Distortion control
Balanced weld sequence, back-step welding, pre-setting parts against shrinkage and strongbacks on long members. Planned before the first tack, not corrected after. See distortion control on heavy plate.
Processes · Heavy fabrication sourcing
How a heavy frame is cut, welded and machined.
A machine base or skid passes through five or six processes before it ships, and the order matters as much as the method. Typical results for each step, so the drawing calls out what the frame needs and the quotes price the same sequence.
00 · Sequence
The usual route through the shop
- Cut and preparePlate and sections cut to size, weld bevels prepared, holes that do not need machining drilled.
- Fit-upParts tacked on a levelled table or fixture, with weld shrinkage allowed for in the setting out.
- WeldTo a qualified procedure, in a planned sequence that balances shrinkage across the frame.
- NDT and repairVisual on all welds, then MT, UT or PT where the drawing calls for it, before heat treatment.
- Stress reliefThermal in an oven, or vibratory where you allow it.
- MachinePads, rails, bores and bolted joints on a gantry or boring mill, after stress relief.
- Inspect, blast and coatDimensional report, then surface preparation and the paint or galvanising system.
The full route, with cost drivers, is in theheavy steel frame fabrication guide.
01 · Cutting
Cutting and edge preparation
The cutting method decides how much dressing a part needs before fit-up and how much heat goes into the edge. Ranges below are typical across machines and operators, not a capability we hold. The shop that quotes confirms its own.
| Method | Typical thickness | Typical tolerance | Where it fits |
|---|---|---|---|
| Oxy-fuel | 6 to 300 mm and more, carbon steel only | ±1 to ±2 mm | Thick base plates and flanges. Wide heat affected zone, slow, square edge. |
| Plasma (high definition) | 3 to 50 mm, best quality under about 40 mm | ±0.5 to ±1 mm | Most frame plate. Slight edge bevel, fast, cuts stainless too. |
| Fibre laser | 1 to 25 mm, thicker on high power machines | ±0.1 to ±0.3 mm | Gussets, stiffeners and brackets that must fit without dressing. |
| Waterjet | Up to 150 mm and more | ±0.1 to ±0.3 mm | No heat affected zone. Slow and costly, used where heat is not allowed. |
| Band or cold saw | Beams, channels, hollow sections | ±0.5 to ±1 mm on length | Section members. Mitres and copes are cut separately or by a beam line. |
Weld bevels on plate over about 20 mm are usually cut with a bevel head or machined, so the root gap and land stay consistent along the joint.
02 · Welding
Fit-up and welding
Heavy frames are welded to a qualified procedure under AWS D1.1 or the ISO 15614 and ISO 9606 pair. Preheat follows from plate thickness, carbon equivalent and hydrogen level, per AWS D1.1 or EN 1011-2. The process mix below is what most heavy frame shops run. Background on the highest deposition process is onWikipedia.
| Process | Typical use on a frame | Notes |
|---|---|---|
| SAW (submerged arc) | Long straight seams in the flat position: beam flanges, box girders, base plate butts | Highest deposition rate. Needs a positioner or a tractor, little spatter. |
| FCAW (flux cored) | Most fillet and groove welds on heavy frames, in position | High deposition, good on thick sections. The usual workhorse. |
| GMAW (MIG/MAG) | Thinner members, brackets, stainless and robot cells | Clean and fast. Sensitive to wind and mill scale. |
| SMAW (stick) | Repairs, site welds, awkward access | Low deposition. Low hydrogen electrodes need heated storage. |
NDT
Visual on every weld. MT on fillet toes, UT on full penetration butts, PT on stainless or where MT cannot reach. The percentage is set on the drawing. See UT, MT and PT on heavy weldments.
As-welded tolerance
General tolerances for welded constructions usually follow ISO 13920. Class B for lengths and F for flatness and straightness are common on frames that are machined afterwards.
03 · Stress relief
Stress relief before machining
Welding leaves residual stress locked into the frame. Machining removes material that was holding it, and the frame moves after the clamps come off. That is why stress relief goes between welding and machining, and why oven length is checked before a shop is shortlisted.
| Thermal, in an oven | Carbon and carbon manganese steel such as S355 or A572 typically held at 550 to 650 °C depending on the governing code, about 1 hour per 25 mm of thickness, with controlled heating and cooling rates. Removes most residual stress. Leaves scale to blast off. |
|---|---|
| Local, with heating pads | For a weld zone on a frame too large for the oven, or a site weld. Treats a band around the weld only, with thermocouples recording the cycle. |
| Vibratory | The frame is vibrated near resonance for a set time. Reduces peak stress partly. Accepted on some machine bases, not where a code or the drawing calls for post weld heat treatment. |
| What to ask for | The time and temperature chart from the furnace recorder, with the frame identified on it, as part of the quality documents. |
Why it matters on machine bases in particular:stress relief for welded machine bases. Background on residual stress is onWikipedia.
04 · Large-bed machining
Machining the frame after welding
Mounting pads, rails, bores and bolted joints are machined after stress relief, with the frame set once on a large bed so the machined features share one datum. Results below are typical for the machine class, not a promise for every shop.
| Feature | Typical machine | Typical result |
|---|---|---|
| Mounting pads and rails | Gantry or bridge mill | Coplanarity typically 0.05 mm/m, 0.02 mm/m on the tighter shops |
| Bores for pivots, shafts and bearings | Horizontal boring mill | Bore H7 typical, position tied to a machined datum face |
| Long coaxial bore lines | Boring mill with outboard support, or line boring bar | Coaxiality typically 0.02 to 0.05 mm over the line |
| Bolted joint faces for split-and-bolt | Gantry mill, both halves to the same datum | Flatness and hole position matched across the joint |
| Drilled and tapped bolt patterns | Gantry or boring mill in the same setup as the faces | Hole position typically ±0.1 to ±0.2 mm |
Leave machining stock
Pads and faces need typically 3 to 5 mm of stock on welded plate so a distorted face still cleans up. Say on the drawing which faces are machined.
Datum on a machined feature
Reference the frame from a machined pad or bore, not from a flame cut edge. The shop can then pick it up again on a second setup.
Measure it properly
Large frames are usually measured with a laser tracker on the machine bed. See laser tracker and CMM inspection and flatness on machined weldments.
05 · Finish
Surface preparation and coating
Machined faces are masked or coated with a temporary rust preventive. The rest is usually blast cleaned to Sa 2½ under ISO 8501-1, then painted to a named ISO 12944 corrosivity category or hot-dip galvanised to ISO 1461. Galvanising heats the frame to around 450 °C, so it goes before machining, never after. Details insurface finish systems for structural steel.
FAQ
Questions about the fabrication sequence
- Can one shop cut, weld, stress relieve and machine the frame?
- Some shops do all four in house. Others weld and stress relieve, then send the weldment to a partner gantry mill and quote it as one job. We tell you which before the RFQ goes out, so every quote already covers the full sequence.
- Does a welded frame always need stress relief before machining?
- Not always. It is needed when machined faces or bores carry tight flatness or position tolerances, or when the code or drawing calls for post weld heat treatment. Without it, residual stress released by machining can move the frame after the clamps come off.
- Is vibratory stress relief acceptable instead of an oven?
- Only where you allow it. Vibratory relief reduces peak stress partly and costs less, but it is not a substitute where a code or the drawing calls for thermal post weld heat treatment. Say on the RFQ whether it is acceptable.
- What fabrication tolerance should the drawing call out?
- For unmachined weldment dimensions, a general tolerance class from ISO 13920 is common, often class B for lengths and F for straightness and flatness. Machined features get their own tolerances. Tight tolerances on as-welded dimensions push cost up quickly.
- Do you fabricate the frames yourselves?
- No. We are a matching service. We match your drawing with up to three vetted Vietnamese fabricators whose crane, oven and machining envelope fit the piece. They quote you directly and we take no commission on the order.
Something else? Email the drawing to and we will say plainly which steps it needs. How shops are qualified for this work is set out onhow we vet.
Know the sequence your frame needs?
Send the drawing with the stress relief, NDT and machining it calls for. Within 24 business hours we tell you which shops clear the envelope, and up to three of them quote you directly.
Get Matched With a Heavy Frame Supplier
We reply within 24 business hours.