How to Build a Plasma CNC Machine
A practical build sequence for engineers who cut plate, not sheet. We cover frame stiffness, motor sizing, torch height control, and the settings that decide whether your edges come out square or slagged. Read it before you buy the first length of steel.
In this article
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Key takeaways
What a plasma CNC machine can and cannot do
Before you learn how to build a plasma CNC machine, decide what it is for. Plasma cuts mild steel, stainless, aluminum and copper plate by melting a kerf with an ionized gas jet. It is fast and cheap per meter on 3–25 mm plate. It is not a milling machine and it will not hold ±0.005 mm. Expect ±0.5 mm on a well-built 1.5 m table and ±1.0 mm on a large gantry without compensation.
The process leaves a heat-affected zone, a slightly tapered kerf edge, and a dross layer on the underside if parameters drift. If your part needs a bearing bore, a sealing face, or a thread, plasma is the roughing step and a CNC mill is the finishing step. We see this pair constantly in fabrication shops: plasma the blank, then send it to a 3-axis or 5-axis machine for the critical features.
Build it if you cut the same families of plate repeatedly and want the nest to run without an operator standing at the torch. Skip it if your work is mostly one-off curved profiles in thin sheet. A fiber laser or a waterjet handles those better, and a hand torch plus a straight edge is cheaper than a bad gantry.
One more boundary. Plasma needs a dry, clean air supply or a gas cylinder. Moisture in the air line is the single most common reason a new build cuts well for ten minutes and then starts leaving holes and heavy dross. Budget for a refrigerated dryer and a coalescing filter before you wire anything.
Frame and gantry: where accuracy is won
The frame carries every other decision. Build a torsion box or a fully welded rectangular frame from square tube, not angle iron. Angle twists under load. Use 100 × 100 × 6 mm tube for tables up to 1.5 × 3.0 m; step up to 150 × 150 × 8 mm beyond that. Weld in diagonals, then stress-relieve or at least let the frame sit a week before you machine the rail mounting faces.
Rails must be parallel to within 0.1 mm over their full length and coplanar to the same figure. Bolt them to a machined pad, not to raw tube. If you cannot machine the pad, shim with ground stock and check with a dial indicator and a long straight edge. Skipping this step is why home-built tables cut a parallelogram instead of a rectangle.
The gantry is the moving mass. A dual-drive gantry with a motor on each side resists racking far better than a single center screw. Keep the gantry tube as short as the cut width allows. Every extra 100 mm of unsupported span adds deflection you will see as taper on the far side of the table.
For the Z axis, 100–150 mm of travel is enough for hand-fed plate up to 25 mm plus a torch body. Do not buy a 400 mm Z axis because it was cheap. Long Z travel means a long cantilever, and the torch tip moves when the carriage accelerates. That motion shows up as a wavy cut edge.
- 1Tube wall6–8 mm minimum; thinner tube rings and transmits vibration into the cut.
- 2Rail parallelismWithin 0.1 mm end to end, checked with a dial indicator on a carriage.
- 3DiagonalsMeasure corner to corner; the two figures should match within 0.5 mm.
Motors, drives and the electronics package
Stepper motors are the usual choice for a first build. They are cheap, they hold position at rest, and they need no tuning. Size them against the moving mass, not the table size. A 3 N·m NEMA 23 on each side of a light gantry moves 20 m/min comfortably. If your gantry weighs more than about 40 kg, move to NEMA 34 at 8–12 N·m or switch to closed-loop steppers.
Servo motors cost three to five times more but hold torque at speed and recover from a bump without losing position. On a table over 2 m wide with a heavy gantry, servos are the calmer choice. Either way, match the drive current to the motor rating and set acceleration in the controller to 300–800 mm/s² for a first run. High acceleration on a light frame is how you find every loose bolt.
The controller turns G-code into pulses. Mach3, LinuxCNC, and GRBL-class boards all work at this scale. Pick one with a breakout board that gives you isolated inputs for the limit switches, the torch height control, and the e-stop. Isolated inputs are not a luxury; plasma noise will trigger false limits otherwise.
Route the torch leads away from the stepper and encoder cables. Separate conduits, or at least 150 mm of spacing. Ground the table to a single earth point, and earth the plasma power supply case separately. Ground loops on a plasma table cause lost steps that look like a software bug.
Torch, table and workholding
Match the torch to the thickness band you actually cut. A 45 A air plasma handles 6–12 mm at good speed. A 65 A unit reaches 16–20 mm. A 105 A machine cuts 25–32 mm but needs a three-phase supply and a serious air dryer. Buying a torch far larger than your work wastes gas and gives you a hotter, wider kerf than the part needs.
The table surface is a consumable. Slats should be 3–5 mm thick, spaced 60–100 mm apart, and replaceable. Mild steel slats are fine; they will spatter and need flipping or replacing every few months. Keep the slat top flat within 1 mm so standoff stays consistent across the plate.
A water table at 25–50 mm below the plate cuts fume, noise, and reflected heat. It also stops thin plate from bowing as the arc passes. Do not submerge the plate. The water should catch the sparks, not quench the cut.
Workholding for plasma is simple. The plate's own weight plus a few magnetic clamps or toggle clamps is enough. Do not clamp across a cut line. And keep the ground clamp on clean metal, close to the cut, or the arc will hunt for a path and mark the plate.
- 1Air qualityDew point below 5 °C; moisture is the top cause of poor edge quality.
- 2Slat spacing60–100 mm; wider spacing lets small parts drop through.
- 3Ground clampOn bare metal within 300 mm of the cut whenever possible.
First cuts: parameters that matter most
Feed rate is the first knob. Too slow and the arc stays in one spot, widening the kerf and building dross. Too fast and the arc lags, leaving a bevel and an uncut edge. Start from the cut chart, then raise feed in 10% steps until dross just disappears on the top edge. Note the number and keep it with the material.
Cut height is the second. A 0.5 mm change in standoff shifts the kerf width noticeably on 6 mm plate. This is why arc voltage THC pays for itself. It samples arc voltage, compares it to a setpoint, and moves Z to hold standoff. Set the THC delay to 0.3–0.8 s after pierce so the initial blow-through does not confuse it.
Pierce delay is the third. Too short and you get an incomplete pierce and a blown nozzle. Too long and you melt a crater at the start point. For 6 mm mild steel at 45 A, start around 0.4 s. For 20 mm at 105 A, expect 1.2–1.8 s. Test on scrap before you cut a real part.
Kerf compensation is the fourth. Measure the kerf on a test cut, then enter that width in the CAM tool table. Without it your holes come out undersize by half the kerf and your outside profiles come out oversize. Recheck it when you change amperage, consumables, or material.
- 1Kerf widthTypically 1.2–2.0 mm at 45 A; measure, do not guess.
- 2Pierce height3.0–4.5 mm; higher on thick plate to protect the nozzle.
- 3Consumable lifeLog pierces per nozzle; replace before edge quality drops.
Common build mistakes and what they cost
The first mistake is a frame that is stiff enough to stand on but not stiff enough to cut. Static stiffness and dynamic stiffness are different things. A gantry that deflects 0.3 mm under hand pressure will deflect more under acceleration, and you will chase the error in software forever.
The second is noisy wiring. Plasma is an electrically loud process. Any signal cable running beside a torch lead will pick up interference, and the symptom looks like random lost steps or false limit trips. Separate the conduits and use shielded cable with the shield grounded at one end.
The third is buying a torch before choosing the air supply. A 65 A plasma needs roughly 200 L/min of clean, dry air at 6 bar. A small shop compressor with a wet tank will not hold that. The cut quality collapses long before the pressure gauge shows a problem.
The fourth is expecting mill tolerances. Plasma is a thermal process. If a drawing calls for ±0.05 mm on a hole, plan a secondary machining operation from the start. We rough plasma blanks and finish them on 3-axis and 5-axis machines every week for exactly this reason, and it is cheaper than fighting the torch.
How to build a plasma CNC machine, step by step
Work in this order. Each step assumes the previous one is verified.
- 11. Fix the cut envelopeDecide the largest plate you will cut. Add 150 mm of margin on each side for clamping and overtravel. Everything downstream follows from this number.
- 22. Weld and stress-relieve the frameUse 100 × 100 × 6 mm tube for tables up to 1.5 × 3.0 m. Weld diagonals, then let the frame sit or stress-relieve it before machining the rail pads.
- 33. Machine or shim the rail padsBring the pads coplanar within 0.1 mm. Bolt the rails down and check parallelism end to end with a dial indicator on a carriage.
- 44. Fit the gantry and drivesUse dual drive for spans over 1.2 m. Set belt or rack tension so there is no more than 2–3 mm of deflection at mid-span under hand pressure.
- 55. Mount motors and set accelerationStart at 300 mm/s² and 15 m/min rapid. Raise in steps only after you cut a test part and check for lost steps against a dial indicator.
- 66. Wire the controller and isolate signalsSeparate torch leads from control cables by at least 150 mm. Use isolated inputs for limits, e-stop and torch height control. Earth the table at one point only.
- 77. Install the torch and set initial standoffSet pierce height at 3.0–4.5 mm and cut height at 1.5–3.0 mm. Use the torch maker's cut chart for amperage, feed and pierce delay for your plate thickness.
- 88. Cut a test coupon and tuneCut a 100 × 100 mm square and a 50 mm hole in the target thickness. Check squareness, dross and hole roundness. Adjust feed in 10% steps and height in 0.5 mm steps.
Component choices and when each one fits
Match the choice to the table span and the plate you cut most often.
| Component | Option A | Option B | Pick B when |
|---|---|---|---|
| Gantry drive | Single center screw | Dual side drive | Span over 1.2 m or gantry over 40 kg |
| Motors | Open-loop stepper | Closed-loop or servo | You cut unattended or the gantry is heavy |
| Z travel | 100–150 mm | 250 mm and up | You stack plates or use a rotary axis |
| Torch class | 45 A air plasma | 65–105 A plasma | Regular cuts above 16 mm |
| Height control | Manual standoff | Arc voltage THC | Any plate that is not perfectly flat |
| Table bed | Dry slats | Water table | Thin plate, fume, or noise limits |
Build it if you cut plate, not if you cut profiles
A plasma table earns its keep on 3–25 mm plate cut in repeated nests. If your parts need mill tolerances, plasma the blank and let a CNC shop finish the critical features.
Questions we get about plasma table builds
How much floor space does a plasma CNC table need?
Add at least 1 m of walkway on the loading side and 0.5 m on the other three sides. A 1.5 × 3.0 m cut area typically needs a 3 × 6 m footprint once you include the control cabinet and the plasma power supply.
Leave headroom for fume extraction above the table. If you run a water table, allow for a drain and a make-up water line.
Can a plasma table cut stainless and aluminum?
Yes, but the settings change. Stainless needs higher amperage and often a nitrogen or air/nitrogen mix for a clean edge. Aluminum cuts fast and reflects heat, so feed rates run higher and dross forms quickly if you slow down.
For both, expect a larger heat-affected zone than on mild steel, and plan a finishing pass if the edge will be welded or sealed.
What tolerance can I realistically hold?
On a rigid 1.5 m table with arc voltage THC and good consumables, ±0.5 mm on profile position is realistic, and ±0.2 mm on hole diameter after kerf compensation.
On a large gantry without compensation, ±1.0 mm is more honest. Thermal distortion in the plate adds its own error, especially on long thin parts.
How often do consumables need replacing?
Electrodes and nozzles are wear items. Track pierces per set, not hours. A nozzle that has done 400–800 pierces usually starts leaving a wider kerf and more dross.
Replace the electrode and nozzle together. Mixing a worn electrode with a new nozzle shortens the life of both.
Do I need a water table or a downdraft table?
A water table is simpler and cheaper for a first build. It controls fume and noise and helps thin plate stay flat. Downdraft needs ducting, a fan, and a filter, but keeps the plate dry and is easier to automate.
If you cut mostly 3–10 mm plate in a workshop with neighbors, water wins on cost. If you cut thick plate all day, downdraft is easier to live with.
Can I send plasma-cut parts to a machine shop for finishing?
Yes, and it is a common workflow. Plasma blanks are often finished on a 3-axis or 5-axis mill for bores, faces, and threads. Send the drawing with the plasma kerf noted so the shop knows how much stock is left.
Two-sided parts benefit from a fixture or a matched set of holes cut in the plasma pass, so the finishing setup has a datum to locate from.
Send us your plasma blank and we will finish it
Upload a drawing and we will return a quotation and a free DFM analysis within 12 hours, covering how much stock to leave after plasma.
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