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.
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.
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:
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.
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.
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.
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.