CNC processing fee fault: why the number on the quote does not match the part

• 1
  What a CNC processing fee fault actually is
• 2
  Setup time is where small quantities break down
• 3
  Cutting time, tool changes and the tolerance tax
• 4
  Inspection, scrap and the cost of proving the part is good
• 5
  Which cost line to question first

A fee fault is not the same thing as an expensive quote. An expensive quote can still be correct. A fee fault is when the quoted number cannot be reproduced by the physics of the part: the tolerance, the material, the geometry and the quantity do not add up to the hours and tooling shown on the paper.

The fault usually shows up in one of two directions. Either the shop under-quoted and will recover the money later through change orders and delays, or the shop over-quoted because it priced a worst case it never intended to hit. Both are faults, and both cost the buyer.

Machining cost is built from a small number of physical quantities: how long the spindle is cutting, how long the machine sits idle during setup, how much fixturing and tooling the part needs, and how many parts get scrapped before the run is stable. Everything else on a quote is a markup on those four numbers.

When a quote cannot be traced back to those four quantities, you are not looking at a price. You are looking at a guess with a currency symbol in front of it.

• 1
  Under-quoteRecovered later through change orders, tooling charges or schedule pressure.
• 2
  Over-quotePriced on a worst case the shop never expected to machine.
• 3
  Real signalCutting time, setup time, tooling and scrap. Nothing else is physical.

Setup is a fixed cost per batch, not per part. On a single prototype it can dominate the whole quote. On a 5,000-piece run it disappears into the noise. This is why the same drawing gets two very different prices at two quantities, and why a fee fault is most visible on low-volume work.

The number that matters is how many distinct setups the part needs. A simple plate on a three-axis mill may need two: one for the top face, one for the back. A housing with ports on four sides can need four or five, or one on a five-axis machine with a rotary table. Each setup carries its own fixture, its own probe cycle and its own chance of a locating error.

Fixtures are the hidden line item. Soft jaws, vacuum plates, custom clamps and sacrificial stock all take design and machining hours before the first good part exists. On complex geometry, fixture design can run longer than the first part's cutting cycle. A quote that lists only machine hours has quietly left this out.

There is a practical test. Ask how many setups the part requires and which faces are machined in each. If the answer is vague, the setup line on the quote is probably a guess too.

• 1
  Two setupsTypical for a plate or bracket on a three-axis machine.
• 2
  Four or moreHousings, manifolds and parts with features on every face.
• 3
  One setupPossible on a 5-axis center with a Ø400 mm rotary table.

Cycle time is the part of the quote you can check with arithmetic. Take the material removal volume, divide by a realistic material removal rate for the alloy and the tool, and you get a floor. Finishing passes, tool changes and retracts add on top. When a quoted cycle time sits far below that floor, the quote is optimistic.

Tolerance changes the arithmetic more than most drawings suggest. Holding ±0.005 mm on a feature usually means a separate finishing pass at light depth, a spring pass, and a slower feed to keep the tool from deflecting. A feature held at ±0.1 mm can be cut in one pass at full feed. The two features may look identical on the drawing.

Material drives the rate. Aluminium 6061 and 7075 cut fast and clear chips well. Stainless 316 and 17-4PH work-harden, so the tool has to stay engaged and the feed cannot be dropped to zero. Titanium TC4 and Inconel run slower again, with more tool wear per cubic centimeter removed.

A quote that prices Inconel at aluminium cycle times is not generous. It is a fault waiting for a change order.

• 1
  Rough floorRemoval volume ÷ realistic removal rate per alloy.
• 2
  Tight toleranceAdd a separate finishing and spring pass.
• 3
  Hard alloys316, 17-4PH, TC4 and Inconel cut at a fraction of aluminium rates.

Inspection is a real cost and it scales with tolerance, not with part size. A bracket checked with calipers and a height gauge takes minutes. A part with true position callouts, a surface finish spec and a first-article requirement takes CMM time, and CMM time is machine time.

Scrap is the other line that gets under-priced. Every new setup produces a first part that may be wrong, and every hard alloy produces some tool breakage. A stable run on 6061 might scrap one or two pieces. A first run on a thin-wall Inconel part can lose more before the process settles.

This is why quantity matters so much to the unit price. Setup, fixturing and first-article inspection are paid once and divided across the batch. At one piece, the buyer pays all of it. At 10,000 pieces, the same burden is a rounding error.

A fee fault often appears when a quote silently assumes a large batch while the order is a one-off. The unit price looks reasonable and the total does not.

• 1
  Calipers and gaugesEnough for loose features checked at the bench.
• 2
  CMM and first articleNeeded when position and finish are called out.
• 3
  Scrap allowanceHigher on first runs, thin walls and hard alloys.