Biesseworks: 7 Costly CNC Machining Mistakes to Avoid

A drawing covered in ±0.005 mm callouts does not make a better part. It makes a slower, more expensive one. Every tight tolerance forces extra passes, more in-process checks, and sometimes a different machine. When a whole print carries the same tight number, the shop has no way to know which features actually matter.

The practical rule is to tolerance by function. Bearing bores, mating faces, and sealing surfaces earn the tight number. Bolt clearance holes, cable routing slots, and cosmetic edges do not. Opening those to ±0.05 mm or ±0.1 mm cuts cycle time and inspection load without touching how the part works.

Tolerance stack-up is the other half of this. If three features each carry ±0.01 mm and they all feed the same assembly dimension, the stack can exceed the assembly allowance. Engineers who mark one critical datum and reference the rest from it avoid most of this. We ask for that datum early, because it changes the setup plan.

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  Do thisMark only functional features tight; leave the rest at general tolerance.
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  Avoid thisOne global tolerance block covering every dimension on the print.
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  Why it mattersTight callouts multiply inspection time, not just cutting time.

Cheap stock is not cheap. A material that costs less at the mill can cost far more at the spindle. Free-machining grades cut faster, hold finish better, and distort less. Alloys with high residual stress may need a stress-relief step between roughing and finishing, and that step has a price.

Take aluminium as an example. 6061-T6 machines cleanly and is the default for most housings and brackets. 7075 gives higher strength but fights the cutter and can move after roughing. Titanium grades such as TC4 (Ti-6Al-4V) cut at a fraction of the speed of aluminium and wear tooling fast, so a design that works in 6061 may be uneconomical in titanium.

The same logic applies to stainless. 303 is the free-machining grade and behaves well on a lathe. 316L resists corrosion better but galls and work-hardens, which means lighter cuts and more time. If the part only sees indoor air, 303 is often the right answer. We compare machinability across grades during DFM and flag the switch before cutting starts.

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  Do thisPick the least exotic grade that meets the mechanical and corrosion spec.
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  Avoid thisSpecifying titanium or Inconel where aluminium would pass the load case.
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  Watch forResidual stress in thick plate; rough, relieve, then finish.

A sharp internal corner is the most common geometry trap. A rotating cutter leaves a radius equal to its own radius, so a true 90° internal corner needs EDM or a different design. Designers who model a zero-radius pocket corner force the shop into a secondary operation or a re-quote.

Depth-to-diameter ratio matters too. A pocket 10 mm wide and 60 mm deep needs a long, thin tool that deflects and chatters. Widening the pocket or splitting it into steps keeps the tool rigid. The same applies to deep slots and narrow ribs, where a taller rib is weaker once the material around it is removed.

Undercuts, cross-drilled holes that break into a bore, and threads that stop inside a blind hole all add setup work. None of them are impossible. They just cost more than the drawing suggests, and the fix is usually a small change made at the design stage rather than at the machine.

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  Do thisAdd the cutter radius to internal corners, or call out EDM.
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  Avoid thisPockets deeper than about four times the tool diameter.
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  CheckBlind-hole threads need a relief groove and enough run-out.

Ra numbers get written loosely and read loosely. An Ra 0.8–1.6 μm finish is standard as-machined territory, produced by a normal finishing pass. Chasing Ra 0.2–0.8 μm needs finer tools, slower feeds, and sometimes a polishing step. A blanket Ra 0.4 μm callout on a part that only needs to look clean drives cost with no functional gain.

Finish also interacts with material and geometry. A polished aluminium face shows every scratch, while the same finish on a bead-blasted surface hides them. If appearance matters, say so on the drawing. If it does not, say that too. Both statements save money.

Missing notes are the quiet killer. Engineers omit the datum, the thread class, the edge break, the material condition, or the surface treatment thickness. The shop then guesses, and a guess that is wrong shows up as a rejected lot. A short notes block on the print beats a long email thread after the parts ship.

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  Do thisList datum, thread class, edge break and finish per face.
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  Avoid thisA single global Ra callout with no face reference.
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  RememberAnodize and plating add thickness that shifts tight dimensions.

Simple parts turn complex when there is nothing to hold. A thin disc, a long shaft, or a part with no flat face forces custom soft jaws, a vacuum plate, or a sacrificial tab. Those are real hours. A small boss or flat pad added to the model can let the part sit in a standard vise, and that change is often free.

Thin walls deflect under clamping pressure. Walls below about 1 mm in aluminium need light passes and low clamp force, which slows the cycle. Adding a few ribs or thickening one wall usually pays for itself in the first run.

Post-processing is the last place teams lose time. Anodizing, electroless nickel, powder coating and laser marking each add lead time and can change dimensions. Hardcoat anodize builds roughly half into the surface and half into the part, which matters on a ±0.005 mm bore. Laser marking needs a minimum character height of 1.5 mm to stay legible. Plan these steps with the machining, not after it.

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  Do thisAdd one flat clamping face or pad for the first operation.
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  Avoid thisWall thickness under 1 mm unless the design demands it.
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  Order of opsMachine, then finish; mask or re-cut tight bores after coating.