It is currently Tue Nov 13, 2018 12:14 am

 

Crud on tip causing poor Ohmic sensing

CNC Plasma Cutters are amazing and there is always plenty of knowledge to share and discuss. Use the CNC plasma cutters general forum for this purpose.
Thor
Posts: 133
Joined: Wed Jun 17, 2015 6:57 pm

Crud on tip causing poor Ohmic sensing

Postby Thor » Fri Sep 28, 2018 9:04 am

Is there anyway to stop crud or small slag balls from getting on nozzle tip and causing the ohmic sensor to fail which then triggers estop and have to clear it. I'm cutting sheet metal alot and on every sheet I probably have to clear an estop and quick wire brush the tip off atleast 20 times, it gets annoying and takes time. Using finecut parts and have it running with almost no slag on underside except in corners and small holes. Have tried antispatter spray, nothing seems to help.

So far the only thing I can think of is to upgrade from the current MP3500 DTHCIV to the newer Command cnc as I see that says it can use the floating head switch as a backup height sense. But I'll have to save up for that.

User avatar
acourtjester
Elite Contributing Member
Elite Contributing Member
Posts: 3206
Joined: Sat Jun 02, 2012 6:04 pm
Location: Pensacola, Fla

Re: Crud on tip causing poor Ohmic sensing

Postby acourtjester » Fri Sep 28, 2018 9:30 am

As far as the Floating head switch you can add that to your system now. Understand when using a floating head switch with an ohmic sensor it is a safety and will not act like the ohmic sensor. By this I mean if the ohmic sensor does not stop downward movement the next thing is the floating head switch and it will act like a limit switch and stop all movement. Many here have their tables setup that way. I cannot speak about the Command CNC, Maybe users or Tom C. will answer that here. I think that it has more switch inputs and that may help with the way you want to hook things up.
DIY 4X4 Plasma/Router Table
CandCNC DTHC-IV & Feather Touch
Hypertherm PM65 Machine Torch
Drag Knife and Scribe
Miller Mig welder
13" metal lathe
Small Mill
Everlast PM 256 tig

Thor
Posts: 133
Joined: Wed Jun 17, 2015 6:57 pm

Re: Crud on tip causing poor Ohmic sensing

Postby Thor » Fri Sep 28, 2018 10:23 am

Thats the way it is now, though its wired as an estop.

jimcolt
Elite Contributing Member
Elite Contributing Member
Posts: 2941
Joined: Mon Jul 20, 2009 11:18 pm
Location: New Hampshire

Re: Crud on tip causing poor Ohmic sensing

Postby jimcolt » Fri Sep 28, 2018 12:01 pm

If material is sticking to your shield and you see arcing between the shield and the material while cutting....then you have shorted the nozzle to the shield. This occurs if you are piercing too close and/or if your pierce delay time is too short and the torch starts moving before the pierce is complete. Too close or too short on delay allows a slug of molten metal to blow back between the nozzle and shield, keep in mind that the shield is supposed to be electrically isolated from the nozzle. When the shield becomes the same electrical potential as the nozzle you will get an arc that jumps from the nozzle to the shield, then to the material. This takes power away from the cut (poor cuts and dross) and destroys the nozzle orifice very quickly. Here is how to minimize this occurrence: 1. Be sure the pierce height that you have set is actually being achieved, and that this height is what the Hypertherm cut chart suggests for the material and thickness being cut. Higher is ok as long as the torch indexes down to cut height while still on the lead in (longer lead in can help). 2. Be sure the pierce delay time is adequate.....any movement before the arc is fully through that material will allow the molten metal to enter the space between the nozzle and shield, shorting these two parts. The book spec for pierce delay is correct assuming that your cnc machine is monitoring the arc transferred signal and starts its time cycle based on that. Some machines have different timing.....so ensure the time is long enough to pierce the material before the x or y axis start to move. 3. Keep the shield face clean and use a light coating of mig welding anti-spatter spray to keep spatter from sticking. 4. Be sure your cut height is correct. Most Hypertherm processes have a book height of .06", any closer may allow spatter and blowback into the nozzle and shield, any higher and you will get worse edge angularity. Keep in mind that as the electrode wears inside the torch that the torch will move closer to the material.....on machines where you must set a preset arc voltage (voltage is proportional to height) the operator must occasionally increase the voltage in order to maintain height. Each volt that you increase on the height control preset voltage will raise the torch by about .004", and the pit on the end of the electrode will wear to as much as .060" when it is at the end of its usable life. This means the operator must increase the arc voltage by as much as 15 volts on the preset before the electrode is spent in order to maintain correct cut height and minimize arcing between the shield and the material. (some cnc machines use auto voltage sampling and do this automatically). Jim Colt Hypertherm

Thor
Posts: 133
Joined: Wed Jun 17, 2015 6:57 pm

Re: Crud on tip causing poor Ohmic sensing

Postby Thor » Fri Sep 28, 2018 8:27 pm

Its not arcing between tip and shield, its more just a black crud coating or occasionally about a 1/32" dia slag ball on the shield face that electrically insulates the torch so that the ohmic doesnt work. I'm thinking its normal but just looking for other ideas on how to limit it. The new linux based control that can use a secondary touchoff when the ohmic fails is probably the best answer, but that doesnt help when trying to gently touch off on warped thin sheet metal. The thicker stuff like 14 ga I'm starting to just set it to go longer between touch offs as it doesnt move much.

jimcolt
Elite Contributing Member
Elite Contributing Member
Posts: 2941
Joined: Mon Jul 20, 2009 11:18 pm
Location: New Hampshire

Re: Crud on tip causing poor Ohmic sensing

Postby jimcolt » Sat Sep 29, 2018 8:13 am

The 1/32" diameter slag ball on the shield face is caused by arcing between the nozzle (note that there is no part called a tip) and the shield, and it occurs for reasons listed in my previous post. I use ohmic sensing daily, and have for well over 20 years (8 years with my current Powermax85), and I have been plasma cutting for 40+ years. While this "slag ball" occurs, it is not normal if you do things right. Just trying to help! If you read my previous post there are suggestions on how to make the ohmic work better for you. There are many cnc machines that have ohmic as a primary sensing device (because it is far more accurate than other surface sense technology, especially on materials thinner than 3/16" or so) and auto back up to torque sensing, limit switch sensing (aka "floating head"), stall sensing and others. Done right, ohmic works great, the backup processes are good when material surface is not conductive.

Jim Colt Hypertherm


Thor wrote:Its not arcing between tip and shield, its more just a black crud coating or occasionally about a 1/32" dia slag ball on the shield face that electrically insulates the torch so that the ohmic doesnt work. I'm thinking its normal but just looking for other ideas on how to limit it. The new linux based control that can use a secondary touchoff when the ohmic fails is probably the best answer, but that doesnt help when trying to gently touch off on warped thin sheet metal. The thicker stuff like 14 ga I'm starting to just set it to go longer between touch offs as it doesnt move much.

User avatar
SeanP
Elite Contributing Member
Elite Contributing Member
Posts: 790
Joined: Thu Aug 01, 2013 11:50 am
Location: Co Kerry, Ireland

Re: Crud on tip causing poor Ohmic sensing

Postby SeanP » Sat Sep 29, 2018 11:45 am

I am plagued by the same troubles as well, so much so that I rarely use the ohmic setup now (luckily I can switch between ohmic & touch off)

I'm not having any problems with spatter getting under the shield it's just the surface that touches down on the plate.

If I'm on a big job I will have a couple of shields on the go, swapping over when needed, using the ceramic mig spray certainly helps it sticking, but still need to tap odd bits off while it's on the move with a paint scraper.

My pierce delay is fine as is pierce /cut height.
Powermax 45 with Duramax torch.
Home built table, Candcnc DTHC 2, Dragoncut 620-4
R-Tech 210 Tig, Jasic 250 mig
Sheetcam, Scanything, Coreldraw X8, Solidworks
Table build gallery

jimcolt
Elite Contributing Member
Elite Contributing Member
Posts: 2941
Joined: Mon Jul 20, 2009 11:18 pm
Location: New Hampshire

Re: Crud on tip causing poor Ohmic sensing

Postby jimcolt » Sun Sep 30, 2018 9:03 am

You do need to wipe the surface that contacts the material every few hundred starts. If you spray the ceramic spray on the face of the shield that touches the surface....expect the ohmic sensing to not work. I spray the ceramic coating on the inside of the shield and let it cure for an hour or so....this minimizes the chance of material shorting between the nozzle and shield....which causes the ohmic to not work correctly. Jim Colt Hypertherm

tcaudle
Posts: 1051
Joined: Tue Feb 28, 2012 6:47 pm

Re: Crud on tip causing poor Ohmic sensing

Postby tcaudle » Mon Oct 01, 2018 3:59 pm

To THOR: Don't spend the money to upgrade to the CommandCNC that lets the Z home be an active backup for the Ohmic sensor. It won't help if the ohmic shorts . That will still cause it to stop and throw an error. It detects if the Ohmic is active (shorted) BEFORE it starts the touch - off and stops. In MACH its a MACRO you could remove but then it would touch off up in the air and the pierce would not work. You might try piercing higher and doing a touch off before every pierce (decrease ref distance to 0) Try to determine when it blows back the slag. Only use fine cut on thinner material (14ga or thinner) . I use conventional 45A tips (ok Jim nozzles) on everything down to 16Ga. Lots more forgiving and works at wider arc gaps. Make sure you have a good workclamp connection to the PLATE you are cutting. Just laying on the slats may not give you full current flow.

There are other reasons to upgrade but fixing your problem with back splash slag is not one of them .

Ironken
Posts: 86
Joined: Thu Nov 30, 2017 7:29 pm

Re: Crud on tip causing poor Ohmic sensing

Postby Ironken » Fri Oct 05, 2018 6:38 pm

My only problem with ohmic is that soot and schmoo will build up on the nozzle and shield after many cuts and my machine will fault out during probing. My solution....a small stainless wire brush and a few passes over the shield/nozzle and a quick wipe with acetone on a rag. Back to cutting.

Thor
Posts: 133
Joined: Wed Jun 17, 2015 6:57 pm

Re: Crud on tip causing poor Ohmic sensing

Postby Thor » Fri Oct 05, 2018 9:22 pm

My problem is more like Ironken's. Its not that the Ohmic is shorting out its that the black crud on the tip is not allowing it to sense the plate when probing, thus driving down till the floating head switch is tripped triggering an e-stop.

islander261
Posts: 146
Joined: Sun Jan 12, 2014 2:30 pm

Re: Crud on tip causing poor Ohmic sensing

Postby islander261 » Sat Oct 06, 2018 11:51 am

Hi

I have had problems with ohmic sensing for years (water table doesn't help) but I need to use it because I only cut thin sheet. My solution is a custom Gcode touch off subroutine that checks for a shorted torch before probing and has automatic fallback to the floating head switch. This is quite a complex piece of code because it has automatic retries and stops on hard errors so the operator can resolve the problem rather than make a bad pierce. I can post the code if anyone wants to see it. One word of warning is that it is for a LinuxCNC control and most likely will not work with Mach 3. I also clean the torch every time when I put a new sheet of material on the table.

John


Return to “CNC Plasma Cutters General Forum”

Who is online

Users browsing this forum: No registered users and 2 guests

Easyscriber engrave attachment for Hypertherm torch's