The Frame That Wouldn’t Stay Flat
We built a machine frame from 100×100×5 mm RHS steel. It looked square on the welding table. We ground the mounting surfaces flat, drilled the holes, and shipped it. When the customer bolted it to their floor, the frame was twisted. The top surface had a 2.5 mm warp across the 2 m length. The linear rails wouldn’t mount flat. We had to send a technician to shim every rail on site.
The problem wasn’t the welding. It was the welding sequence. We welded all four corners from the outside, which pulled the frame out of square as it cooled. Nobody had planned the weld order. Weldment frame design for machine bases isn’t just drawing tubes and telling the welder “weld it up.” It’s controlling the distortion so the frame arrives flat.
This article is what I’ve learned about designing and detailing weldments that actually stay flat after welding, machining, and shipping.
Material Choice: Tube Steel vs. Plate vs. Extrusion
Machine frames come in three basic materials. Each has its place, and mixing them wrong causes problems.
RHS/SHS Tube Steel (The Default)
Rectangular Hollow Section (RHS) or Square Hollow Section (SHS) is the standard for machine frames. It’s stiff in both bending axes, welds easily, and is cheap. The wall thickness matters:
| Size | Wall Thickness | Best For |
|---|---|---|
| 80×80 mm | 4–5 mm | Small machines, light frames, guarding posts |
| 100×100 mm | 5–6 mm | Medium machines, most custom automation |
| 150×150 mm | 6–8 mm | Large gantries, heavy press frames |
| 200×200 mm | 8–10 mm | Machine tool bases, heavy loads |
Wall thickness below 4 mm on an 80×80 tube flexes. Wall thickness above 8 mm on a 100×100 tube is overkill and adds weight without stiffness. For most custom machines, 100×100×5 mm is the sweet spot.
Welded Plate (The Machined Base)
For high-precision stations (vision, dispensing, assembly), a machined plate on top of a welded frame gives a flat reference surface. The plate is typically 10–20 mm thick steel, machined flat after welding. It bolts to the tube frame on top. This separates the structural frame from the precision mounting surface — the frame can flex slightly without affecting the precision plate.
Aluminum Extrusion (The Fast Build)
Aluminum extrusion (80/20, item, MiniTec) is fast to assemble and flexible. It’s not welded — it’s bolted together. The downside is stiffness: extrusion flexes under load and gets expensive at machine heights above 1.5 m. Use it for guarding, light sub-frames, and adjustable fixtures. Don’t use it as the main structural frame for a machine that carries a press or a heavy gantry.
Welding Design: Controlling Distortion Before It Happens
Welding distortion isn’t a surprise. It’s predictable. The weld heat expands the metal, which expands it in the heat-affected zone. As it cools, it contracts — and if the contraction is constrained by the surrounding material, the part warps. The welder can’t fix this by welding harder. The designer controls it through geometry and weld detail.
Weld Symmetry: Balance the Heat Input
If you weld one side of a tube joint heavily and the other side lightly, the frame pulls toward the heavily welded side. Balance the welds. If you have a fillet weld on the outside of a corner, put an equal fillet on the inside. If you’re welding a cross member, weld both sides alternately — not both sides of one corner, then both sides of the next.
Weld Size: Smaller Is Better
A 6 mm fillet weld doesn’t need to be an 8 mm fillet weld. The required weld size is determined by the load, not by what looks strong. A weld that’s too big puts more heat into the joint, creates more distortion, and adds stress concentration. Specify the minimum weld that carries the load. A 3–5 mm fillet on a 5 mm wall tube is usually enough.
Stitch Welds vs. Continuous Welds
For non-critical joints (guarding brackets, cable tray supports), use stitch welds — 20 mm weld, 50 mm gap. This cuts the heat input by 70% and reduces distortion. Continuous welds are only needed where strength or sealing matters. For most frame-to-crossmember joints, stitch welds on both sides are fine.
The distortion rule: Every weld bead is a heat source. Less total weld length = less distortion. Don’t weld every seam on the frame. Weld the joints that need strength. The rest can be stitch welded, bolted, or skipped.
Welding Sequence: The Instruction You Put on the Drawing
The drawing should tell the welder the weld order. “Weld all seams” is an invitation for a warped frame. A good weld sequence:
- Tack weld first. The welder tacks all four corners, checks diagonals with a tape measure, and adjusts if they’re off. Only then starts the full weld.
- Weld opposite corners alternately. Weld corner 1, then corner 3, then corner 2, then corner 4. This balances the heat input and prevents the frame from pulling one direction.
- Weld in small passes. Don’t run a 300 mm weld bead in one pass. Break it into 50 mm segments, let it cool, then continue. This prevents localized overheating.
- Check diagonals during welding. Every few weld beads, the welder checks the diagonals. If they’re pulling out of square, they add a spacer or adjust the weld order.
Put this sequence on the drawing. Don’t assume the welder knows. The shop that builds one frame a month doesn’t have the muscle memory for optimal weld sequence. The one-page instruction saves you the shim job on site.
Stress Relieving: Is It Needed?
Weldments have residual stress locked in from the welding. This stress doesn’t cause immediate failure, but it causes the frame to move after machining — you mill the surface flat, ship it, and a month later it’s warped as the residual stress relaxes.
Stress relieving (heating the weldment to 580–650°C in a furnace, then cooling slowly) relieves the residual stress. It’s expensive and adds lead time. But for precision machines (vision, dispensing, precision assembly), it’s worth it. The alternative is a frame that moves after install.
| Application | Stress Relief? | Why |
|---|---|---|
| General machine frame, ±0.5 mm tolerance | Usually not needed | Normal variation is acceptable |
| Precision station, ±0.1 mm tolerance | Recommended | Prevents post-machining movement |
| Machine tool base, precision rails | Required | Rail mounting surface must stay flat |
| Large weldment (>2 m) | Recommended | Bigger frames move more |
Machining After Welding: The Flat Surface
A welded frame is never flat enough to mount linear rails or precision tooling directly. The top surface needs to be machined — either milled or ground.
What to Machine
- Rail mounting surfaces: The pads where linear rails bolt down should be milled flat to ±0.05 mm across the rail length. Don’t rely on the weldment’s natural flatness.
- Mounting holes: Drill and ream (or bore) the mounting holes after welding. Drilling them before welding means they move during welding.
- Datums: The frame should have a defined datum surface (a machined pad) that all other measurements reference. This is what you use to set up the machine on the floor.
How to Machine Without a Big Mill
If the frame is too big for your mill, use a portable milling machine (gantry mill) on the floor. Or, more commonly, use shim stock. The frame mounts on levelling feet. After installation, you check the top surface with a precision level and shim under each foot until it’s flat. This works for general machines, but it’s a maintenance task — the customer has to re-shim if the floor settles.
Mounting the Frame: Feet, Anchors, and Floor Interface
A frame is only as flat as the floor it sits on. The mounting interface matters.
Levelling Feet
Every frame needs adjustable levelling feet (pads) so you can set the machine level on an uneven floor. The foot should have a bolt that screws into a welded nut on the frame, with a swivel pad that sits on the floor. Size the foot for the load — a 100×100 frame on a concrete floor needs a foot with at least 50 mm diameter pad.
Anchor Bolts: Bolt It Down or Not?
For most custom machines, you don’t anchor the frame to the floor. The machine’s own weight holds it in place. Anchor bolts are needed for:
- Machines with high dynamic loads (a press that cycles fast, a reciprocating saw).
- Machines on an upper floor (vibration to the floor below).
- Machines that move (conveyors that push against the frame).
For general assembly and inspection machines, levelling feet are enough. The customer can slide the machine for layout changes without drilling the floor.
A Weldment Design Checklist
- Is the tube size and wall thickness appropriate for the load? (Not overbuilt, not underbuilt)
- Are welds balanced symmetrically? (Avoid one-sided heat input)
- Is the minimum weld size specified? (No oversized fillets)
- Is the weld sequence on the drawing? (Tack, alternate corners, small passes)
- Does the frame need stress relieving? (Based on tolerance requirements)
- What surfaces need machining after welding? (Rail pads, datums, holes)
- Are levelling feet specified? (Foot size, pad diameter)
- Does the frame need anchor bolts? (Or is weight enough?)
- Are cable chases and guard brackets included in the weld model? (Not afterthoughts)
- Have you specified the weld standard? (ISO, AWS, or internal standard)
The Bottom Line
Welded steel frame design isn’t just drawing tubes. It’s controlling the welding heat, specifying the right weld sequence, stress relieving when needed, and machining the mounting surfaces after welding. The frame that arrives flat on the customer’s floor isn’t a miracle — it’s the result of a drawing that tells the welder how to weld, a stress relief cycle if needed, and a machined mounting surface. Spend the time on the weld detail now. The shim job on site is expensive, embarrassing, and avoidable.
