The short answer
On steel CNC RFQs, **magnetic-particle inspection (MPI)** is not the same as a [dye-penetrant check](https://3dwk.com/resources/when-to-specify-dye-penetrant-after-cnc), a [CMM sampling plan](https://3dwk.com/resources/cmm-sampling-plans-for-cnc-lots), or a general “NDT as needed.” MPI finds surface and near-surface cracks in ferromagnetic steel by magnetizing the part and reading a particle indication. It does not replace dimensional inspection, and it is not a default method for austenitic stainless such as 304 or 316. Silence on whether MPI is required, on which faces or fillets must be inspected, and on whether it comes before or after heat treat or coating, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the particle bench time alone.
This guide is for buyers packaging steel parts where a specification cloud says “crack check” or “MPI if needed” with no zone or sequence. Illustrations use an **Example part** only—not a customer bracket. Sibling notes live in [first-article inspection reports](https://3dwk.com/resources/first-article-inspection-reports) and [when to specify heat treat after CNC](https://3dwk.com/resources/when-to-specify-heat-treat-after-cnc); here we focus on **MPI packaging** so estimators do not have to guess whether a 1018 bracket needs wet fluorescent MPI on two high-stress fillets, or no NDT at all.
What MPI means for quoting
An **MPI pack** names the method class you actually need (for example wet fluorescent versus visible dry), the zones or fillets to inspect, the acceptance idea (no linear indications in zone A, or inspect-and-report), and the sequence versus heat treat, grinding, or coating. It is not a dye-penetrant pack, and a general “NDT to applicable standards” cloud does not tell the shop which faces to magnetize or when. Process pages for carbon-steel milling ([CNC 3-axis Steel 1018 aerospace](https://3dwk.com/en/cnc-3axis/steel-1018/aerospace.html), [CNC 5-axis Steel 1018 aerospace](https://3dwk.com/en/cnc-5axis/steel-1018/aerospace.html), [CNC 3-axis Steel 1018](https://3dwk.com/en/cnc-3axis/steel-1018.html)) assume MPI is called only when the alloy is ferromagnetic and the zones are named.
Do not bury “MPI as needed” inside a notes cloud. Put **MPI WET FLUORESCENT ON FILLETS A AND C AFTER MACHINING, BEFORE BLACK OXIDE; NO MPI ON COSMETIC FACE D; ACCEPT: NO LINEAR INDICATIONS IN A/C** on the PDF and mirror it in RFQ text. Background: [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote), [heat-treating steel after CNC](https://3dwk.com/resources/heat-treating-steel-after-cnc-machining).
Where MPI applies—and where it does not
MPI needs a ferromagnetic path. Steel 1018 and many low-alloy steels respond; austenitic stainless generally does not, so a 316 housing should be planned for dye penetrant or another method, not a silent MPI note. Use MPI where machining, grinding, or heat treat could leave a surface crack in a fillet, a hole edge, or a thin wall—not as a blanket cosmetic scan of every milled face. [Mill certs](https://3dwk.com/resources/when-to-request-mill-certs-with-cnc-parts) and MPI are different documents: a cert speaks to the bar, MPI speaks to the finished surface.
Steel 1018 bar $/kg planning bands sit in the common carbon-steel range under **rates.v1.1** material tables; material cost is rarely the MPI story—zone count, method, sequence, and whether a third-party report is required are. Do not invent laboratory certificates; if you need a named procedure or a third-party report, write that requirement rather than assuming the shop already holds a particular accreditation.
Sequence with heat treat, grinding, and coatings
If the part will be heat-treated or ground, say whether MPI is after those steps. A check done only on the soft milled state can miss a crack that opens later. Coatings such as [black oxide or zinc](https://3dwk.com/resources/zinc-plating-vs-electroless-nickel-vs-black-oxide) usually come after MPI, because a film can hide an indication. If only two fillets are critical, mask the requirement to those zones so the shop does not plan all-over inspection.
Tolerance and inspection traps
Calling “MPI as needed,” mixing MPI with dye penetrant on a non-magnetic stainless, or asking for 100% photography of every particle indication without an acceptance rule, all force **needs_manual**. Default non-critical lots to dimensional inspection only, and put MPI on named zones when a crack would be a functional reject. Related: [ISO 2768 general tolerances on CNC RFQs](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq), [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Illustration 1 shows an **Example part** steel bracket with labeled MPI on fillets A and C versus cosmetic face D left to visual/dimensional checks only.
Example part walkthrough
**Example part:** Steel 1018 milled bracket ~120 × 60 × 12 mm, two high-stress fillets A and C, cosmetic face D, black oxide after inspection, ISO 2768-m general. With STEP + **MPI WET FLUORESCENT ON A AND C AFTER MACHINING, BEFORE BLACK OXIDE; NO MPI ON D; NO LINEAR INDICATIONS IN A/C**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent “NDT as needed, then coat” often lands DFM notes or **needs_manual**—without inventing any customer PO or certificate.
Illustration 2 contrasts a vague RFQ (“MPI everything / applicable standards”) versus a labeled zone-and-sequence pack and the clearer estimate path.
What widens bands fastest
Unlabeled “NDT,” MPI called on non-magnetic stainless, inspection planned before a heat-treat step that should be first, and coating-before-MPI sequences. Quantity breaks help unit minutes but do not erase method ambiguity—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [CMM versus optical measurement](https://3dwk.com/resources/cmm-vs-optical-measurement-machined-parts).
What to do next
Mark whether **MPI is required, on which ferromagnetic zones, and in what sequence**, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the MPI questions rather than forcing a fake price.
Common questions
Can I specify MPI on stainless 304 or 316?
Usually no. Those alloys are typically austenitic and do not provide a useful magnetic path. Use [dye penetrant](https://3dwk.com/resources/when-to-specify-dye-penetrant-after-cnc) or another method suited to non-magnetic metals, and say so on the drawing.
Is MPI a substitute for a CMM report?
No. MPI looks for cracks. A CMM or a first-article report looks at size and geometry. Keep the two packs separate, and only order both when you need both.
Should MPI come before or after black oxide or zinc?
Plan MPI on the bare steel, then coat, unless your procedure explicitly inspects through a thin film. A coating applied first can hide or smear an indication.
Do I need MPI on every steel prototype?
No. Many fixtures and non-safety brackets are fine with dimensional checks. Use MPI when a fillet, a hole edge, or a heat-treated section would be a functional or safety concern if it cracked, and name those zones.