The short answer
On stainless and other CNC lot RFQs, **dye-penetrant inspection (PT / LPI)** is not the same note as a [CMM sampling plan](https://3dwk.com/resources/cmm-sampling-plans-for-cnc-lots), a [first-article report](https://3dwk.com/resources/first-article-inspection-reports), or a general “100% crack check” cloud. Penetrant testing is a surface method for finding surface-breaking discontinuities after machining, [heat treat](https://3dwk.com/resources/when-to-specify-heat-treat-after-cnc), grinding, or welding—not a substitute for dimensional inspection, and not the same as magnetic-particle inspection (MPI), which needs a ferromagnetic alloy. Silence on whether PT is required, on the sampling plan, and on whether it happens before or after finish, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the CMM minutes alone.
This guide is for buyers packaging stainless or similar CNC RFQs where a drawing or email mentions “crack check,” “dye check,” or “NDT as needed” with no method or sample size. Illustrations use an **Example part** only—not a customer aerospace fitting. Sibling notes live in [CMM sampling plans](https://3dwk.com/resources/cmm-sampling-plans-for-cnc-lots) and [when to request mill certs](https://3dwk.com/resources/when-to-request-mill-certs-with-cnc-parts); here we focus on **PT packaging** so estimators do not have to guess whether every cosmetic witness needs a penetrant line.
What dye-penetrant inspection means for quoting
PT uses a penetrant, a dwell, and a developer to highlight surface-breaking cracks or porosity. It does not measure size, and it does not see subsurface flaws. Process pages for aerospace stainless ([CNC 3-axis SS 316 aerospace](https://3dwk.com/en/cnc-3axis/ss-316/aerospace.html), [CNC 5-axis SS 316 aerospace](https://3dwk.com/en/cnc-5axis/ss-316/aerospace.html), [CNC 3-axis SS 316](https://3dwk.com/en/cnc-3axis/ss-316.html)) assume method, timing, and sample-size notes when lots mix structural arms with cosmetic covers. This page does not invent a NADCAP stamp, a lab name, or an acceptance class you did not write.
Do not bury “NDT as needed” inside a notes cloud. Put **PT (VISIBLE DYE) ON ARM A AND WELD PREP B, 5 PCS PER 25, AFTER MACHINING AND BEFORE PASSIVATION; COVER TABS FAI ONLY** on the PDF and mirror it in RFQ text. Background: [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote), [when to specify passivation after CNC stainless](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless), [stainless 304 vs 316 vs 17-4PH](https://3dwk.com/resources/stainless-304-vs-316-vs-17-4ph).
When PT helps, and when it does not
PT is most useful on faces where a surface-breaking crack would matter: a thin arm after roughing or heat treat, a ground seat, a weld prep, or a heavily milled pocket corner. It is a poor default on every as-machined cosmetic face, on a surface that is still packed with cutting fluid, or after a coating that can hide or fill a tight indication. If you already specified MPI, check the alloy: 316 is generally not a magnetic-particle candidate; 17-4PH may be, depending on condition—write the method that matches the material.
Stainless 316 bar and plate $/kg planning bands sit in the mid-to-upper commodity range under **rates.v1.1** material tables; material cost is rarely the PT story—cleaning, dwell time, and lot sampling are.
Timing, finish, and sampling
Say **before or after** passivation, bead blast, or paint, and whether PT is first-article only, a sample per lot, or 100%. Cleaning before penetrant is part of the method; “PT after unknown finish” is a common **needs_manual** trigger. If a face will be [passivated](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless) or coated, decide whether PT is the last look at the bare metal.
Tolerance and inspection traps
Calling “100% crack check” without a method, mixing PT with a CMM of the same faces in one unlabeled “inspect all,” or asking for fluorescent PT plus a named aerospace spec you have not attached, all force **needs_manual**. Default non-critical covers to dimensional inspection only, and put PT only on the features that need a surface-crack look. Related: [CMM vs optical measurement](https://3dwk.com/resources/cmm-vs-optical-measurement-machined-parts), [first-article inspection reports](https://3dwk.com/resources/first-article-inspection-reports).
Illustration 1 shows an **Example part** stainless arm with labeled PT sampling on the structural arm versus first-article-only PT on a cover tab.
Example part walkthrough
**Example part:** Stainless 316 block ~90×40×16 mm, structural arm A and a cover tab, PT (visible dye) on arm A for 5 pieces per 25 after machining and before passivation, cover tab first-article PT only, ISO 2768-m general, no MPI. With STEP + **PT VISIBLE DYE, ARM A, 5 PER 25, AFTER MACHINE / BEFORE PASSIVATION; COVER TAB FAI ONLY; NO MPI**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “100% NDT / crack check as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“dye check everything”) versus a labeled PT pack and the clearer estimate path.
What widens bands fastest
Unlabeled “NDT as needed,” 100% PT on every cosmetic face, method left as MPI on a 316 part, and PT after an unspecified finish. Quantity breaks help unit cost 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 [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
What to do next
Mark **method (PT vs MPI), faces, sample size, and before/after finish**, 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 NDT questions rather than forcing a fake price.
Common questions
Do all stainless CNC lots need dye-penetrant inspection?
No, and not as a silent default under **rates.v1.1** estimates. Many dimensional lots never need PT. Use it when a surface-breaking crack on a named face would matter, and write the sample size.
Can I write “MPI or PT as needed” and let the shop choose?
That is a common **needs_manual** trigger. 316 is a poor MPI candidate; a heat-treated 17-4PH part might use MPI instead of, or in addition to, PT. Name the method that matches the alloy and the face.
Should PT come before or after passivation?
Usually after machining and **before** passivation or coating, so the bare metal is inspected. If your specification says otherwise, attach that specification. “PT after finish, finish unstated” widens the band.
Is PT a substitute for a CMM or a mill cert?
No. PT looks for surface-breaking discontinuities; a CMM checks size and form; a mill cert traces the stock. They can appear on the same pack, but one does not replace the others.