The guide to building CNC plasma cutting machines

Blatantly copied and pasted from:
https://forum.linuxcnc.org/plasma-la...-a-lot-of-info

From extensive experience on this forum and on cnczone.com and cncecke.de, the main problem for most users wanting to build any kind of cnc machine are the plans and dimensions, it makes it easier to understand how everything fits together and the interaction of parts therein.
I am aware that there are a lot of plans on the net, most of them are utterly useless, some are usable and anything that needs to be paid for ends up being even more useless than the free ones, some are way to complicated and ............
Moving on,
The main idea is to have some basic information with some drawings and dimensions, a few versions of it with the bill of materials for each of them and the estimated cost.
For the materials to be used:
- suggest using HiWin or THK rails on bigger ones, round bar can be used on smaller ones, no ball bearings ( skate bearings ). The reason for not using ball bearings is that most of hobby builders do not have a drill press or a mill to drill precise holes and hand drilling usually ends up in frustration and giving up all together. All the other versions, like using precise ground plates coupled with ball bearings or V groove bearings will end up costing nearly the same as linear guides, sometimes more expensive. Also using chains is not an option, they tend to vibrate and those vibrations will show on the final cut.
- rack and pinion are usable and easy to set up to a certain extent, the reduction needed is a bit trickier to make, buying ready made reductions costs a lot, raising the total cost beyond hobby budget.
- using belts is the best option for several reasons, like no vibrations, easy to set up and cheap, but very functional.
- belts can be driven directly from the motor shaft with a small pulley ( GT2 belt with a 14 tooth pulley would yield a 28mm movement for one revolution of the motor making it nearly perfect for plasma, GT3 with 12 tooth would be 36mm, not ideal but still usable), but this implies using micro stepping drives for smooth motion.
- moving portion of the machine ( gantry with the Z axis and floating head ) has to be as light as possible and as rigid as possible to enable fast moves and direction change and not allow for vibrations and consequent distortions on the resulting cut.
- THC should be included, hence the floating head mentioned above, for the price of today's THC systems it would be a pity not to have it.
- Gantry can be made out of a single square tubing with dimensions ranging from 80x80x2 to 120x120x3 and anything in-between, i mostly use 80x80x2 as it is sturdy enough and very light for up to 3 meters length. Using aluminium profiles is also nice but much more expensive, again i use rarely Bosch Rexroth 90x45mm profiles.
- motors, steppers are much better and easier to work with for beginners as they have no feedback and the subsequent EMI isolation problems.
- Plasma power supplies produce huge amounts of interference during use, so basic grounding rules have to be followed, shielded cables for signalling are a must and keeping the computer and drives as far away from it is essential.
- Routing cables to the motors to avoid going in parallel with the plasma cable is also very important to avoid induced currents messing up things badly.
- computers, almost anything can be used, from older ones ( even P1@1000Mhz ) to new ones, but the new ones may have more trouble with latency as they tend to have a lot of power saving options, although with the Preempt-RT kernel they do work nicely. When using Mesa or Pico hardware motion control that becomes less important as the step generation and/or feedback is handled in the external hardware. If using bare main boards it becomes implicit to use metal enclosures for all the electronics.
- frame and table, made out of metal tubing or profiles are easy to make, require welding or drilling and bolting, or aluminium are easier to assemble but more expensive. I exclusively make the frame and the table separate to avoid having the plasma and the electronics ground to the same potential as the computer and electronics have the minus side wired to the ground and the plasma will end up having the positive side wired to the ground , the table always with water as it will hugely reduce the amount of smoke and prevent warping.
Part 2
- usually on smaller machines i do the gantry on the wide side so it makes it a lot easier to handle bigger material, case in point, here we can get 2x1m or 2.5x1.25m or 3x1.5 or 6x2m plates, so on machines with 2x1m work area i do the gantry on the 2 meter side so material 2 meters wide can be cut, resulting in the capability of using anything found here for material. This requires 2 motors on one axis and that can be handled in several ways: having both motors wired in parallel to a single drive ( imperative use of the same cable length on both motors ), in series ( cables do not matter but the max velocity is limited as the overall induction of the motors is larger and it takes more time to reach max. current draw from the drive ), two drives, one for each motor and controlled by the same outputs, or two drives controlled by separate outputs ( very nice for squaring the machine if limit switches are provided, requires using version 2.8 of Linuxcnc).
- for the Z axis "floating head" is a very nice feature as it enables probing for the material automatically, this coupled with a THC is a must for thinner plates. Usually on smaller machines i use drawer slides with ball bearings found on older photocopiers or in shops, use the "double" one so it can do the probe move on one rail and the Z axis movement on the other rail. This makes for a very light and functional head/torch holder, with the added magnets taken out of old computer hard drives ( neodymium or rare earth magnets, very strong and light ), used as a break away in case torch hits something while cutting. The probe part of the torch holder uses a light spring so any torch collision will trip the probe switch and bring the machine to a stop preventing incidents that might occur.

Plasma needs as a minimum 4 inputs to work properly WITH THC,
- Probe or floating switch
- ARCOK signal from plasma or THC ( Proma THC 150 provides that from analysing the voltage )
- UP from THC
- DOWN from THC
That is with no limit switches or home switches. For that even the parallel port has enough inputs.
Some parallel ports can be configured for IN or OUT or X mode so the number of inputs and outputs can be changed as needed. Adding another parallel port PCI or PCI-express card is always a cheap option for more I/O.
7i96 from Mesa has 11 inputs so that will provide for the above and as below
- E-STOP
- 3x Home switches for all three axis that should also be used as limits for their respective axis
- 3x Limit switches for the negative side of their respective axis.
That should complete a normal machine.
On machines that use DC servo motors i use always another set of limit switches ( 2 for each axis ) as extreme limit switches that are wired to a relay that will cut the power to the machine in case one of them is triggered. This is in case the drives fail, usually they will short one of the outputs and send all the power to the motor so even stopping the enable signal will not work in some cases.
Adding some physical switches to the control panel is also very useful so that would require more inputs or a Mesa 7i73 that has also encoder inputs for MPG and can control a small display and some keyboards. All this is still cheaper than a touch screen monitor.

Will leave this post for further general info on cnc plasma cutting and add separate posts for actual machine info.
If anyone has any idea regarding what should be here, you are welcomed to add it. Thank you in advance.
As a guideline , a small 1x1 meter work area machine should be a nice starting point, a 2.5x1.5m should be as the middle and a 6x2m is the big one. Open to advice on the naming and any other info.
Regards,
Tom

- - - Updated - - -

Info on the small machine.

As a guideline,
- 1x1 meter or thereabouts is a nice starting point for a usable machine.
- using HiWin or THK linear rails, 15 or 20 mm ones is a nice compromise between cost and functionality and a very long life, they do not get blocked from dust as they have vipers
- unsupported round rails are also usable from 20mm and up, they do sag a bit under load but that is not important if using a THC
- supported round rails will end up costing nearly the same as the above mentioned linear ones as there must be 4 rails, 2 for each axis and 2 carriages per rail.
- a single carriage per rail is more than enough for a plasma, so the total cost for 3 meters of rail and 3 carriages can be from 250 to 300 Euro ( usually there are 2 carriages for one rail at that price )
- GT2 belts 6mm wide are more than enough for this size and weight, cost for 5 meters with 5 pulleys (16 or 20 tooth) is 5 to 10 Euro
- stepper motors, Nema 17 or Nema 23 work fine, as long as the nema 17 are at least 40mm high, or 1.2A. Some Nema 17 from older photocopy machines have GT2 14 teeth pulleys, 2 of them can move a 20KG gantry easily up to 12 meters/minute without ever loosing position. Using Micro stepping drives is essential for direct drive, info bellow at the motor section. Cost anything from 40 to 100 Euro for all 4 of them.
- Z axis and floating head, drawer slides with ball bearings ( 2 of them or one dual slide ), M6 or M8 all thread with a coupling to the motor, nema 17 are always enough, even 0.8A ones. A single limit switch for probing, more if limits and homing are to be used. Cost around 20 Euro.
- A simple welded or bolted steel structure is all good for the machine, same goes for the table. Welding a water table is not an easy task so ask for help or leave it to a professional. It works without the water table, but the dust created during cutting is very fine heavy metal dust and breathing it is not healthy at all. Cost anything from 50 to 200 Euro.
- THC preferably Mesa THCAD, more complicated to install and set up, much quicker and more control for making finer cuts. Proma THC 150 is easier to set up and use, it works perfectly with Linuxcnc, it is a bi slower, Generates it's own ARCOK signal so no need to fiddle with the plasma power for that. Cost 100 to 200 Euro.

Estimated total cost from 400 to under 1000 Euro for a fully functional, fully automatic machine that will last a long time happily cutting all day every day. Add to this the cost of the plasma power source and a decent compressor.

Some detailed pictures of such a machine are here:
forum.linuxcnc.org/plasma-laser/37614-ne...ry-complicated-build

- - - Updated - - -

Info on the medium sized machine.

Anything from 2x1 meter to 3x2 meters work area i would consider a medium sized machine, so any info below pertains to those dimensions.
-- Explanation: -- a wide machine is the one having the wide gantry and short sides, i.e. a wide 2x1 meter machine has the gantry on the 2 meter side, while a narrow one has the gantry on the 1 meter side. Besides being much more usable for cutting wide plates, the total length of rails is shorter. A wide 2x1m requires 4 meters of rails, the narrow one needs 5 meters, same goes for belts or rack.
- rails, 15mm are enough but i would prefer 20mm, cost would depend on the size and would range from 350 to 500 Euro.
- motors, again for the X and Y axis nema 17 can be used but the 1.4A to 1.7A variety, nema 23 works perfectly, nema 34 are to much as they would vibrate to much. Cost is anything from 60 to 200 Euro.
- belts, again GT2 6mm wide is enough even for the 3x2m one, anything wider would be less flexible, hence better for this use and the price difference would be negligible in the overall price of the machine. Aim for 10mm wide. Cost 30 to 50 Euro.
- Z axis and floating head, drawer slides with ball bearings ( 2 of them or one dual slide ), M6 or M8 all thread with a coupling to the motor, nema 17 are always enough, even 0.8A ones. A single limit switch for probing, more if limits and homing are to be used. I would suggest using two floating heads in case you get 2 plasma cutters later on or use 1 for plasma and 1 oxyacetylene torch, the latter would not need a floating head just a normal Z axis. Separate configs can be made for plasma and for oxyacetylene cutting so it would switch between heads as needed. Cost 20 to 40 Euro.
- A simple welded or bolted steel structure is all good for the machine, same goes for the table. Welding a water table is not an easy task so ask for help or leave it to a professional. It works without the water table, but the dust created during cutting is very fine heavy metal dust and breathing it is not healthy at all. Cost anything from 150 to 600 Euro.
- THC preferably Mesa THCAD, more complicated to install and set up, much quicker and more control for making finer cuts. Proma THC 150 is easier to set up and use, it works perfectly with Linuxcnc, it is a bi slower, Generates it's own ARCOK signal so no need to fiddle with the plasma power source for that. Cost 100 to 200 Euro.

Estimated total cost from 700 to under 2000 Euro for a fully functional, fully automatic plasma and oxyacetylene machine that will last a long time happily cutting all day every day even 100mm thick plates depending on the oxyacetylene torch. Add to this the cost of the plasma power source and a decent air compressor and the acetylene torch with the required hoses and oxyacetylene bottles.

One such machine, a bit bigger at 3.6X1.35 meters cutting area is here with the obligatory pictures:
forum.linuxcnc.org/plasma-laser/37691-3-...rs-cnc-plasma-cutter

- - - Updated - - -

Info on the big machine. 6x2 meters.

Just to be clear, this is not something that can be done as a hobby or home made, this requires a lot of tools and experience, costs a lot, so it is not to be taken lightly, especially not to be started as a project and later giving up on it.
The end result should be a fully functional machine that can be used 24 hours a day every day, and that implies using heavy duty parts, reliable electronics, a lot of safety features especially if using servo motors and drives. Also a lot of compromises are inevitable for cost/functionality/reliability to get to an acceptable point.
- This has to be a long machine as making a sturdy 6 meter gantry is not an easy task.
- Rails, 20mm are enough but i would prefer 25mm for the long axis, 15mm or 20mm for the short one. The only problem is there are no 6 meter lengths of those types of rail, only 4 meters, so the minimum for a 6 meter rail is 35mm for THK and HiWin, so this is where the supported round rails come in as they can end up cheaper but much harder to find at that length. cost would depend on the size and would range from 1500 to 2500 Euro.
- Motors, again for the X and Y axis nema 23 can be used but the 2 N/M variety for the long axis with reduction, nema 23 works perfectly for the short axis, nema 34 are also nice with reduction for the long axis. Cost is anything from 150 to 500 Euro for steppers, at least 1200 to 2000 for servo motors with included drives.
- Belts, again GT2 9mm or 10mm wide is enough for the 2m axis, preferably 15mm, at least 25mm wide for the long axis, preferably 40mm wide. Cost anything from 150 to 300, pulleys included.
- Rack and pinion is much better for the long axis with reduction, This would require 14 meters of rack for both axis and 3 pinions. Cost 250 to 400 Euro
- Z axis and floating head, drawer slides with ball bearings can also be used here but i would prefer something much sturdier, M8 all thread with a coupling to the motor, nema 17 is enough for the slides, nema 23 for anything bigger and stronger, A limit switch for probing and 2 for limit/home. I would suggest using two floating heads in case you get 2 plasma cutters later on or use 1 for plasma and 1 oxyacetylene torch, the latter would not need a floating head just a normal Z axis. Separate configs can be made for plasma and for oxyacetylene cutting so it would switch between heads as needed. Cost 50 to 400 Euro.
- A welded steel structure is all good for the machine, a bolted one will not be easy to make. Same goes for the table. Welding a 6x2 meter water table is not an easy task so professional help would be required. It works without the water table, but the dust created during cutting is very fine heavy metal dust and breathing it is not healthy at all. Cost anything from 450 to 1500 Euro.
- THC preferably Mesa THCAD, more complicated to install and set up, much quicker and more control for making finer cuts. Proma THC 150 is easier to set up and use, it works perfectly with Linuxcnc, it is a bi slower, Generates it's own ARCOK signal so no need to fiddle with the plasma power source for that. If anyone has a better idea please do share. Cost 100 to 200 Euro.

Estimated total cost from 2500 to under 5000 Euro for a fully functional, fully automatic plasma and oxyacetylene machine that will last a long time happily cutting all day every day even 100mm thick plates depending on the oxyacetylene torch. Add to this the cost of the plasma power source and a decent air compressor and the acetylene torch with the required hoses and oxyacetylene bottles.

- - - Updated - - -

Info on plasma power supply, torches, consumables, wiring, troubleshooting, HowTo get the ARCOK signal form older ones etc.

Most of the plasma sources can be used, the list follows
- Scratch start ones, or cheap chinese ones should be avoided, they can be made to work with a little programming, but to unreliable.
- Blow back ones, can be used with the required software modifications, but i do not use them, again not reliable enough due to miss fires.
- High voltage start ( HV start ) ones can be used always, they do work nicely, are reliable and produce less interference
- High voltage/high frequency start ( HV/HF start ) can be used always, very reliable but produce a lot of interference making shielding and grounding of utmost importance.
--- notice --- some plasma sources lack the current limiting inductors so care is needed as not to have a situation where the output can be short circuited. As some of them have the main lead wired to the nozzle, having that nozzle touch the material while cutting for prolonged times can result in thermal shut down, or dead transformer if the thermal shut down is removed. Seen a lot of these issues.
- wiring torch on control from the computer is easy, a resistor + a darlington transistor and a relay is all that is needed
- getting the ARCOK signal is a bit trickier and can be done in 2 ways, with a HAL sensor or with an induction coil, more on that later.
- Getting the voltage is also easy enough and making a voltage divider requires some resistors, again easy enough.
more to come...

- - - Updated - - -

Drives, motion control, feedback, wiring, using relays to avoid EMI problems, etc.

- Drives, for small sized version any microstepping drive can be used that can provide 1.2 to 2A and 1/8 micro steps, 1/4 can be used but it will have a bit more noise and vibrations.
- for the medium sized machine drives should have 2 to 4A and 1/8 or 1/16 micro stepping.
- for the above two machines, even the TB6600 drives can be used, they do work although they tend to be a bit tricky as they do produce a lot of electrical noise and have no protection against it built in, adding capacitors to the power inputs and sometimes to signal inputs makes them usable.
- the big one should use high quality stepper drives, at least 3A and 1/4 or 1/8 micro stepping as they have to be used with reductions or servo drives with reduction, servos need properly shielded cables or they will fail to work as soon as the torch fires.
- Personally i use Lam Technologies DS1044 and DS1076 drives as they are protected against anything, even reverse polarity or shorts on the outputs and cross wiring of the motor leads. The downside is you have to buy a usb dongle to program them that costs 28 Euro's and is usable for all their drives, only one needed. The micro stepping is by far the best i have used and in over 6 years i had no failures, and the rating on them is the real Amperage, not peak or RMS or whatever, so do use lower values for testing.
Wantai drives are also usable but the rating on them is way to optimistic so anything less than the DM860 is not recommended, except for the small one that can use the DM542 type drives. DM420 is to small and to sensitive, they crap out easily. Same goes for Leadshine, nice drives, usable for sure but at a price point near the Lam technologies drives.
- Powering the drives, there are plenty of power supplies that can be used, for the first two versions 19V to 24V is enough at 4 to 6 Amps, usually i use laptop power bricks that work perfectly, for the big one 48 Volts at 10 Amps is plenty. Properly grounding the computer and the drive power supply is a must, they must be at the same potential.
-Motion control -
- A computer with a parallel port or an add on PCI or PCI-Express card will do just fine, but it has limited number of inputs so no limit switches can be used with a single parallel port, 2 parallel ports will provide enough Inputs and outputs.
- Using Mesa or Pico boards is nice if you can afford it as it will provide much faster step rates for stepper systems and full feedback control for servo systems, with plenty of I/O.
- Break out boards, in most cases i do not use them at all, the cheap ones are way to slow and the expensive ones are well, expensive. So get a Mesa board, namely the 7i96 or 7i76E or, 5i25 or 6i25 with 7i76 for stepper systems and a 5i25 or 6i25 with 7i77 for stepper systems.
- There are also other possibilities like using steppers with encoders or clear path servos that are not servos etc etc.
- Wiring -
- This part is essential for a reliable machine.
- Plasma cutters make a lot of electrical noise, induce huge amounts of electricity and HF/HV start causes a lot of RF interference at every ignition, so shielded cables for all signal wiring should be used. Shield should be tied to ground only on the electronics side, not the machine side. Shielded motor cables should be used when using HV/HF start, otherwise can make do without shielding.
- Relays should be used on all inputs for all types of machines, so at least two, one for probe input and one for ARCOK input are a must. If using limit switches, do use relays for each axis or for each switch. For big machines, especially when using DC servo drives, extreme limit switches wired through relays to cut the machine power are a must, meaning another set of limit switches besides normal limit switches, to cut ALL power to the machine.

There is so much confusing and plain wrong informations about pilot arc and everything related to HF HV plasma etc, that i am baffled ! How did that happen? Where was i when that happened? Who allowed that to happen? ...... :) :) :)
Oh nevermind, internet is slowly but surely becoming the worst place to get proper informations, just sifting through that makes the head hurt.
Do i sound a bit grumpy ? :)

Scratch start = the torch has to physically touch the material it is cutting to strike an arc

Blow Back = has a partially moving nozzle that when pressed against the material it shorts to ground and as it is released it strikes and arc.

Pilot arc = has the means to strike an arc all on it's own even when far from material and more importantly it can maintain that arc for some time without touching or moving any torch parts. This makes it very reliable and that is why almost all expensive ones have one of two types of pilot arc, and this is why cheap Chinese ones insist they have pilot arc although most of them do not.
The implications of using a pilot arc are far from simple and it takes quite a bit of explaining so i will try a short version:
there has to be some provision inside such power sources to protect the rectifier inside, the rectifier can not be disconnected and connected again after a successful arc transfer as that would cause much more trouble etc etc.

Then there are 2 types of pilot arc start systems the HV or High Voltage and the HV/HF or High Voltage and High Frequency
The HV uses a transformer to raise the voltage to some 10 to 20 thousand Volts ( this is why the protection above mentioned ) and use that voltage to strike a maintainable arc between the electrode and the nozzle. This is done at 50 or 60 Hz frequency so it is much easier to isolate and causes less interference to other equipment.
The HV/HF is a bit different as it also changes the frequency of that high voltage to several KHz, hence the inherent eagerness to induce itself on anything and everything around it at some fairly long distances.

Now some more info about the interference caused by plasma power sources:
They all make interferences ! ALL! Some more some less, but for us who use them with CNC machines the most important thing regarding the two types we use, namely the HV and the HV/HF types is : NOT IMPORTANT, it absolutely is not important what of the two types you have, they are very reliable and work perfectly well given the proper grounding and shielding.
As a reference of why they are the same in terms of interference, is the fact that the HF is a bit harder to isolate, but the HV makes much stronger interference.

Hi, Tommy
Im thinking of building very budget cnc plasma for diy hobby stuff. Very valuable information in your post and as you reposted there is really a confusion about pilot arc etc.
I have a budget plasma cutter i bet its HF because every cheap one is HF - I have upgraded it to have a Pilot Arc. So my question is - will a HF plasma cutter produce less interferences with pilot arc vs touch start or it will be the same?
Thanks again for the information you shared.