Please Note:

The below information is provided to the best of my knowledge and understanding. I’ve gathered this info through sheer trial/error and countless hours of internet searching. These are the most common questions I’ve gotten through youtube and instagram based on my machines. All guidance should be vetted and if there is any incorrect or additional information that should be added, feel free to drop me a note and I will get it updated.

What is a floating torch head and do I need one?

A floating torch head is an additional linear motion attachment that is mounted to the Z axis plate to allow the torch to travel a short distance. While your plasma table may be perfectly square and true to the gantry, steel plate is almost never perfectly flat. Prior to each cut, its recommended to first find the surface of the steel plate before indexing back up to pierce height.

Lats say for example you use a small linear guide that has 0.25″ of travel. You mount your torch to this linear guide and you mount the linear guide to the Z plate. Now, with the Z axis stopped, you will be able to lift the torch by hand that 0.25″ or let it hang at the end of the linear guide. At the top of the 0.25″ stroke, a switch is mounted. Now when you push the torch up by hand, when it gets to the top of that 0.25″ stroke, the switch is tripped. This switch is a reference that will be used to find the surface of the plate.

Lets say you start a program and rapid to your first cut, when you are at the starting point, prior to firing the torch, you will probe towards the steel sheet and use the switch as the trigger to stop. The Z axis will lower, the torch will touch the sheet, and the Z axis will continue downward until the switch is tripped. Now the control software is told that the current Z position is -0.25″ because we know the travel of that floating torch head is 0.25″ from a dead hang to when it trips the switch. If your pierce height is 0.125″ the control software now knows it will need to retract 0.375″ to get to the correct pierce height.

If you get a steel sheet that has a slight warp in it or becomes warped during the cut, every pierce will now be at the proper height since the surface is being found before each pierce. The above is just one way to build a floating torch head. See my YouTube channel for different types of floating torch head designs.

What is arc voltage torch height control and do I need it?

Arc voltage torch height control is an external module that measure the voltage between the tip of the plasma torch and the metal it is cutting. This voltage corresponds to a height, the closer to the metal the torch is the lower the voltage, the further away the torch is, the higher the voltage.

Since steel plate is never truly flat and may warp during cutting, we need a way to keep the torch at the proper cut height the entire length of the profile cut or edge quality will suffer. Most plasma cutter manufacturers will provide a chart of recommended cut speeds, heights and voltages to cut at. These voltages are their measured voltages for the cut height they list, these may vary from your observed voltages due to hardware and steel property differences.

With arc voltage torch height control, to determine your voltage, the best thing to do is shut off the torch height control feedback and make a test cut at the height recommended by the plasma cutter manufacturer. While cutting, note the voltage shown on the THC unit. You can now set this voltage to the desired voltage on the THC unit. When you enable THC feedback and begin a new cut, the THC unit will send move up/down signals back to your breakout board to raise and lower the torch to keep it at the optimal height for edge cut quality.

Smaller tables such as a 2’x2′ table might be able to get away without a THC unit simply using the touch and go of a floating torch head but anything over this size, I would say, must have some form of Torch Height control.

What plasma cutter should I get?

The most direct answer to this one is going to be, the best one you can afford. Something to pay attention to when choosing a plasma cutter is the start type. Plasma cutters that use high frequency to start the arc cause massive amounts of electrical interference and you’ll be pulling your hair out trying to shield and ground everything. I’ve seen a handful of people get these types of cutters working but the lengths they had to go through to keep that electrical noise out of the CNC controls was quite excessive.

Plasma cutters that use a blowback style start or a start cartridge produce little to no electrical interference. I’ve never shielded a single motor or sensor cable on any of my tables and have never had any issues with noise. Any little bit of interferrence I’ve seen could be tuned out with debounce (see this link for a description of debounce Link) or by simply switching the sensor to a ground trigger.

Some plasmas I’ve used or have seen working are Hypertherm, Thermal Dynamics, Everlast, Longevity, and the Lotos Supreme Cut60D just came out which is a blowback style plasma. There are others but the key is to look for the blowback style start. What I’ve done in the past is leave the plasma cutter as the last item to source, while you are building your table, keep an eye on Craiglist for a used Hypertherm or Thermal Dynamics. While these are the best, I’ve had very good results with Everlast and my 4×2 table is still running to this day, issue free with the Everlast Powerplasma 60S.

Should I use Mach or LinuxCNC?

This all boils down to personal preference but I’ve strictly used linuxcnc. When I first started in CNC I was not only on a tighter budget than I am now but also wasn’t sure I could actually get a table together and working. Right out of the gate you’re going to pay a few hundred for mach, linuxcnc is 100% FREE. There is no better price for budget builds than free.

Next, I feel Linux is just a much more stable operating system. I’ve run linuxcnc on my router table for jobs that are 14+ hours, not a single hiccup. I cant tell you how many times I’ve seen the windows blue screen of death or had it crashed because it felt like it. If the PC is going to be controlling a rotary cutter or plasma torch, I want something stable.

LinuxCNC is incredibly flexible if you are willing to spend some time to learn the ins and outs. Most of the logic takes place in the .HAL files. HAL will give you access to the connections between the software side functions and the physical hardware inputs and outputs. I’ve been slowly learning over the past few years how to manipulate these files to add new features, tweak settings, and add UI screens. To me, these commands or functions just look like a bunch of logic gates. I use the below resource to help edit and change what I need.

http://linuxcnc.org/docs/html/man/man9/

Lastly, despite Mach being more popular out of the two there is still a large user base for LinuxCNC and a support forum where I’ve turned to and gotten help many times. Its is still in the end personal choice but I’d recommend taking a look, wont cost you anything.

Website: http://linuxcnc.org/

Forum: https://forum.linuxcnc.org/

What is best: Belt, Rack and Pinion or Ball Screw Drive?
How do I get my drawing from a sketch to a cut ready file?
How do I setup my Limit switches or Inputs (Pull Up/Down)?

This explanation is going to be based on linuxcnc but I would assume that mach3 has similar if not identical concepts.

Most breakout board inputs operate on the basis that there are two states, True and False.

  • 5V = True
  • Ground = False

Your jumper settings for the inputs can either be pull up or pull down, what this means is that the selection you choose will be the state the input is held at.

  • Pull Up – Inputs are held in the TRUE state at 5v. They are “Pulled Up” to 5v.
  • Pull Down – Inputs are held in the FALSE state at ground. They are “Pulled Down” to Ground.

The important thing to remember is that TRUE/FALSE really does not matter much as you can configure a switch or signal to be considered “True”  regardless of whether it is at 5v or ground, let me explain:

For instance, lets take a limit switch example. We have one side of a normally open switch hooked up to pin 11. We have the jumper set to “Pull Up” so the pin is at 5v and linuxcnc sees this pin in the TRUE state. On the other side of the normally open switch, we connect it to ground. Now when the switch is tripped, it passed the ground signal to pin 11 pulling that input to “False”. This state change is what is important. If I were to configure the above pin in linuxcnc the line would look as follows:

  • parport.0.pin-11-in-not

By specifying “not”, it’s telling linuxcnc that I want the action for this pin to be done when the signal in not there, or when its grounded. So the limit switch is tripped, ground is sent to pin 11, Pin 11 is now seen as FALSE, pin-11-in-not is triggered and a limit is triggered.

If you were to examine Pin 11 with the HAL meter in linuxcnc you would see the following pins to monitor.

  • parport.0.pin-11-in
  • parport.0.pin11-in-not

If you had the “Pull Up” jumper set and no input on pin 11, the HAL meter would show the following:

  • parport.0.pin-11-in = TRUE
  • parport.0.pin-11-in-not = FALSE

Change that jumper to pull down and you would get the inverse:

  • parport.0.pin-11-in = FALSE
  • parport.0.pin-11-in-not = TRUE

The control software does not care what type of switch this is, if its a limit, Estop button or Move up/down, arc-ok signals from a THC, it is just waiting for any of the above inputs you configured to go and do what it is supposed to do.

One last example, lets take the Proma THC. It has a common input and then outputs for arc-ok, move up and move down. If you set the Jumper on the breakout board to “Pull Up” you would hook the Proma’s common input to Ground. When the proma needs to send any of these signals, it simply passes the ground signal to the inputs on the BOB and pulls it down to ground.

The best way I have found to configure lease for the lease amount of noise is to always have the pin at the 5v state and trigger on ground. Sometimes this is not possible due to the selection of components but I have found my debounce delay (signal filter in linuxcnc) always has to be much higher if triggering on 5v due to noise in the lines.

Do I need a Water Pan?

Plasma cutting creates a very large amount of fine metal dust and smoke. When I built my first table, I had planned on a water pan but ran a test cut or two open air. Just with a few small artwork pieces, the garage was covered in a thin film of metal dust and the taste and smell of metal hung in the air.

I would advise some sort of dust collection system, be it a water pan or downdraft system. A water pan, in my opinion, is the easiest route to go. I’ve normally run pan from 3-4 inches deep with slats spaced every 2″ to keep tip ups to a minimum. A drain with a holding tank is good to have so you can clean out the table from time to time or retrieve small pieces that fall to the bottom without fishing around in the dirty water. I’ve used the following additive in the past to keep the rust and mildew to a minimum and it has worked well.

https://www.lubeclean.ca/product/greencut-plasma-arc-fluid-lubecorp/?c=81128f937718

Downdraft is basically having the slats open underneath the table but funneling the underside down to an opening that has a suction motor to bring down the dust and smoke. I’ve never run one of these and would assume you would filter the air before exhausting it back to your shop or garage but it is not something I have looked into at length.

What motors should I buy?
What type of linear motion should I use?
Do I need ohmic sensing?

The first three plasma tables I build ran on touch and go (floating torch head) alone. Ohmic sensing is for the most part a continuity circuit. The shield of the torch tip is electronically isolated from the rest of the table. Some manufacturers have a special retaining cap with a clip that allows the isolated shield to be affixed with a wire. The other side of the ohmic module gets attached to some metal that has electrical continuity with the plate being cut. When the Z axis probes the torch down, the instant the tip touches the metal, the circuit is complete and the ohmic module sends a signal to the control board to stop the probe motion.

The reason one would want this over a floating torch head is due to steel sheet deflection. When you cut thinner sheets, the weight of the torch pushing down on the sheet during the floating torch head probe motion may push the sheet down some before tripping the floating torch switch. When this happens, your calculation to retract to pierce height will not be accurate. Piercing too low, or even on the surface of the steel if the deflection is a large amount, is a sure way to damage torch consumables.

There are a few downfalls of ohmic. Since water is conductive, if you use a water table, the ohmic sensing could trigger off of a droplet of water instead of the steel surface which would make the pierce height too large. Its also possible for the ohmic sensing to not trip if the metal or torch shield is dirty so a floating torch head must be used as a backup.

The 4×4 table was the first table I’ve outfitted with an ohmic sensing option and I’ll be doing some testing with it in the future, this is the module I purchased.

https://www.candcnc.com/store-home/feather-touch-ohmic-sensor/

This module is an NPN style sensor so the trigger wire will short to ground. The breakout board it is connected to must have its inputs pulled up to 5v.

21 Comments

  1. Hi, I planned to build my own plasma cutter and I am looking for an affordable plasma. I found a Hypertherm 45 and I would like to know if is compatible with CNC and if it’s worth. It is not an Hypetherm 45 XP. Thank you.

    Like

    1. Yes, the Powermax 45 is an excellent choice. I use an older powermax 1000 with the latest duramax torch, just like the torch design the 45 uses and it work fantastic. You will be happy with the cut quality of the 45.

      Like

      1. Thank you for your fast reply. And what do you think about a Everlast Powerplasma 50S? That Hypertherm is second hand and I don’t know how it was used, and the price is 1200 euro. With that price I can buy a new Everlast…. I don’t know… On the other hand the seller have also a Hypertherm 30… the price is more reasonable….about 700 euro… I don’t know what to do.
        I have no experience with plasma, and I have a lot of questions…

        Like

      2. I found an Everlast provider in Europe and now I have the opportunity to choose between Everlast Powerplasma 50S without CNC connector and one Everlast Powerplasma 60S with CNC connector. What do you think? It is a real advantage to have that CNC connector?

        Like

      3. I am preparing for a build based on your videos. (BTW Fantastic Job) I noted your reply above regarding the Powermax 45. I have located a Powermax 45, with machine torch, but it is an XP. Is this designation model one I should avoid? Purchasing this one would get me the torch and machine for $2070.00 including shipping!

        Thank you.

        Like

  2. Unless there was a change that I am unaware of, the 50s has the cnc port. I had a 50, the model prior to the 50s and it had the port. Everlast’s website now shows the 50s with the cnc port so either one will be fine.

    Like

  3. Sorry I left out one question in my previous comment/question about the Powermax 45XP. Does this cutter use a blowback style start or a start cartridge which produce little to no electrical interference?

    Like

    1. Craig, the 45XP is the newer and improved model of the powermax 45, you should have no issues if this is the cutter you go with. As for the start type, yes, all the duramax series torches are blowback style which aren’t noisy compared to the high frequency start type. I Use a durmax torch on my hypertherm and have zero shielding. The very small amount of interference I get is filtered in linuxcnc with a few debounce statements.

      Like

  4. Got the table running with the 45XP.
    Having a rough time getting the Price THC to work.
    Anybody have tricks to get the price unit to work

    Thanks
    Jim s

    Like

      1. The voltage that reads on the price unit is in the range of 300 volts . The output from the hypertherm 45 the 50 :1 green plug show about 2 volts as tested with multimeter. At 50:1 I would expect to read 100 v on the Price….not 300 . There is no movement of z motor
        I have tried to get some tech support from price but there has been no response to my numerous emails …..is price still in business ?
        I am about to shit can the price.
        What is your THC choice at the present
        Thanks for all your help
        The table works great…….except for the price CNC

        Like

  5. Jim,

    On the main screen, with the torch off you have three values, 0 measured volts, a default of 100 desired volts and then a tolerance value. When you fire the torch, what does the measured voltage read? I make test cuts at my desired cut height and speed with the THC shut off in the software to observe the voltage. Voltage correlates to height and speed. Once the edge quality is dialed in you set the desired voltage to the measured voltage of your cut using the jog wheel. I also recommend you adjust the arc-ok with a wide value since I believe the price only sends adjustment signals when the voltage is within the arcok range. My price unit has been working great since day one, cut a bunch of 1/4″ plate yesterday without issue.

    Like

  6. Hello, I’m building a cnc plasma table , replicating your Lighter Duty 4×4 CNC Plasma table, Are you selling any steel plates yet? I would love a set for my build

    Like

  7. Hi there,

    Currently I’m also building a CNC plasma cutter. There frame and gantry are almost done so I started soldering the HobbyCNC chopper board. (This would be the 3-axis hobbyCNC EZ REV0 board). I was wondering how I’m going to control the plasma cutter via this board because no pins are actually left. How do you control the plasma cutter from linuxCNC? Did I buy the wrong board? Meaning I should buy one of the PRO boards?

    All help would be welcome!

    Like

    1. I used the pro board on the first build I did. Linuxcnc or any other machine controller needs an output pin to drive a relay to turn the torch on. You may be able to white wire a connection to the bottom of the dsub connector on the board to grab one of the “unused” pins and define it as an output in the software. Just as an FYI the hobbycnc board, even the pro board, did not provide enough voltage to drive a 5v relay, I ended up having to get a buffer chip to go inline with the 25pin cable to boost the voltage before it entered the hobby board.

      Like

  8. Hi Stefan, great work do you make and great web page too!
    I´m building a cnc plasma cutter, but in your experience, which THC do you recommend? the proma compact THC SD / 150 or the AVHC10 that do you use in your machine? Thanks
    Jose Luis

    Like

  9. I’m trying to plan a 4×8 table for both plasma and cnc if I buy the electronics/motor kit from candcnc will I still be able to use a spindle and do 2.5d carving along with halftoning ? I cant find anyone using candcnc linux with a router setup seems like mach3/4 is what is normally used for this

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s