How to Make CNC Plasma Cutting Work on a Table You Build
A step-by-step build guide for engineers and shop owners. You will see how to size the table, drive the gantry, set torch height and pick cut parameters. You will also learn when building is the wrong choice.
In this article
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Key takeaways
Frame, rails and drive: what decides accuracy
Start with the frame. A plasma table carries a torch that moves at 3,000–6,000 mm/min and throws sparks and heat downward. The frame has to stay flat while the bed heats and cools. Welded steel tube at 100 × 100 × 4 mm is the usual choice for tables up to 3,000 mm wide. Aluminium extrusion looks clean in a drawing, but its thermal expansion is roughly twice that of steel, so a warm bed can pull the gantry out of square.
Rail choice sets the achievable tolerance more than the controller does. Linear profile rails (for example 20 mm or 25 mm wide) give low friction and repeatable preload, but they must sit on a machined or shimmed surface. On a welded frame, the rail pad is usually a separate steel plate that is milled flat before bolting. If you bolt rails straight onto raw tube, expect height variation of 0.3–0.8 mm over 2,000 mm and a cut that wanders.
Drive layout: below 1,500 mm of travel, a 5 mm pitch ball screw is stiff and simple. Above that, rack and pinion with a module 2 rack and a 3:1 to 5:1 planetary gearbox is the practical answer. Belt drives are cheap and quiet, but they stretch under the acceleration a plasma torch needs on corner-heavy nested parts.
One more rule. Keep the torch lead and the motor cables in separate cable chains, and ground the table frame to a single point. Plasma arcs are electrically noisy, and a shared ground path is the most common reason a new build loses steps on the Y axis.
Torch height control and the pierce cycle
Pierce height and cut height are two different numbers and they are not interchangeable. Typical values for mild steel are 3–5 mm pierce height and 1.0–1.5 mm cut height. The pierce happens while the arc is still establishing, so the torch sits higher to keep molten splash off the shield. Once the arc is stable, the Z axis drops to cut height and stays there.
A torch height control (THC) reads arc voltage and adjusts Z in real time. Arc voltage rises when the torch is too high and falls when it is too low, so the controller holds a setpoint that corresponds to your cut height. On 1–3 mm sheet the plate warps as it heats, and without a THC the nozzle will drag within a few hundred millimeters of travel. On 12 mm plate and above, the THC still helps, but the plate moves less.
Set the THC delay so it does not react during the pierce. A delay of 0.5–1.5 s after arc transfer is normal. If the delay is too short, the Z axis dives while the pierce is still blowing, and you get a hole that is wider than the kerf.
Keep a small gap between the work lead clamp and the cutting area. Clamp on clean metal, close to the cut, and never on painted or rusty stock. Voltage sensing is only as good as the return path.
Air supply, consumables and cut charts
Clean, dry air matters more than most builders expect. Plasma cuts by blowing ionized gas through a constricted nozzle, and moisture or oil in the line erodes the electrode and nozzle fast. Use a refrigerated dryer plus a coalescing filter, and size the line so pressure at the torch stays at the cutter's rated value, often 0.5–0.7 MPa (about 75–100 psi). A 5 psi droop during a long cut is enough to change the edge quality.
Consumables set the cut quality. A 1.1 mm nozzle on a 45 A cutter handles roughly 1–6 mm mild steel; step up to 1.3 mm and higher current for 8–12 mm. Every torch maker publishes a cut chart with the current, feed rate and voltage for each thickness, and that chart is the starting point, not a suggestion. Deviating by 20 percent on feed rate is visible as dross on the bottom edge.
Water tables and downdraft tables solve different problems. A water table traps smoke and cools the plate, which reduces warping on thin sheet. A downdraft table keeps the plate dry and is easier for nested jobs with fine detail. For mixed work, a water table with adjustable water level is the flexible option.
Keep a log. Record nozzle part number, current, feed rate, pierce delay and the resulting edge. After a few weeks you will have your own chart, tuned to your air and your material.
The machined parts you cannot weld your way around
Some brackets on a plasma table have to be machined. Gantry end plates carry the rail blocks and the pinion, and their bores must line up so the gantry stays square. Torch mounts hold the torch perpendicular to the plate, because even 1° of tilt shows up as a beveled edge on thick material. Rail pads and motor mounts also need flat, parallel faces.
Typical tolerances are ±0.05 mm on bore spacing and 0.02 mm on the flatness of a rail pad. That is well inside what a 3-axis mill delivers. Material is usually 6061-T6 or 7075 aluminium for gantry plates because of weight, and 1045 or 4140 steel for motor and pinion mounts where stiffness matters.
This is where a build stalls. A shop can weld a frame in a weekend, then wait two weeks for a local machine shop to cut the gantry plates. If your design is not final, that wait repeats.
We machine these parts every week for machine builders: gantry plates from 6 mm to 20 mm thick, torch mounts, rail pads and pinion housings, held to ±0.005 mm when the drawing calls for it. Upload a STEP file and you get a quotation plus a DFM review within 12 hours.
Step by step: build order that avoids rework
- 11. Fix the cut envelope and material rangeDecide the largest sheet and the thickest plate first. A 1,500 × 3,000 mm bed with 1–12 mm mild steel covers most job-shop work. Moving up to 2,000 × 4,000 mm forces bigger drives and a heavier frame.
- 22. Weld and stress-relieve the frameUse 100 × 100 × 4 mm steel tube, tack, check diagonals within 1 mm, then finish weld. Let it cool slowly and re-check. Skipping stress relief means the frame moves after the rails are set.
- 33. Machine the rail pads before mounting railsFace the pad plates flat, then bolt them on. Shim to within 0.05 mm over the full rail length. Never shim a rail with washers; use ground shim stock.
- 44. Install rails and set preloadTorque rail bolts to the maker's figure in a cross pattern. Run a dial indicator along the block travel: 0.02–0.05 mm variation per 300 mm is a workable target.
- 55. Fit the rack, pinion and gearboxSet backlash at 0.05–0.10 mm and check it at both ends of travel. A rack that is too tight will whine and heat the gearbox on long jobs.
- 66. Wire the controller, drives and THCKeep plasma leads and step/direction cables in separate chains. Ground the frame at one point. Set motor current to about 70 percent of the drive rating and tune acceleration until corners no longer round off.
- 77. Square the gantry and run a test couponCut a 200 × 200 mm square and measure both diagonals. Adjust the A axis or the gantry coupling until the difference is under 0.3 mm. Then cut a 10 mm hole and check roundness.
- 88. Tune from the cut chartStart at the maker's feed rate and voltage for your thickness, then adjust in 10 percent steps. Log every change. Settle on settings that give a clean bottom edge with minimal dross.
Build versus buy, and drive choice by travel
Use this when deciding scope and hardware.
| Decision | Option A | Option B | Pick when |
|---|---|---|---|
| Table frame | Welded steel tube | Aluminium extrusion | Steel for beds over 1,500 mm |
| X travel drive | Ball screw to 1,500 mm | Rack and pinion above 1,500 mm | Rack for long nested sheets |
| Torch height | Fixed standoff | Arc voltage THC | THC for sheet under 3 mm |
| Table type | Water table | Downdraft table | Water for thin plate, downdraft for fine detail |
| Build or buy | Build in-house | Buy a used table | Buy if cutting under 20 hours a week |
| Gantry plates | Weld and drill | CNC machined | Machine when bores must stay aligned |
Questions builders ask before starting
How much floor space does a 1,500 × 3,000 mm table need?
Allow about 2,000 × 3,800 mm for the table itself, plus 800 mm of walkway on the long sides and room at the ends for sheet loading.
Height matters too. A water bed at 700–800 mm puts the cut line at a comfortable working height and leaves room for the slats and the catch tray.
Can I run a plasma torch on a router frame?
Sometimes, but the router frame is usually lighter and its belts stretch under the acceleration plasma needs.
The bigger problem is heat and sparks. A wooden or thin aluminium bed will not survive long. Keep router frames for routing.
What accuracy should I expect from a home-built table?
A careful build with machined gantry plates and squared rails holds about ±0.2 mm on part features and ±0.5 mm on hole position.
The plasma process itself adds kerf variation of 0.2–0.5 mm, so chasing tighter than that on the frame is wasted effort.
Do I need a water table for thin sheet?
It helps a lot on 1–3 mm plate, where heat warping is the main cause of a failed cut.
If you cut mostly 6 mm and above, a downdraft table is cleaner and easier to maintain.
Which parts should I have machined instead of making myself?
Gantry end plates, torch mounts, rail pads and pinion housings. These set the geometry of the whole machine.
Frame tubes, slats and the water tray can be fabricated and welded in your own shop.
How do I keep the cut square on thick plate?
Check torch perpendicularity first, then feed rate. A torch tilted by 1° produces a visible bevel on 12 mm steel.
Slow the feed until dross disappears, then check the top edge. Too slow gives a rounded top corner and a wider heat-affected zone.
Send us your gantry plates and torch mounts
Upload a STEP file and get a quotation with a DFM review within 12 hours. Prototypes and 10,000-part runs, no minimum order quantity.
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