Where the Money Goes in a Stainless Machining Quote

September 21, 2026

A stainless quote that looks high is usually correct. The useful question is which line is responsible, because two of the six drivers are yours to change and four are not.

1. Grade

The same part in 303 and in 316L are two different prices, and the difference is machine time rather than material.

Published machinability ratings against free-machining carbon steel B1112 at 100: 303 sits near 78, 304 near 45, 316L near 36, 2205 duplex near 30. That is a factor of more than two between 303 and 316L on the same geometry, and wider in practice on parts with many small features.

The trap is that the cheap grade is often the wrong one. 303 pits where 316L holds, which is the subject of 303, 304 and 316L compared. The saving is real only when the part stays dry, unwelded and unpolished.

What you can do. Write the environment on the drawing and leave the grade open where you can. A shop that machines these grades every week will tell you when a cheaper alloy still meets the requirement, and that conversation is worth more than negotiating on the quoted number.

2. Work hardening and tool wear

This is the one you cannot negotiate away. Austenitic stainless hardens where a tool rubs, holds heat in the part and eats inserts. It shows up as longer cycle times and as a tooling line that is a real cost rather than an overhead.

It also explains why a shop that does not run stainless quotes higher, or quotes lower and then requotes. Uncertainty gets priced. Work hardening covers the mechanism and, more usefully, the five geometry changes that reduce it.

What you can do. Open internal corner radii. Reduce the depth of the deepest pocket. Thicken the thinnest wall. None of these cost anything in function on most parts and all of them come off the cycle time.

3. Tolerance and the fact that the part moves

Stainless grows while it is cut, shrinks as it cools, moves when residual stress releases and moves again if it is aged or welded. Holding a tight dimension through that means an extra operation, a settling period and inspection time.

The cost steps are not linear. ±0.1 to ±0.05 mm is close to free. ±0.02 to ±0.01 mm can double the cost of a feature. What each decimal place costs has the bands.

What you can do. Tighten the four features that mate with something and leave the other forty at a general note. This is the single largest saving available on most stainless drawings and it is the one most often left on the table.

4. Finishing, passivation and the second supplier

Bead blast, a Ra callout, passivation to a standard, electropolishing. Each is a real process with a real price, and most of them happen at a finisher rather than at the machine shop. That means a second queue, transport and a second set of paperwork.

Two specific cost traps:

A blanket finish note. Ra 0.8 micrometre in the title block applies the expensive requirement to every surface. Mark the surfaces that matter instead. See the surface finish guide.

An unspecified passivation. The word passivate without a standard means each shop assumes something different. One quotes a citric bath with a copper sulphate test and a certificate, another quotes a rinse. The quotes are not comparable. Citric against nitric covers how to write it.

5. Certificates and traceability

Mill certificates with heat numbers, first article inspection reports, CMM reports, passivation certificates, lot control. These are office hours and they are not free.

On a batch of 500 the paperwork disappears into the unit price. On a batch of 5 it can be a large share of it. That is not a shop being difficult, it is the same fixed cost spread over fewer parts.

What you can do. Ask for the documents you will actually read. A full dimensional report on every feature of every part is rarely what anyone needs. A first article with the critical dimensions, plus mill certs, usually is. Say which features are critical.

6. Quantity and setup

Stainless setups take longer to prove out than aluminium ones, because the first part tells the shop how this particular geometry behaves in this particular material.

That setup cost is fixed. Spread over one part it dominates. Spread over 200 it nearly disappears. The curve is steep at the start and flat by the time you reach a few hundred, which is why two of a part rarely costs twice one, and why 50 often costs far less than ten times five.

What you can do. If you know a part will repeat, say so in the RFQ. A shop that knows an order recurs quarterly can justify fixturing that it would not build for a one off, and it will often price the first batch differently because of it. Prototype against production covers where the curve flattens.

Reading a quote that looks high

Work through it in this order:

1. Is the grade right, or is it habit? If the part is dry and indoors, ask whether 303 or 304 would do.
2. How many tight tolerances are on the drawing, and how many mate with something? If those numbers differ a lot, that is the saving.
3. Is the finish note global or local?
4. Is passivation specified with a standard, or left open?
5. Are you asking for documents you will not read?
6. Is the quantity a real quantity, or a test order that will repeat?

Then, and only then, compare prices. Three quotes against an incomplete drawing are three different scopes, not three prices for the same thing. What to put in a stainless RFQ is the checklist that makes them comparable.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• Why Five Stainless Parts Cost Nearly as Much as Fifty
• How Tight a Tolerance Does Your Stainless Part Need?