshold

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03 Jan 2020 10:22 #153881 by thefabricator03

rodw wrote: But what I've found is that Plasmac recovers so perfectly I don't stress about it.


My main problem with the false triggers is I need to walk back down to the machine and restart it. Gets old after the 8th or 9th time. But most hobbyist would not have a problem its production runs that it causes issues.

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