Scrap Rate in Machining: Where Losses Come From and Who Pays
No machine shop quotes a scrap rate, but every quote contains one. A shop that expects 2% of a run to fail builds that 2% into the price; a shop that expects 8% builds in 8%, and if you cannot tell the difference you are paying for someone else's process problem.
Understanding where scrap comes from — and which causes are yours — is one of the fastest ways to take cost out of a program without touching the drawing.
The rates, by part class
Scrap on a stable, well-understood part is small. A turned bushing in brass with a ±0.05 mm bore and one thread runs at 0.5-1.5% across a 5,000-piece lot. A prismatic aluminum bracket in a two-setup process runs 1-3%. A thin-wall housing with a 1.2 mm wall and a flatness callout of 0.05 mm can run 6-10%, and the first lot can run 15% while the process is being dialed in.
The first lot always costs more than the tenth. A shop quoting a new part will allow 3-5% on the first run and 1-2% on repeat runs, which is why the second order often comes in 3-6% cheaper for reasons nobody explains.
| Part class | Typical scrap, repeat lot | Typical scrap, first lot | Dominant cause |
|---|---|---|---|
| Turned bushings, simple | 0.5-1.5% | 2-4% | Tool wear, bore size drift |
| Prismatic brackets, 2 setups | 1-3% | 3-6% | Setup error, datum shift |
| Thin-wall housings | 5-10% | 12-18% | Distortion, chatter |
| Deep-pocket manifolds | 3-6% | 6-10% | Tool deflection, chip recutting |
| Cosmetic anodized covers | 4-8% | 8-12% | Handling and finish damage |
| Tight-tolerance bearing seats | 6-12% | 15-20% | Measurement dispute, temperature |
First-off and setup scrap
The first part off a new setup is a test, not a product. A shop will typically machine one part, measure it, adjust offsets, and machine a second before releasing the run. That is one or two parts consumed per setup, plus the fixture proving time.
On a 50-piece order with two setups, two consumed parts is 4% of the lot before any real failure. On a 2,000-piece order it is 0.1%. This is why small lots carry a scrap allowance that looks disproportionate, and why the same part ordered in larger releases gets cheaper per unit.
You can reduce setup loss by paying for a proper fixture rather than a vise-and-shims setup. A USD 1,500 fixture that removes two prove-out parts from every lot pays back in three lots at 500 pieces.
See also: Machining Cycle Time Calculation: A Worked Example on an Aluminum Bracket and CNC Capacity Planning: Reading a Supplier's Promise Date.
Thin walls, deep pockets, and the geometry that fails
Distortion is the most expensive scrap cause because it usually appears after the part looks finished. A 1.2 mm aluminum wall machined from solid will move when the residual stress in the plate is released; the part measures correctly in the vise and 0.06 mm out of flat when unclamped.
Prevention is process, not inspection. Rough, stress-relieve, then finish; take equal stock from both faces; use climbing cuts with light radial engagement for the final pass. All of it costs machine time, and a shop quoting 15% less than everyone else is usually skipping it.
Deep pockets fail differently. As depth exceeds 3× tool diameter, deflection grows and the wall tapers; recutting chips raises temperature and burns the surface. A part that is 0.02 mm out at the bottom of a 60 mm pocket is scrap unless the tolerance allows it.
Both problems are cheaper to design out than to machine around. If your design has walls under 1.5 mm or pockets deeper than 5× cutter diameter, expect a scrap allowance in the quote and a DFM note suggesting a change.
Finish damage and handling
Anodized parts can be scrapped by a scratch that happened after machining, in deburring, in packing, or in your own receiving bay. On cosmetic covers the finish-damage rate can exceed the machining failure rate, and the cost is not just the part — it is the re-anodizing lead time of 5-7 days on a replacement.
Practical controls: specify protective film on cosmetic faces, individual bagging rather than bulk cartons, and a 100% visual check at final packing rather than a sample. All three add 1-2% to the piece price and remove a much larger loss.
Measurement disputes
Some scrap is not scrap. It is a part that meets the drawing measured one way and fails measured another.
The classic cases: a thin wall measured with a caliper under clamping pressure, a bore measured at 18°C in the shop and 24°C in your inspection room, a flatness measured on a surface plate that is not flat. A 100 mm aluminum part grows about 0.0023 mm per degree Celsius; a 6°C difference moves it 0.014 mm — more than the tolerance on a ±0.01 mm callout.
Agree the measurement method in the PO: instrument, temperature, clamping state, and datum. This single paragraph prevents more rejection arguments than any inspection upgrade.
How scrap is priced into a quote
Scrap enters the price twice. The direct cost is material plus machine time for the lost parts: on a 30-minute part at USD 32/hour with USD 6 of material, a scrapped piece costs about USD 22. At 5% scrap on a 1,000-piece run that is 50 parts, or USD 1,100 spread over the good 950 — about USD 1.16 per part, roughly 3% of a USD 40 price.
The indirect cost is capacity. Scrapped parts consume machine hours that were scheduled for revenue, so a shop with 8% scrap is effectively running 8% less capacity than it appears to have. That is why high-scrap parts get longer lead times as well as higher prices.
When you review a quote, ask two questions: what scrap rate is built into this price, and what would reduce it? A shop that can answer both is managing the process. A shop that says "we don't have scrap" is either very good or not measuring, and the invoice will eventually tell you which. Our about page describes how first articles and process sign-off work here, which is where most scrap is designed out.
Frequently asked questions
What is a normal scrap rate for CNC machining?
Between 1% and 3% for stable prismatic and turned parts on repeat lots, and 5-10% for thin-wall or cosmetic parts. First lots typically run two to three times the steady-state rate while the process is proven.
Who pays for scrapped parts?
The supplier, if the scrap is caused by its process. The buyer, if the cause is a design that cannot hold the tolerance, an unclear drawing, or damage after delivery. This is why the acceptance criteria in the PO matter more than the scrap rate itself.
Does a higher price mean lower scrap?
Not automatically, but it often correlates on difficult parts. A quote 15% below the field on a thin-wall housing usually means less finishing time, less stress relief, and a scrap rate the buyer will discover at receiving inspection.
Can I negotiate the scrap allowance out of the price?
You can negotiate the rate, not the existence of one. Asking a shop to quote with zero scrap allowance just moves the risk into a contingency the shop will not honor later. Better to ask what rate is included and what design change would lower it.
Related reading
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