Bead blast finish on stainless: what to put on the drawing
Put six things on the drawing, not just the words "bead blasted": which faces are cosmetic, the blast media and grit, the order of operations with passivation last, what gets masked, how the parts will be inspected, and how they are packed. Then ask every shop to price against one approved sample. A note that says only "bead blasted" lets each shop quote its own idea of the finish, and you cannot compare the prices.
Blasting is not one finish. The Nickel Institute's handbook on cleaning stainless steel says that with blasting "a variety of finishes can be obtained through selection of abrasives and adjustment of pressures". So one shop prices fine glass bead on every face. Another prices coarse media on the faces it can reach. A third adds a polish first. All three can say they quoted "bead blasted".
Which faces should you mark as cosmetic?
Only the faces a customer will see. Mark them on the drawing as A surfaces, and leave the rest as B. Protolabs Network uses the same split: the A side is "the most aesthetically important surface, typically the visible side of your product". The B side is hidden after assembly. Every cosmetic face costs handling time, fixturing and inspection, so a part with every face marked cosmetic is priced as if every face were visible.
A visible face needs one more instruction: whether machining marks may show through the blast. Blasting hides fine tool marks. It does not always hide steps, chatter or witness lines from a second setup. If those must disappear, say so, because it can mean an extra finishing pass before blasting.
What should the finish note say?
Write it as a note the shop can quote without asking a question:
- Process and media. "Glass bead blast, [grit size], uniform matte, A surfaces." Glass bead gives a satin look. Coarser or angular media gives a rougher, duller one.
- Clean media only. Media used on carbon steel must not be used on stainless. The Nickel Institute warns that iron particles from steel shot or grit "may become imbedded in the surface". Left there, they rust.
- Passivate after blasting. Call out passivation to ASTM A967 as the last step before packing. The handbook lists "iron from cutting tools and contaminated sandblast materials" as the contamination passivation removes. A967 also gives a free iron test, so the shop can show the treatment worked.
- Masking. List the threads, bores, sealing faces and datum faces that must not be blasted. Protolabs Network says blasting leaves dimensions "not affected except for specific features". Make sure your fits are not among those features.
- Packing. Ask for each part to be wrapped or separated. A cosmetic face scratched in the box is a reject, and nobody will agree on who caused it.
Fit a roughness value (Ra) only if you or the shop will actually measure it. Ra measures roughness, not sheen or colour, so the sample does more to control the look than the number does.
How do you stop arguments when the parts arrive?
Write down how a face is judged. Without that, one inspector holds a part to a bright lamp and another glances at it across the bench. Protolabs Network publishes an inspection condition you can copy onto your drawing or purchase order:
| Condition | Protolabs Network criterion |
|---|---|
| Viewing distance | 50 cm ± 10 cm |
| Lighting | 500-1000 lux, 3300-5300 K neutral white |
| A side | No scratches, stains, blemishes or hang marks |
| B side | No scratches, stains or blemishes; minor hang marks and up to 2 minor defects allowed |
Then agree a reference sample. Ask the first shop you shortlist for one blasted coupon or first-off part in your grade. Approve it, sign it, and send a photo of it with the RFQ to every other shop. The written note tells them what to do, and the sample shows them what you will accept.
Does all this make the quote dearer?
It makes the price honest. Media, masking, passivation and careful packing are real operations. A shop that leaves them out of a vague quote either adds them later as a revision or skips them. You then find out when the parts arrive, or when they show rust spots in service. A full finish note lets you compare like with like. It also shows you which shop has priced the work and which has guessed. It belongs in the finish line of the RFQ, alongside the material, quantity and tolerances covered in what to send with a machining RFQ.
Ready to price a batch of visible stainless parts? Send the drawing with its finish note to Vesprr Workshop and you will get a quote for that exact specification.
Sources
- Nickel Institute / American Iron and Steel Institute, Cleaning and Descaling Stainless Steels, Designers' Handbook Series No. 9001 (sections on sand blasting, vapor blasting and passivation).
- Protolabs Network (Hubs), Surface finishing services: cosmetic finishes, A and B sides, and cosmetic acceptability criteria.
- ASTM International, ASTM A967/A967M: Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts.