HF VS Blowback

[rehoward]

My question now is about the advantages or disadvantages of HF VS Blowback as a method of torch ignition when using a CNC cutting table.
I have a Lotos LTP 5000D plasma cutter. It has a max of 50amps and features an HF starting arc. I have read that either method will work for a CNC table plasma cutter.

I have also have been told that the HF system will be a real chore to get working properly in this application. I have also been told that the HF will destroy my electronics if I do not shield everything.

So, here enters the blowback system which does not use HF. So no electrical interference to destroy my control stuff. But then I was told that my plasma unit will not be able to use a Blowback type of arc starter. Why wouldn't my machine be able to use a blowback torch?

So to me this is beginning to seem like two guys arguing over the virtues of Ford against Chevy. Would some unbiased folks please help clarify what the best choice is for my DIY plasma table and help me to discover the path of enlightenment?

Randy

[adbuch]

Hi Howard,
Here is a quote from Jim Colt: "Make sure the plasma you buy has a “blowback” start (nearly all newer systems do) rather than a high frequency start, which will interfere with the PC based machine control and electronics."
You can read Jim's article here.
David

https://www.hypertherm.com/en-US/learn/ ... sma-table/

[adbuch]

Howard,
Here is a quote directly from Lotos Technology: "All of our machines have a High Frequency start meaning that we do not recommend that our machines and torches are used for CNC. Generally for best results and for ease of set up, it is recommended to have a Low Frequency start for your plasma cutting machine and torch in order to prevent blow back."
You can read the entire article here.
David

https://www.uwelding.com/blog/lotos-ltp ... alex-gall/

[rehoward]

Well it looks like I am screwed. Lotos says it can be converted to a low frequency pilot arc but do not say who does that or what it costs. I have yet to understand what is special about a blowback start machine. Why won't it work on my machine?

[adbuch]

What is special about the "blowback" start is that is does not create large amounts of high frequency noise. For hand cutting it doesn't matter, but for cnc work with a machine controlled by a pc and associated electronics - this high frequency noise can disrupt the electronics and cause the cnc table to malfunction. There are less expensive plasma cutters that do use the "blowback" start. A friend of mine purchase one from Harbor Freight several years ago with the "blowback" start for around $400 with coupon. You can easily tell, because when you push on the electrode with your finger it is spring loaded and will move back up into the torch housing. The high frequency start plasma cutters don't do this.

Here is an example of a low priced plasma cutter with "blowback" start. Price is $529 on Amazon. Perhaps you could purchase on like this and sell your old HF machine. It would probably be much cheaper than the cost to convert your machine to "blowback" operation - if it is at all possible to do so.
David
https://www.amazon.com/Friendly-Non-HF- ... B076FDQC2P

HeroCut55i.jpg

[Rodw]

There is no doubt that HF start fills the environment with electrical noise and also the 25,000 volts or so it generates on startup is prone to fry electronics. I can give you a list of things I would do to use it but as said, it might be cheaper to buy another plasma machine.
1. Be very careful when designing you control panel to lay it out defensively for noise.
2. Use an ethernet connected control card so it is galvanically isolated from your PC
3. Use industrial standard 24 volt logic circuits becasue it is much harder for noise to alter the voltage enough to cause a spurious fault.
4. Be sure you tap into the arc voltage before the HF start circuit so you don't have to contend with that 25000 volts
5. If you have Ohmic sensing, fit a decoupling capacitor where the Ohmic signal enters the control panel
6. Follow best practice star ground wiring (which is worth doing on any machine)

You can achieve most of these things is you use Linuxcnc and a Mesa ethernet card (the 7i96 is the cheapest option) and their THCAD-10 torch voltage board (subject to 4 above).

[adbuch]

There is no doubt that HF start fills the environment with electrical noise and also the 25,000 volts or so it generates on startup is prone to fry electronics. I can give you a list of things I would do to use it but as said, it might be cheaper to buy another plasma machine.
1. Be very careful when designing you control panel to lay it out defensively for noise.
2. Use an ethernet connected control card so it is galvanically isolated from your PC
3. Use industrial standard 24 volt logic circuits becasue it is much harder for noise to alter the voltage enough to cause a spurious fault.
4. Be sure you tap into the arc voltage before the HF start circuit so you don't have to contend with that 25000 volts
5. If you have Ohmic sensing, fit a decoupling capacitor where the Ohmic signal enters the control panel
6. Follow best practice star ground wiring (which is worth doing on any machine)

You can achieve most of these things is you use Linuxcnc and a Mesa ethernet card (the 7i96 is the cheapest option) and their THCAD-10 torch voltage board (subject to 4 above).

Hi Rod,
Have you personally done this? If so, have you ever had any sort of noise problems that have affected your electronics? We don't know if Howard is planning to build a system from scratch, or if he is planning on purchasing a turn key system.
Thanks,
David

[Rodw]

Well he was quite clear about DIY I thought.

Would some unbiased folks please help clarify what the best choice is for my DIY plasma table and help me to discover the path of enlightenment?

Most of the things I mention I have experience with. Except Item 4 (becasue I spend the money on blowback systems) and 5. which is a solution from a HF electronics specialist to solve an issue with my neighbour's MAXPRO 200 which triggered a probe fault from time to time due to noise. It was on a $70k commercial table and clearly the manufacturer had no clue.. and a $15 part would have solved it.

And yes, I bought a oscilloscope to investigate noise in my arc voltage signal with a cheap plasma cutter.
And yes, I added a star ground system at that time.
It seems crazy but the source of noise seemed to be coming through the mains and it was controlled once I added an EMI filtered IEC connector.
And yes, I designed my control box from the ground up with noise in mind.
And yes, I use a 24 volt logic system which is galvanically isolated from the control box.
I'm quite confident I can build a reliable CNC plasma table with a HF start machine. But its not worth doing on a entry level machine. The one I had died before I built my table.

The Herocut you mention is an interesting machine at a crazy price but I don''t know anybody using one yet. IT includes raw arc voltage, arc OK and torch on pins so well worth considering on a budget. I;d love to get some feedback on it.

[adbuch]

Rod,
I missed the DIY - sorry. Thanks for your feedback. You are in the UK? I once used my Tektronix 2235 to investigate a HF noise situation that was causing my Plasmacam "store-bought" table to do the "sewing machine" routine. It turned out to be the coiled up work clamp cable under the table. Jim Colt set me straight on that one. It was not a problem when using my 45xp, but showed up when using my 85. Apparently the Hypertherm 85 model produces much more high frequency switching noise than the 45xp.
David

[rehoward]

Learning about the differences of HF VS blowback leads me to another question. Let's say I have several CNC machines in my shop and none are powered up. Would use of a manual troch with HF still endanger the CNC units nearby even though they are not active?

[rehoward]

PS - I am making plans to sell my HF plasma machine. It is a Lotos model LTP5000D and it is completely brand new, never used, and has resided in a box under my small breakfast table in the kitchen for 1-1/2 years. I am willing to sell it cheaper than it can be purchased on Amazon, and no shipping costs if you are local.

Randy

[adbuch]

Learning about the differences of HF VS blowback leads me to another question. Let's say I have several CNC machines in my shop and none are powered up. Would use of a manual troch with HF still endanger the CNC units nearby even though they are not active?

No

[adbuch]

PS - I am making plans to sell my HF plasma machine. It is a Lotos model LTP5000D and it is completely brand new, never used, and has resided in a box under my small breakfast table in the kitchen for 1-1/2 years. I am willing to sell it cheaper than it can be purchased on Amazon, and no shipping costs if you are local.

Randy

Perhaps post it in the for sale section of this forum. Or your local Craiglist. Or keep it as a second machine for non cnc applications.
David

[rehoward]

I sold the Lotos machine last night. So far the HeroCut is the only budget machine I have found with a blowback ignition system. There is very little info on the web for it.

Randy