Writing an ITP a Fabricator Can Actually Follow
What belongs in an inspection and test plan for a machine base: hold points, witness points, acceptance criteria, NDT scope and the documents you get in the box.
An inspection and test plan is a table that says, for each stage of manufacture, what gets checked, against what criterion, by whom, and what document results. On a machine base bought from a shop you cannot visit weekly, it is the main instrument of control you have.
Why it matters more on a heavy structure
A machine base is expensive, heavy and slow to replace. By the time a four-tonne weldment is finish machined, the cost of a mistake is the whole part plus the freight plus the schedule. There is no reworking your way out of a structure that was welded in the wrong sequence.
So the value of an ITP is not the final inspection. It is the intermediate checks, where a drift is still correctable.
The columns
A workable ITP has these:
| Column | What it holds |
|---|---|
| Stage | The operation, in sequence |
| Activity | What is inspected or tested |
| Reference | Drawing, procedure or standard clause |
| Acceptance criteria | The number or condition that passes |
| Method | How it is checked |
| Responsibility | Fabricator, buyer, third party |
| Intervention | Hold, witness, review or monitor |
| Record | The document produced |
The columns people leave out are acceptance criteria and record. Without acceptance criteria the check is an opinion. Without a named record you have no evidence afterwards.
Intervention points
- Hold (H). Work stops until released. Expensive in schedule, so reserve it for irreversible steps.
- Witness (W). You are notified and may attend; work continues regardless.
- Review (R). You review the document afterwards.
- Monitor (M). Performed and recorded by the fabricator, available on request.
A common error is marking everything a hold point. The fabricator prices the waiting, the schedule stretches, and you do not attend most of them anyway. Two or three genuine hold points is usually right.
A workable ITP for a machine base
Materials
- Plate and section material certificates reviewed against the specification. Record: mill certificates. Intervention: R.
- Casting material certificate and grade verification, if applicable. Record: foundry cert. R.
Pre-fabrication
- Welding procedures and welder qualifications verified current. Record: WPS, WPQR, welder certs. R.
- Fixture and build sequence approved, if the sequence is contractual. Record: sequence drawing. H.
Fabrication
- Fit-up check before welding at defined assembly checkpoints. Record: dimensional report. M or W.
- Visual weld inspection on all welds. Record: VT report against ISO 5817 level. M.
- NDT to the specified scope: MT or PT on full-penetration and critical welds, UT where thickness and criticality justify it. Record: NDT reports. R or W.
- Dimensional check of the completed weldment against the ISO 13920 class. Record: dimensional report. H: this is one worth holding.
Stress relief
- Thermal cycle or vibratory treatment to the specified procedure. Record: furnace chart or treatment certificate. R.
Rough machining
- Dimensional check confirming stock remains on all functional faces. Record: report. M.
- Second stress relief where specified. Record: chart. R.
Finish machining
- Final dimensional and geometric inspection against the drawing’s GD&T, with the support condition and temperature stated. Record: full dimensional report. H.
- Surface finish check on functional faces. Record: report. M.
Finishing and dispatch
- Surface preparation and paint system, including dry film thickness. Record: coating report. M.
- Protection of machined faces verified. Record: photographs. M.
- Packing, crating, lifting points and marking checked. Record: packing report with photographs. W or H on a first order.
- Document pack compiled and complete. Record: the pack itself. H.
Acceptance criteria that work
Write numbers and references, not adjectives.
- Not “good weld quality” but “ISO 5817 quality level B”.
- Not “flat” but “0.03 mm over the pad, measured by precision level, supported on the six points shown, at 20 ± 3 °C after a four hour soak”.
- Not “well painted” but the system, the coats and the dry film thickness range.
- Not “properly crated” but the crate specification, lifting point locations and whether heat-treated timber marked to ISPM 15 is required.
The test is whether two people in different countries reading the same line would accept the same part. See measuring flatness for why the support and temperature clauses are not padding.
The document pack
Agree at order stage exactly what arrives with the part. A typical machine base pack:
- Material certificates for plate, section and castings.
- WPS, WPQR and welder qualification records.
- Visual and NDT reports.
- Stress relief chart or certificate.
- Dimensional and geometric inspection report, with method and conditions.
- Coating report.
- Packing list and crate photographs.
- Any concession or deviation requests, with your written disposition.
Ask for what you will actually read. Every document is real hours in the price, and a pack nobody opens is a pure cost.
Third-party inspection
On a first order with a new supplier at distance, an independent inspector at the fabrication dimensional check and again before crating is cheap relative to the value of the part and the cost of return freight. It also changes behaviour: a shop that knows an inspector is coming prepares differently.
On repeat work with a shop that has performed, the fabricator’s own documented inspection is normally enough, and the cost is better spent elsewhere.
Common questions
What is the difference between a hold point and a witness point?
At a hold point, work stops until the specified party has inspected and released it. At a witness point, the party is notified and may attend, but work proceeds if they do not. Hold points give you control and cost schedule; witness points give visibility without stopping the job. Use hold points sparingly and where a mistake would be irreversible.
Do I need third-party inspection on a machine base?
It depends on distance, value and how well you know the shop. On a first order with a new supplier several thousand kilometres away, an independent inspector at two or three points is inexpensive insurance against a multi-tonne mistake. On repeat work with a proven shop, the fabricator's own documented inspection is usually sufficient.
When should dimensional inspection happen?
At several stages, not just at the end. After fabrication and before stress relief, after rough machining, and after finish machining. Inspecting only the finished part tells you whether to accept or reject it; inspecting through the process tells you in time to correct it, which is the entire point.
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