Protech CNC Machining: How Tolerance Is Actually Held

• 1
  What Protech CNC Machining Really Means on the Floor
• 2
  Why Five-Axis Motion Changes the Cut, Not Just the Reach
• 3
  Thermal Drift: The Error Nobody Sees at Setup
• 4
  How Material Choice Rewrites the Machining Plan
• 5
  Inspection Is Part of the Process, Not a Final Gate
• 6
  Choosing the Right Machine for the Part

The phrase gets used loosely. In practice, Protech CNC machining is a set of decisions made in a fixed order: which machine, how the part is held, how the tool path is sequenced, and how the result is measured. Get that order wrong and no amount of spindle speed rescues the part.

A tolerance is not a machine specification. It is the sum of every error source between the spindle and the surface: spindle runout, thermal growth, fixture deflection, tool wear, and the resolution of the probe or gauge used to check the result. A machine rated to ±0.005 mm only produces that number when the other four items sit well below it.

That is why two shops can quote the same part at the same tolerance and deliver very different results. The difference is rarely the machine. It is what surrounds the machine.

This page is for engineers and buyers who need a way to read a quote. It covers the mechanics of five-axis motion, when a part belongs on a three-axis mill instead, how heat moves a dimension, and what the inspection report should actually contain.

• 1
  Machine choiceDetermines reach and setups, not accuracy on its own.
• 2
  WorkholdingUsually the largest single error source on thin parts.
• 3
  Thermal stateA 5 °C shift moves steel roughly 0.006 mm per 100 mm.
• 4
  InspectionIf it is not measured the same way, it is not the same part.

A three-axis mill moves the tool in X, Y and Z. The cutter orientation stays fixed, so the tip of a ball nose tool travels at near zero surface speed at its center. That leaves a witness mark and forces the programmer to step over at 5 to 10 percent of tool diameter to keep the scallop height down. On a contoured surface, that is a lot of passes.

Five-axis machining adds two rotary axes. On our simultaneous centers, the rotary table is Ø400 mm. Tilting the tool lets the cutter engage the flank rather than the tip, which raises effective cutting speed and spreads wear across a longer edge. Stepovers of 20 to 30 percent of tool diameter become practical, so a curved face finishes in fewer passes with a better Ra.

The second gain is setup count. A part with features on five faces can be reached by rotating the table instead of re-fixturing. Every re-fixture is a chance to lose 0.02 mm. Removing three setups often buys more accuracy than tightening the machine spec.

The cost is programming and rigidity. Rotary axes add compliance, and the post processor has to handle singularity points where two axes try to align. For a flat bracket with holes on two faces, five-axis is expensive overhead.

A machine does not hold size the moment the spindle starts. Ball screws, spindle bearings and the part itself all grow as they warm. A cold machine and a machine that has run for four hours are different tools. That is why the first part off a Monday morning shift is the one most likely to drift out of tolerance.

The material matters as much as the machine. Aluminum expands about 23 × 10⁻⁶ per °C, steel about 11 to 13, and Inconel is closer to steel but cuts far hotter. A 100 mm aluminum bore measured at 25 °C will read about 0.02 mm smaller when it cools to 20 °C. If the drawing calls out a dimension at 20 °C, that gap is real.

In-process gauging and probing catch drift that a pre-set offset cannot. We probe critical features between passes, and for long runs we re-check the first article after the machine reaches steady state, not before.

For parts held to ±0.005 mm over long dimensions, the practical rule is simple: let the machine warm, measure at a controlled temperature, and treat any measurement taken on a cold part as provisional.

The same geometry machines very differently depending on stock. Aluminum 6061 and 7075 cut fast and hold a fine finish with little effort. Stainless 316 work hardens, so the cutter has to stay engaged and never rub. Titanium Ti-6Al-4V conducts heat poorly, so the tool edge absorbs most of it and lives are short.

Inconel is the extreme case. Cutting speeds drop by an order of magnitude against aluminum, and the material springs back under the tool. That means more passes, more tool changes, and a longer quoted lead time for the same drawing. A feature that is easy in 6061 can be a genuine constraint in Inconel.

Plastics bring a different problem. POM and PEEK move with temperature and clamp pressure, so the fixture has to be gentle and the finishing pass light. Deep pockets in unfilled plastics tend to chatter regardless of feed, so we reduce radial engagement rather than push the speed.

None of this changes the tolerance a shop can hold in principle. It changes how much time and how many setups it takes to get there. That is the number that shows up in the quote.

• 1
  AluminumFast, stable, good finish. 6061, 7075, 6082, 2024.
• 2
  StainlessWatch work hardening. 303, 304, 316L, 17-4PH.
• 3
  TitaniumShort tool life, heat stays in the edge. TC4.
• 4
  PlasticsLight clamping, small radial stepover. POM, PEEK, PA.

A part is only as good as the measurement behind it. If the shop checks a bore with calipers and the customer checks it with a CMM, the two numbers will disagree. Datum choice, probe stylus diameter and probing force all shift the reading slightly. Agree on the method at the quoting stage.

Good practice is three checkpoints. Incoming stock is verified before it is cut, because a wrong alloy or a hard spot ruins the run. In-process probing catches drift while the part is still in the fixture. Final inspection confirms the drawing before the part leaves the building.

We inspect 100 percent of parts before shipment and issue reports on request. For first articles, the report should list the nominal, the tolerance band, the actual reading and the instrument used. A single pass or fail column tells the buyer nothing.

Surface finish needs the same discipline. Ra 0.8–1.6 μm is a normal machined finish; Ra 0.2–0.8 μm needs a deliberate finishing pass and often a different tool. Specify which one you need, because the second costs more and only matters if a seal or a bearing runs against it.