CNC accuracy tips for parts that must hold tolerance

Most accuracy problems start at the title block. A drawing that says ±0.005 mm everywhere forces every operation into the tightest setup, and the price follows. On a 100 mm aluminium bracket, the mounting hole pattern may genuinely need ±0.005 mm, but the outer profile rarely does. Split the drawing: locate critical features from a shared datum, then open the rest to ±0.1 mm or a general tolerance note.

Ask what the feature does. A bearing bore, a dowel hole, a seal groove and a mating face carry function. A clearance slot, a cable pass-through or a cosmetic edge does not. When a feature only needs to clear another part, geometric tolerance on position usually describes the requirement better than a linear ± on every dimension.

Remember stack-up. If three parts assemble, the tolerance budget is divided, not repeated. A common mistake is calling ±0.005 mm on each of five stacked components; the assembly then misses by far more than the individual prints suggest. Assign the tightest band to the one feature that sets alignment and let the others breathe.

Material matters here too. Aluminium 6061 and 7075 cut cleanly and hold size well in a temperature-controlled shop. Titanium Ti-6Al-4V and Inconel 718 spring back, generate heat and need slower passes, so a 0.005 mm band costs more in cycle time. Stainless 316 work-hardens if the feed is too light, which shows up as taper and chatter rather than a clean cut.

A datum is a promise the machine can keep. Put the primary datum on a surface that will be machined flat in the first setup, not on a raw cast surface with draft. If the datum is a rough surface, every later operation inherits that variation, and no amount of machine accuracy recovers it.

One setup beats three. Every re-fixture adds an error that is invisible on the machine but visible on the CMM. For parts with features on four or five faces, a 5-axis machine with 16 simultaneous centers in our shop cuts the part in fewer setups, so the relationship between bores stays tight without manual alignment. Where a part reaches 4,000 mm, machine travel and part rigidity, not the control, usually set the limit.

Thin walls are a workholding problem before they are a cutting problem. A 1 mm wall on a 100 mm pocket will deflect under clamping and under cutting force. Support it, take light finishing passes, and consider leaving sacrificial ribs that come off last. On mill-turn work, doing the OD and the face in one chucking removes a concentricity error you would otherwise chase in inspection.

Sequence the job so stress relief happens early. Remove the bulk, let the part settle, then finish. Parts hogged from plate and finished in the same pass often move overnight and measure differently the next morning.

• 1
  Datum firstMachine the primary datum flat before locating anything else from it.
• 2
  Fewer setupsEach re-fixture adds positional error that no control can correct.
• 3
  Support thin wallsLight finishing passes and temporary ribs beat heavy clamp pressure.
• 4
  Rough, settle, finishLeave stock for a second pass on parts cut from plate.

A machine that cuts ±0.005 mm at 8 a.m. can drift by 0.02 mm after four hours of roughing. Spindle growth, ballscrew heating and chips carrying heat into the table all move the tool relative to the part. Shops that hold tight bands keep the floor temperature stable, warm the spindles before the first finish pass, and separate roughing from finishing so the finish cut starts from a settled machine.

Tool wear is the slower version of the same problem. A carbide end mill that cuts a 20 mm pocket on the first part will cut it slightly smaller on the two hundredth. Radius wear of 0.01 mm is normal and predictable. On long runs we measure the first part, a middle part and the last part, then offset the tool before the drift reaches the tolerance limit.

Chip evacuation is often the quiet cause of a bad finish. Recutting chips in a deep pocket scores the wall and raises the measured Ra. Through-spindle coolant or a high-pressure jet fixes it, and so does a toolpath that lifts out of the cut instead of dragging back.

For materials like Inconel 718 or hardened tool steel, the cutting edge dulls fast and the cutting force rises with it. That force deflects the tool and the part, so the dimension walks in one direction. Planning a tool change before the wear limit, not after the surface tells you, keeps the band predictable.

Measure the way the part works. A bore checked with a caliper reads across two points on the wall; a bore that has to take a bearing should be checked with a bore gauge or on a CMM at the same temperature as the mating part. An aluminium part measured right after machining is warm and reads larger than it will be at 20 °C.

Define the inspection before the first cut. If the drawing calls a true position of Ø0.05 mm, the CMM needs the datum system the drawing names, and the report needs to show it. Reports are available on request, and every job gets a raw material check, in-process monitoring and a final inspection before shipment.

Watch for the errors that look like machine faults. A part that measures out of round often sat on three points in the fixture. A hole that is straight at the top and tapered at the bottom is usually a dull drill or poor chip clearance. A face that reads flat on the machine and dished on the CMM was clamped against a surface that was not flat.

Surface finish sits next to size for a reason. A Ra 0.8–1.6 μm sealing face on a turned part is not a cosmetic call; it changes how the seal seats. State the finish on the drawing and state how it will be checked, or the shop will pick a finish that passes size and fails function.

A tight first article proves the process, not the run. What keeps a run accurate is the setup sheet: which machine, which fixture, which tool, which offsets, and which measurement triggered the last adjustment. When a job repeats six months later, that sheet is the difference between hitting the band on the first part and spending two days dialing it in.

Capability is measured over parts, not minutes. We track the qualification rate on the parts we ship, and a 99.99% figure means the process holds a band rather than passing a single inspection. For automotive and medical work that requires ISO 9001, IATF 16949 and ISO 13485, the process records are part of the deliverable, not paperwork added afterward.

Volume changes the plan. A single prototype lets you hand-fit and adjust. A 10,000 part run needs a fixture that loads the same way every time and a tool life plan that replaces cutters on a schedule. There is no minimum order quantity here, so the same shop can take a job from one piece to a full run, which keeps the process knowledge in one place.

If a feature keeps drifting, the answer is usually process, not tighter tolerance. Change the datum, change the setup, or change the finishing pass. Writing a smaller number on the print does not make the machine more accurate.