Chicago CNC machining: making accuracy possible on real parts

A tolerance of ±0.005 mm (±0.0002 in) is a total band of 10 μm. For scale, a human hair is roughly 70 μm. When a Chicago engineer puts that number on a drawing, it has to survive the whole process: the blank, the fixturing, the cut, the coolant, the chip evacuation, and the measurement itself.

Tolerance means nothing without a datum. Most tolerance failures we see start at the drawing, not the machine. If datum A is a rough cast surface and datum B is a hole drilled later, the inspector and the machinist are measuring two different parts. Fix the datum scheme first, then talk about the machine.

On our 16 simultaneous 5-axis machining centers we can reach ±0.005 mm on features cut in the same setup. That single-setup condition matters more than the axis count. Every re-clamp adds stack-up error, and stack-up is what eats a 10 μm band.

Not every feature needs that band. A mounting face at ±0.05 mm and a bearing bore at ±0.005 mm can sit on the same part. Tolerancing each feature to its function keeps cycle time and cost under control.

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  Same setupFeatures cut without re-clamping hold position relative to each other.
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  Clean datumsMachined datums inspect faster and repeat better than as-cast surfaces.
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  Function-based toleranceOnly tighten the features that control fit or motion.

A 3-axis mill cuts from one direction. If your part has holes on four sides, that means four setups, four fixtures and four chances to lose position. Our 3-axis machines handle flat plates, housings and simple brackets very well, and they are the cheapest way to make them.

A 4-axis mill adds a rotary table, Ø400 mm on our machines. Parts that are essentially cylindrical with features around the circumference, like shafts, flanges and cam profiles, belong here. One rotation replaces several re-clamps, and the angular position stays locked to the part.

Simultaneous 5-axis is for contoured surfaces and features that cannot be reached from a straight approach: turbine blades, impellers, medical housings with undercuts, and any part where a long thin tool would chatter if it had to reach around a corner. The tool tip stays normal to the surface, so the cut is shorter and the finish is more even.

If a part is simple and flat, 5-axis adds nothing but cost. We say so. The honest answer for a lot of Chicago work is a well-fixtured 3-axis job with a good vise and a probe.

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  Plate work3-axis, single or two setups, lowest cost per part.
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  Round parts4-axis with Ø400 mm rotary table, angular features in one clamping.
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  Contoured or deepSimultaneous 5-axis keeps the tool short and normal to the surface.

A spindle running at 12,000 rpm gets hot. So does the ball screw, and so does the part while the tool is cutting it. Aluminum expands about 23 μm per meter per °C. A 300 mm aluminum part that warms 5 °C during roughing grows roughly 35 μm before you even measure it.

That is why the order of operations matters. Rough, let the part rest, semi-finish, rest again, then finish. On tight work we leave stock for a final pass after the part has returned near room temperature. Cutting to final size while the part is warm means it shrinks out of tolerance when it cools.

Coolant helps and hurts. Flood coolant pulls heat out of the cut zone, but it also creates a temperature gradient across the part if it only hits one side. For thin walls we often slow down and use air blast or a light mist instead of flooding one face.

Shop temperature is not a detail. A shop that swings 10 °C between morning and afternoon will hold different numbers at 7 a.m. and 3 p.m. Climate control is part of the tolerance, not a comfort feature.

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  Rough, rest, finishLeave stock for a final pass after the part cools.
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  Even coolingAvoid cooling one face of a thin wall harder than the other.
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  Stable roomA 10 °C shop swing shows up directly in the measured size.

A part will move if the fixture lets it. Thin plates bow when a vise clamps them. Long shafts deflect under tool pressure. Round parts spin or lift when the cut direction changes. Most of the out-of-tolerance parts we rework were not cut wrong; they were held wrong.

For thin plates we use vacuum chucks or support the underside with a soft backing so the cutter is not pressing the part into open air. For long parts we add a steady rest or support blocks at the midpoint. For 5-axis contoured parts we often cut soft jaws from the same stock batch so the grip matches the part geometry.

Clamping pressure is a variable you can control. A torque wrench on the vise screw gives repeatable clamping, and repeatable clamping gives repeatable size. It sounds small. On a ±0.005 mm part it is not small.

Datum transfer is the other half. When the part moves from the first setup to the second, the second setup must reference a machined surface from the first, not a raw edge. That rule alone removes a large share of position errors.

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  Soft jawsCut from the same stock batch so the grip matches the part.
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  Support the middleLong and thin parts need a steady rest or support blocks.
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  Repeatable clampingA torque wrench on the vise keeps the pressure the same every cycle.

Inspection is not a final gate. It is a loop. We check incoming material, monitor the cut in progress, and inspect the finished part before it ships. Reports are available on request, and every part is inspected before shipment.

A CMM report and a caliper reading can disagree and both be right. The CMM measures at 20 °C on a granite table; the caliper measures a warm part on a bench. If the drawing says ±0.005 mm, the measurement method has to be agreed before the first cut, not after the parts arrive in Chicago.

For bores and shafts, gauge repeatability matters as much as the tolerance. A bore gauge that reads to 1 μm is useful; one that reads to 10 μm on a 10 μm band is not. We match the instrument to the band.

When a feature is hard to measure after assembly, we measure it in-process and record the offset. That is how position stays controlled on parts that get buried inside a housing later.

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  Measurement standardAgree on the instrument and temperature before the first cut.
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  In-process checksCatch drift during the run, not after 500 parts.
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  Reports on requestRaw material, in-process and final inspection records.

Aluminum is forgiving. It cuts fast, holds a good finish, and 6061 or 7075 will sit at ±0.005 mm without drama as long as you manage heat. Thin aluminum walls are the exception; they deflect and vibrate, and no amount of machine rigidity fixes a 0.8 mm wall.

Stainless moves more. 304 and 316 work-harden if the tool rubs, so the cut has to stay under the hardened layer. 17-4PH holds dimension well after heat treatment, which is why it shows up in medical and aerospace work. Titanium TC4 (Ti-6Al-4V) needs low cutting speed and a lot of coolant, and it springs back against the tool.

Plastics are the opposite problem. POM and PEEK cut clean but move with temperature and absorb moisture. A PEEK part measured right off the machine can be a different size the next morning. For tight plastic parts we machine, let them normalize, then take the final pass.

Inconel and magnesium sit at the ends of the range. Inconel is slow and hard on tools; magnesium cuts easily but needs chip control because fine magnesium chips are a fire risk. Both are doable. Both need the drawing to say what actually matters.

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  Aluminum6061, 7075 cut fast and hold ±0.005 mm with heat control.
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  Stainless304 and 316 work-harden; keep the tool under the hard layer.
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  PlasticsPOM and PEEK need a rest before the final pass.