CNC Parts Custom Made: How Tolerance Actually Gets Held

• 1
  What "custom made" changes about the machining process
• 2
  Where the error comes from
• 3
  Choosing a tolerance band that the part can actually hold
• 4
  Material behavior sets the real ceiling
• 5
  Matching machine travel to part size and setup count
• 6
  When a tight tolerance is the wrong call
• 7
  Which process route fits your part

A stock part comes off a line with one fixed setup. A cnc parts custom made order starts from your model, so every decision downstream belongs to that one job: stock size, workholding, tool sequence, inspection plan. Nothing is inherited from a previous run.

That has a practical consequence. Accuracy on a custom part is not a property of the machine alone. It is the sum of the machine, the fixture, the tool, the material and how many times the part gets moved between operations.

Take a simple aluminium bracket with six holes and two pockets. On a three-axis machine it needs three setups. Each setup reintroduces the same error stack: vise jaw deflection, chip left under a locating face, operator clamping force. A five-axis machine with the right fixture can reach five of those six faces in one setup, so the stack is applied once instead of three times.

This is why two shops quoting the same drawing can both claim five-axis capability and still land at different results. The hardware matters, but the setup plan matters more.

• 1
  One setup is worth more than one tight specFewer re-clamps means less stacked error, even on a moderate-tolerance part.
• 2
  Custom means the fixture is designed for the partA dedicated soft jaw or vacuum plate is often the cheapest accuracy upgrade available.

Positioning error is the floor you cannot go below. A machine with linear scales holds tighter than one relying on ball screw pitch alone, and thermal growth over a long run shifts the zero point by a few microns per degree. On a warm afternoon that shows up on the last parts of a batch, not the first.

Tool deflection is the next layer. A long, thin end mill pushed at a heavy radial depth will bend and cut oversize, then spring back and leave a tapered wall. The fix is not a slower feed alone. It is a shorter gauge length, a larger diameter where the geometry allows, and lighter radial engagement with higher spindle speed.

Workpiece movement is the most common cause of a part that measures well on the bench and fails on the CMM. Thin walls, tall ribs and unsupported bosses all deflect under clamping pressure and then relax after unclamping.

The last layer is measurement itself. If the inspection setup holds the part differently from the machining setup, you are measuring the fixture, not the part.

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  Thermal driftLong unattended runs shift zero; in-process checks catch it.
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  Tool push-offLong reach tools cut oversize on deep cavities.
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  Clamp relaxationThin sections move after unclamping.
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  Measurement biasInspection fixturing must match machining fixturing.

Tolerance is a cost lever long before it is a quality target. Moving a bore from ±0.1 mm to ±0.01 mm usually adds a finishing pass, a temperature-controlled check and possibly a jig-bore step. It does not double the price, but it does change the process plan.

A useful default for machined metal parts is ±0.05 mm on critical features and general tolerances of ±0.1 mm on everything else, called out through a title-block note. That keeps the drawing readable and stops the shop from applying a tight band across faces that do not care.

GreatLight holds ±0.005 mm on selected features when the geometry supports it: short bores, flat faces, features reachable in one setup. It is not a blanket number for any drawing. Deep bores with a length-to-diameter ratio above 5:1, thin webs under 1 mm, and free-form surfaces blending into a sharp edge are where the limit moves outward.

Surface finish travels with tolerance. A bore held to ±0.01 mm usually needs Ra 0.8–1.6 μm or better, because a torn surface will not repeat on the gauge. As-machined at Ra 1.6–3.2 μm is fine for non-fitting faces.

• 1
  Tight where it fitsApply the tight band to mating and locating features only.
• 2
  State your datumTolerance without a datum reference is an argument waiting to happen.

Aluminium 6061-T6 machines clean and holds ±0.02 mm comfortably on most prismatic parts. Its thermal expansion is roughly 23 µm per metre per degree Celsius, so a 300 mm part checked straight off a warm machine can read 0.01 mm off before it settles.

Stainless 304 work-hardens at the cut. Light passes with a sharp tool and constant feed work; dwelling in the cut does not. 17-4PH in the H900 condition is dimensionally stable and common for shafts, but it is hard on tooling and slow to cut.

Titanium Ti-6Al-4V is the classic case where the part moves more than the cutter. It conducts heat poorly, so the tool edge runs hot, and the material springs back against the flank. Rough, stress-relieve, then finish is the normal sequence for anything with a thin section.

Plastics are their own problem. POM and PEEK move after machining as internal stress releases, so hold the finish cut for a day and check again. ABS and PC are fine for fixtures and covers but will not hold a bearing fit.

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  Aluminium 6061 / 7075Good stability, fast cutting, the default for prototypes.
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  Stainless 303 / 304 / 17-4PHWatch work hardening and tool wear.
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  Titanium, InconelPlan a stress-relief step between rough and finish.
• 4
  POM, PEEK, PAMachined dimensions drift after stress release.

Setup count drives accuracy more than spindle speed does. A part that fits inside a single five-axis envelope with one workholding position will beat a part that gets re-clamped four times, even on a tighter machine.

For large frames and long profiles, GreatLight runs up to 4,000 mm of processing travel, with a 4,000 × 400 × 150 mm envelope on the long-bed machines. Medium work in the 750 × 1,150 × 550 mm and 600 × 600 × 600 mm range covers most enclosures and manifolds.

Compact parts in the 500 × 500 × 450 mm and 500 × 310 × 200 mm envelopes are where the tightest work happens, because the part is small relative to the machine and thermal drift matters less across the feature. A Ø400 mm rotary table with 16 simultaneous 5-axis centers handles angled ports, undercuts and blended surfaces without a second op.

Size also decides whether a feature is machinable at all. A deep internal pocket narrower than the smallest available cutter shank cannot be reached regardless of tolerance.

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  Long parts4,000 mm travel for frames and rails.
• 2
  Medium housings750 × 1,150 × 550 mm and 600 × 600 × 600 mm.
• 3
  Compact tight work500 mm class envelopes hold the finest features.

Not every drawing needs the tightest band the shop can hold. If a feature sits under a gasket, a ±0.2 mm band does the same job as ±0.02 mm and costs less to inspect, less to scrap and less to rework.

The warning sign is a tolerance with no partner feature. A hole specified at ±0.01 mm with no mating pin dimension usually means the tolerance was copied from a template rather than calculated from a fit. Ask what it mates with before you tighten anything.

The opposite failure is a loose band on a locating feature. If two parts are doweled together and the dowel hole is called out at ±0.3 mm, the assembly will fight on the line even though every individual part passes inspection.

A free DFM review catches both cases. Send the model and drawing, and we mark the features where the band can open up and the features where it needs to close.

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  Loose where nothing matesOpen the band on clearance and cosmetic faces.
• 2
  Tight where alignment mattersDowel holes, bearing seats, sealing faces.