Part 6: Frame & Linear Motion
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The table and mechanical setup of your CNC plasma cutter is just as important as the plasma torch itself. A good table ensures accurate, reliable cuts — and makes your whole system easier to maintain and upgrade down the line.
Let’s break down the key choices you’ll need to make.
Frame Material: Aluminium Extrusion vs Welded Steel
Aluminium Extrusion
Aluminium extrusion (often 40x40mm or 80/20 profile) has become popular for CNC builds — especially DIY and hobby-level plasma tables.
Pros:
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Ease of Assembly: No welding — cut to length, square it up, bolt it together.
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Modular: Easy to make design changes, expand the table, or add accessories (cable management, tool holders, etc.).
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Lightweight: Reduces overall machine weight, helping with faster acceleration.
Cons:
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Rigidity Limitations: Less torsional stiffness compared to a welded steel frame — may flex under high-speed changes in direction (high acceleration).
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Cost: Quality extrusion can be pricey, especially if you're using larger profiles (e.g., 80x80mm).
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Bolted Joints: Can loosen over time if not properly torqued and Loctite'd.
Welded Steel Frame
Welded steel (typically square hollow section — SHS) is the more traditional choice for plasma cutting tables — especially for commercial or heavy-duty builds.
Pros:
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Strength and Rigidity: A well-welded steel frame resists flexing and provides long-term stability.
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Cost-Effective: Steel tubing is generally cheaper per metre than aluminium extrusion — especially in the common 50x50mm x 3mm wall thickness size.
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Durability: With good welds and proper finishing (rustproofing), it’ll last a lifetime.
Cons:
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Fixed Dimensions: Once it’s welded, it’s permanent — adjustments require cutting and re-welding.
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Requires Welding Skills and Tools: Not ideal if you don’t have access to a welder or experience with square, flat weld-ups.
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Transport and Assembly: Large welded frames are heavy and harder to move or modify.
Tip: Plasma cutting doesn’t generate the same forces as milling — you don't need extreme rigidity, but you do need enough stiffness to handle quick acceleration without twisting.
Linear Motion and Gantry Setup
While the frame holds everything together, linear motion is what moves the torch — and getting it right is key for speed, accuracy, and long-term reliability.
Gantry and Actuator Basics
The gantry usually holds the torch mount and runs side-to-side (Y-axis), while the X-axis moves the gantry along the length of the table. The Z-axis moves the torch up and down.
Main Gantry Types:
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Single Drive (Y-axis): One motor moves the gantry — simpler, but may rack or skew if not perfectly balanced.
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Dual Drive (Y-axis): Motors on both sides of the gantry — prevents racking, better for wider tables.
Key Considerations:
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Keep it lightweight — plasma cutting needs fast acceleration, not brute strength.
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Minimise gantry twist — use strong cross-bracing or larger box-section materials.
Linear Motion Options
Your frame supports the rails or guides the gantry runs on — here are common choices:
V-Wheels on Aluminium Extrusion
Budget-friendly, simple setup for light DIY tables.
Cons:
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Can wear faster.
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Susceptible to dust and grit from plasma cutting.
Linear Rails (Hiwin-style MGN or HGH)
Precision ground rails and bearing blocks.
Pros:
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High precision, smooth movement.
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Good resistance to contaminants if properly shielded.
Cons:
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More expensive.
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Needs careful mounting — misalignment can cause binding.
Steel Tube and Carriage Bearings
DIY-friendly — roller carriages riding on steel tubing.
Pros:
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Cheap, easy to source and fabricate.
Cons:
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Not as smooth or durable over time.
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Exposed bearings need maintenance.
Note: Linear rails are ideal for plasma — the fast, smooth movement they provide really helps when making long cuts at high speed.
Drive Mechanisms for Motion
We already discussed transmission types (rack and pinion, belts, ball screws, lead screws), but your frame design impacts how you mount these:
Rack and Pinion:
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Often mounted along the side rails — strong and efficient for large tables.
Belt Drives:
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Pulley systems run along lightweight extrusions — compact but less durable.
Lead Screws (ACME Screws):
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A cheaper alternative to ball screws, often used in DIY CNC builds.
Pros:
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Simple, easy to source.
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Good for short-travel applications.
Cons:
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Slower speeds and higher friction — not great for the rapid movement plasma cutting needs.
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More prone to wear and backlash over time, especially on larger tables.
Ball Screws:
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High precision, but expensive and slow.
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Better for routers or mills, less ideal for plasma's high-speed needs.
Linear Travel Systems
The guide systems that support the gantry movement:
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Linear Rail (Hiwin, HGH types):
Accurate and durable, but require regular cleaning due to plasma dust. -
V-Wheels (Xtreme wheels, etc.):
Budget-friendly and less sensitive to plasma dust. -
Roller Bearings (carriage bearings on steel profile):
Simple, durable for DIY builds, but needs regular maintenance. -
Hardened Steel Rods with Cylindrical Bearings:
(Also called round linear rails) Common in some router-style machines, but less typical in plasma setups.
Design Considerations: Plasma ≠ Router
Acceleration Matters More Than Rigidity
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Plasma cutters need to move fast — often 5,000–15,000 mm/min or more.
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Heavy gantries slow down acceleration and cause overshoot.
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Lighter, balanced designs perform better.
Flexibility Over Mass
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Plasma torches don’t exert cutting forces like router bits.
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The torch just floats above the work, so lighter, quicker machines deliver better results without sacrificing cut quality.
Water Table vs Downdraft
Water Table
A shallow tray filled with water sits under the workpiece.
Pros:
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Traps smoke, dross, and sparks — keeps the workshop cleaner.
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Simple, passive system — no fans or ducts.
Cons:
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Adds weight — your frame must support the extra load.
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Water needs treatment to avoid rust and algae.
Downdraft Table
Uses fans or ducting to suck smoke and fumes downward.
Pros:
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Can be lighter and more portable.
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No water to manage.
Cons:
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Requires a powerful fan and ducting system.
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More complex to build.
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More expensive.
Water Treatment Tip:
Use plasma table conditioners to prevent rust and slow down algae growth.
Transmission Choices
How the table moves matters a lot for speed, precision, and durability.
Rack and Pinion:
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Reliable, low maintenance, good for longer tables.
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Standard for mid- to large-sized plasma tables.
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Gear ratios help balance speed and torque.
Belts:
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Affordable and easy to set up.
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Fine for smaller tables or hobby use.
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Stretch over time — not ideal for heavy machines.
Lead Screws (ACME Screws):
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Simple and cheap, used in DIY CNC builds.
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Good for short travel but not ideal for plasma's rapid movements.
Ball Screws:
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High precision, but slow and expensive.
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Better suited to routers/mills than plasma tables.
Slats: Flat vs 'Gator Teeth'
The work surface of a plasma table is usually a series of removable steel slats.
Flat Slats
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Easier to replace, simple to fabricate.
Cons:
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Molten metal can pool and stick, creating more dross on cuts.
'Gator Teeth' Slats
Angled slats that reduce contact points.
Pros:
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Less dross build-up.
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Better airflow for downdraft systems.
Maintenance Tip:
Design your table so slats are easy to remove and replace — they wear out over time and regular maintenance keeps cuts clean.
Floating Head Design Basics
A floating head allows the torch to “float” vertically on a spring or sliding system. It protects the torch from crashing into warped material and enables Initial Height Sensing (IHS) — the system finds the surface of the material before starting a cut.
Why It Matters
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Metal sheets aren’t always perfectly flat.
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A floating head ensures the torch starts at the right height, improving pierce quality and reducing consumable wear.
Simple Setup
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Spring-loaded Z-axis.
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Mechanical or magnetic switches to detect contact.
