A cost-down redesign changes the drawing, material or process of a machined part to lower its price while the part keeps its function. The main levers are looser tolerances, a more machinable material, fewer setups, a closer stock size and a different process, and each one trades a saving against a specific risk.
What is a cost-down redesign of a machined part?
A cost-down redesign starts from function. For every feature you ask what it does, what the drawing demands of it, and whether those demands are higher than the function needs. EN 12973:2020 is the European standard on value management, the discipline behind this question, and describes how organisations plan and apply it.
Machining cost follows machining time, tooling, handling, inspection and material. A requirement that nothing depends on raises one or more of these for no return. Redesign removes such requirements, or replaces a costly way of meeting a real one.
This article gives the direction of each effect and not a percentage. Savings depend on part, machine, material and quantity, and no standard or authority publishes a general figure. The only numbers below are values from standards and geometry we calculated from them.
Which redesign levers cut cost, and what do they risk?
Table 1. Redesign levers, cost effect and risk for machined parts
| Lever | Cost effect | Main risk | Check before release |
|---|---|---|---|
| Looser tolerance on non-functional features | Fewer finishing passes, less measuring | Fit, seal or alignment fails at assembly | Tolerance stack of the assembly |
| Roughness stated only where it matters | Finishing pass no longer the default | Wear or leakage on a functional surface | Which faces seal, slide or bear |
| Free-cutting or easier-machining material | Shorter cycle, lower tool wear | Corrosion, weldability, strength or approval changes | Material approval and certificate type |
| Stock size closer to the finished part | Less material bought and removed | Size not stocked, or straightness lost in thin bar | Bar and plate sizes the shop stocks |
| Fewer setups | Less handling, fewer datum transfers | Position tolerance between faces gets harder | Datum scheme and position tolerances |
| Different process, for example sheet metal instead of milling | Lower cost per part at suitable quantity | Tooling cost; different tolerance class and material | Break-even quantity, general tolerance standard |
Source: Ertaş Engineering assessment of the effect directions; standards named in the sections below.
Two of the six rows rarely need anything beyond a drawing revision: tolerance and roughness. Material, stock and process changes also reach purchasing, certificates and tooling. Try those two first.
How far can you loosen a tolerance?
Start with the general tolerance. ISO 2768-1:1989 gives four classes (f, m, c and v), three of which are usual, and the step between them is large. Our network works to ISO 2768-m as standard, tighter per drawing. A drawing that asks for class f, or for an individual tolerance on every size, moves away from that default. ISO 2768-1 is listed as to be revised, and the 1989 edition stays valid meanwhile.
Table 2. ISO 2768-1 linear tolerances and band width, mm
| Nominal size | Class f | Class m | Class c | Band m → c |
|---|---|---|---|---|
| over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | 0.4 → 1.0 |
| over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | 0.6 → 1.6 |
| over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | 1.0 → 2.4 |
Source: ISO 2768-1:1989, Table 1. Band width = twice the deviation. The last column is our calculation. Detail in the ISO 2768 article.
For a 50 mm length, class c lets the part vary by 1.6 mm against 0.6 mm in class m, a band 2.7 times wider. That is a calculation, not a saving: class c is for features whose size does not matter, such as a clearance edge or a cut-out. A part's functional dimensions keep their individual tolerances.
Where a feature does have an individual tolerance, the IT grade of ISO 286-1:2010 sets its width. For 18 to 30 mm the standard tolerance is 21 µm in IT7, 52 µm in IT9 and 130 µm in IT11. Moving a non-critical seat from IT7 to IT9 widens the band 2.5 times. ISO 286-1 was confirmed in 2026.
Surface roughness works the same way. Indicate it to ISO 21920-1:2021, which replaced ISO 1302:2002, and only on the faces that seal, slide or bear a load. A roughness value on every face makes the finishing pass the default for the whole part.
Can a more machinable material lower the price?
Sometimes. ISO 683-4:2016, confirmed in 2022, gives delivery requirements for free-cutting steels in three groups: for parts not heat treated, for case-hardening and for quenching and tempering. Free-cutting grades exist to machine more easily, and the shop confirms the gain on the actual part. They are not interchangeable with the grade they replace, and the added elements change other properties.
Stainless steel shows the trade-off clearly. Free-cutting 1.4305 (303) has 0.15 to 0.35 % sulphur, according to one supplier datasheet. The same datasheet reports lower corrosion resistance than grade 304, pitting and crevice corrosion in chloride environments above 60 °C, and poor weldability. A part that sees chlorides, or is welded after machining, should keep 1.4301. For stainless bar, the current delivery standard is EN 10088-3:2023, adopted in Germany as DIN EN 10088-3:2024-04, and a revision draft dated 2026-12 is already listed.
A material swap must pass four checks: mechanical properties against the load case, corrosion in the real environment, welding or heat treatment after machining, and the customer's material approval.
How do stock size and setups change the machining effort?
Material removed is stock volume minus finished volume. A solid round part turned from bar shows the effect in a few lines of geometry. Take a part with a 50 mm diameter and 80 mm length.
Table 3. Stock removal for a Ø50 × 80 mm turned part from two bar sizes (calculation)
| Bar diameter, mm | Bar volume, mm³ | Removed volume, mm³ | Change in removal |
|---|---|---|---|
| 65 | 265,465 | 108,385 | — |
| 60 | 226,195 | 69,115 | −36 % |
Source: Ertaş Engineering calculation, V = π/4 × d² × 80 mm; finished part simplified to a plain cylinder of Ø50 × 80 mm.
The saving exists only when the smaller bar size is stocked and the finished part fits inside it. Ask the shop which sizes it holds before you shorten a diameter by a few millimetres. See DFM for CNC milling for the milling rules on corner radius, pocket depth and wall thickness, which are the next place to look.
Setups follow the same logic. Every face the cutter cannot reach needs another clamping and another datum. Moving a feature to a face that is already machined removes a setup. The risk is a position tolerance between features that now sit on different faces, so check the datum scheme first.
When geometry is the cost driver, change the process. A bracket milled from a block may be cut flat and bent from sheet instead. Process selection sets out the break-even between tooling and unit price. Our design engineering service reviews drawings for these changes before they reach the shop.
How do you release a redesign for a part already in series?
A redesign of a part in series is an engineering change, and the customer who owns the drawing approves it. Send the change as a revised drawing with a change note that lists what changed and why. Ask the supplier to confirm in writing that nothing else moved.
Agree on proof before volume: a first article on the new revision, as in first article inspection to AS9102, or the PPAP elements your quality agreement names. Keep the old revision available until the new one is approved, so you can fall back if the first parts fail.
Change one lever at a time on critical parts. If tolerance, material and process change together, a failed first article cannot be traced to one cause.
Frequently asked questions
- What is a cost-down redesign?
- A change to the drawing, material or process of an existing part that lowers its price while the part keeps its function. It starts from the function of each feature, then removes requirements that nothing depends on.
- Which change usually costs the least effort to make?
- Removing requirements that no function needs, such as a tolerance or a roughness value on a free surface. Such a change touches the drawing only. Material and process changes also touch certificates, tooling and approvals.
- How much does a looser tolerance save?
- No standard gives a percentage, because the saving depends on part, machine and shop. What a standard does give is the width of the band: for a 50 mm length, ISO 2768 class c allows a band of 1.6 mm against 0.6 mm in class m, which is 2.7 times wider.
- Is free-cutting steel a safe replacement for a standard steel?
- Only after a check. ISO 683-4 covers free-cutting steels for parts not heat treated, case-hardened or quenched and tempered, but the added sulphur changes other properties. Free-cutting stainless 1.4305, for example, resists corrosion less well than 304 and welds poorly.
- Does a redesigned part need a new first article?
- Treat it as yes when form, fit, function, material or process changes, and agree the scope with your customer. The customer's quality agreement decides which documents are due.
Sources
- EN 12973:2020, Value management (EVS, national CEN member)
- ISO 2768-1:1989, life cycle: published 1989-11-02, stage 90.92, revision project listed (ISS, national ISO member)
- ISO 2768-1 linear tolerances, classes f, m, c (AmesWeb table)
- ISO 286-1:2010, ISO catalogue, status 90.93, confirmed 2026-06-19
- ISO 286-1:2010 standard tolerance grades IT6 to IT11 (table reproduced on Wikipedia)
- ISO 8015:2011, GPS fundamentals, confirmed 2021-12-10 (ISO catalogue)
- Envelope requirement and the ISO 8015 default that size is controlled independently of form (GD&T Basics)
- ISO 21920-1:2021, life cycle: published 2021-12-20, replaces ISO 1302:2002 (ISS, national ISO member)
- ISO 683-4:2016, Free-cutting steels, status 90.93, confirmed 2022-01-26 (ISO catalogue)
- DIN EN 10088-3:2024-04, stainless steel bars, current, replaces DIN EN 10088-3:2014-12 (DIN Media)
- Stainless steel 1.4305 / 303 bar: sulphur 0.15–0.35 %, corrosion resistance lower than 304, weldability (Aalco datasheet)
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