How to specify tolerances for welded frame assemblies (Gd&T)

Welded frames carry machines, Förderer, Karren, and fixtures. Getting the right tolerance scheme is the difference between smooth installation and costly rework. As a China-based OEM/ODM factory, Able Hardware builds precision frames using Automatische/Robotermig (and TIG when specified), integrated CNC machining, and export-ready quality documentation. This guide shows how to apply GD&T to welded frames so you can buy once and install fast.

Where GD&T Adds Value

• Communicates function (fit, form, and orientation) instead of guessing “tight everywhere.”
• Prevents over-tolerancing that inflates weld time, fixturing, Bearbeitung, and coating rework.
• Enables scalable inspection—from weld gauges to CMM/laser scanning—aligned to risk.

For production examples, see our Custom Metal Frames und Welding Trolley Carts.

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Datum Strategy for Frames

Choose Stable, Functional Datums

1. Primary (EIN): The mounting plane or base pads that interface with the floor, machine bed, or skid. Define A as a broad, stable plane after any post-weld machining.
2. Secondary (B): A long reference rail or machined edge that drives part orientation along length.
3. Tertiary (C): A cross-member or machined hole/slot that locks rotation.

Tip: If the frame is powder-coated, define datums on non-coated machined pads to avoid stack from coating thickness.

Sequence and Fixturing

Call out “Datums after weld, before coating” if pads are spot-faced post-weld. We use dedicated fixtures and robotic beads to stabilize heat input.

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Gd&T Controls That Work Best on Welded Frames

Straightness & Flatness

• Straightness of tube rails: 0.5–1.0 mm per metre typical for as-welded members.
• Flatness of base plane (EIN): 0.3–0.8 mm per 300 mm as-welded; 0.1–0.3 mm after machining.

Parallelism & Perpendicularity (Squareness)

• Parallelism between upper deck and base A: 0.3–0.6 mm per 300 mm as-welded; 0.1–0.2 mm machined.
• Perpendicularity of uprights to A: 0.2–0.5 mm per 100 mm.

Position for Hole Groups

Use true position with MMC for bolt patterns on pads and brackets:

• Ø0.3–0.6 mm (machined) or Ø0.8–1.5 mm (as-welded pierced plates) to datums A|B|C.
• For long rails, apply composite position to hold pattern-to-pattern repeatability while allowing rail growth.

Profile for Envelopes

• Profile of a surface 0.8–1.5 mm controls overall tube frame envelope without over-constraining every member.

Runout (When Needed)

• Use circular/total runout on shafts/bosses welded to frames (rare). Andernfalls, position + perpendicularity is cleaner.

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Weld Quality Level & Symbole

• Specify symbols per ISO (or AWS if your standard). Reference ISO 5817 quality levels:
  • Level C (moderate) for non-critical cart frames.
  • Level B (streng) for machine bases or precision mounting pads.
• Define throat size, Länge, Tonhöhe for intermittent welds to manage heat input.
• State acceptance standard and inspection method (visual, fillet gauges, dye-pen, measurement plan).

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Realistic Tolerances by Process Route

As-Welded (Fixture-Controlled)

• Flatness of base A: 0.5–1.0 mm per 300 mm.
• Perpendicularity of uprights: 0.3–0.6 mm/100 mm.
• Position on pierced holes: Ø0.8–1.5 mm.

Weld + Post-Machining

• Spot-faced pads and reamed holes: flatness 0.1–0.2 mm; position Ø0.2–0.4 mm.
• We often weld oversize pads, stress-relieve if needed, then CNC to tolerance.

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Distortion Control Plan

Materialien & Dicke

• Kohlenstoffstahl (S235–S355) is most predictable for robotic MIG.
• Rostfrei (304/316) needs tighter heat control and sometimes larger profiles to resist pull.
• Balance wall thickness across opposing members; asymmetry increases bow.

Process & Wärmeeingang

• Staggered, symmetric bead sequence with robot paths; interpass spacing to cool; copper backing for critical edges.
• Intermittent welds where feasible to reduce mass and heat.

Stress Relief & Bearbeitung

• For heavy frames or tight pads: low-temp stress relief, then face mill/ream.
• Define machining allowance (z.B., +1.0–1.5 mm on pads).

Beschichtung & Masking

• Powder coat adds 60–100 µm Dicke; mask datum faces and threaded holes.
• Note any Rz targets after coating for sealing surfaces. Sehen Pulverbeschichtung.

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What to Put on the Drawing/RFQ

• 3D model (Schritt/IGES) + fully dimensioned 2D with datums A|B|C.
• Gd&T feature control frames for base flatness, Quadratismus, hole positions, and key profiles.
• Weld callouts with ISO 5817 level, throat, Länge, Tonhöhe, Verfahren (MIG/TIG), and sequence notes if critical.
• Critical-to-Quality (CTQ) list highlighting features we must measure 100%.
• Material grade, tube size, Wandstärke, and any pre-galv or pickling requirements.
• Coating spec (Pulver, shot-blast class, color/texture) and masked areas.
• Inspection level (Fai, PPAP-like, or batch) and required records.

For engineering templates and checklists, visit engineering.

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Inspection Plan & Reporting

We align inspection with risk and your audit needs:

• First Article with full layout to datums (portable CMM or laser scan for large frames).
• Go/No-Go gauges for squareness and envelope checks on the floor.
• Weld gauges + visual per ISO 5817; optional NDT on critical joints.
• Pack with shock/tilt indicators and corrosion protection; export-grade crating.

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Kosten & Lead-Time Drivers (No Absolute Prices)

• Tolerance class: tighter flatness/position and Level B welds increase fixturing, bead time, und Bearbeitung.
• Member count & Schweißlänge: more joints = more heat = more rework risk.
• Material & Größe: stainless and thick-wall sections raise cycle time and consumables.
• Post-machining & stress relief: adds operations but enables tighter GD&T.
• Finish: powder texture, masking, and color changes influence takt.
• Volume & Wiederholbarkeit: repeat orders justify dedicated jigs for lower unit cost.
• Documentation depth: FAI/PPAP-style packets add metrology time.

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Warum Able-Hardware

• Automatische/Robotermig lines with consistent heat input; TIG available for thin or visible joints.
• In-house Laserschnitt & CNC-Bearbeitung for post-weld pads and precision holes.
• ISO-aligned QC with export-ready documents and packaging.
• Proven supply for carts, Gestelle, and machine bases worldwide.

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FAQ

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Upload your drawing and get a quote now

Send your STEP/PDF with target GD&T, Schweißklasse, and finish notes. We’ll return a manufacturability review and a firm lead time.

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