Basic Machining Knowledge: If You Skip It, the Part Fails

A part is a collection of surfaces, and each surface carries two kinds of requirements: its own size, and its position relative to other surfaces. Position requirements cover the distance between surfaces plus geometric relationships such as coaxiality, parallelism, perpendicularity, and circular runout. Those relationships only mean something when you name a datum.

Here is the trap. If the drawing shows a tight perpendicularity callout but no datum feature symbol, the inspector has to guess. So does the machinist. One shop may measure from the bottom face, another from a side face, and both will report different numbers from the same part. That argument costs days.

A good drawing sets the primary datum as the surface that will locate the part in its final assembly. Then secondary and tertiary datums follow, usually in the order the part is clamped. When we quote a job, the first thing our process engineers check is whether the datum scheme matches the setup plan. When it does not, we send a DFM note before cutting metal.

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  Primary datumThe surface that seats the part in assembly. It should be flat and reachable.
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  Datum orderMatch the clamping order. Out-of-order datums force extra setups.
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  Mixed datumsA datum on a curved or as-cast surface is hard to repeat. Machine it first.

A single tolerance is easy. A chain of them is not. If four features stack in one direction and each carries ±0.05 mm, the assembly can drift by ±0.2 mm before any process variation is added. Engineers who tolerance every feature in isolation often find this out during first-article inspection.

Our general machining tolerance is ±0.005 mm on critical dimensions, with typical surface finishes from Ra 0.2–0.8 μm for fine work up to Ra 1.6–3.2 μm as-machined. Those numbers are not free. Holding ±0.005 mm means slower passes, more inspection, and sometimes a temperature-controlled room. Specifying it on a clearance hole wastes money.

The practical rule: tolerance only what the function needs. A bearing bore, a sealing face, and a dowel pin hole earn tight numbers. Bolt clearance holes, chamfers, and non-critical pockets do not. When a tight tolerance is unavoidable, tell the shop which dimension is the driver so the setup can be planned around it.

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  Stack before you specAdd the chain first, then decide which link needs tightening.
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  Functional onlyTight tolerances on non-functional features add cost with no benefit.
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  Flag the driverMark the one dimension the setup must protect.

Cutting tools are round, and they have length. A deep pocket with a small corner radius forces a small-diameter tool, and small tools deflect. A 6 mm end mill reaching 60 mm deep will chatter long before it holds a tight tolerance. When we see a depth-to-diameter ratio above about 4:1, we flag it.

Undercuts, internal threads near a shoulder, and features on five faces all push the part into more setups. Each setup adds a new datum shift. On a three-axis machine, a part with features on six sides might need three or four fixtures. On a simultaneous five-axis center, the same part can often be done in one or two. That changes both accuracy and cost.

Sharp internal corners are another common one. A square pocket corner cannot be cut by a round tool. Either the drawing names a corner radius, or the shop picks one and the part does not match the mating piece. Standard practice is to call out the largest radius the function allows.

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  Depth ratioKeep depth under 4× tool diameter when accuracy matters.
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  Corner radiusSpecify it. The tool will leave one anyway.
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  Setup countEach extra setup adds datum shift and cost.

Aluminum 6061-T6 cuts fast and holds tight tolerances well. It is the default for prototypes and most brackets. 7075 is stronger but more prone to distortion after heavy material removal, so roughing and finishing are often split into two operations with a stress-relief step between them.

Stainless 304 work-hardens. Light passes with a sharp tool work better than heavy passes that rub. Titanium Ti-6Al-4V and Inconel are worse: they hold heat at the cutting edge, so tool life drops and the process runs slower. Those jobs get quoted with more time, not less.

Surface finish follows the same logic. Anodizing, electroless nickel, and powder coating all add a thin layer. A hardcoat anodize layer can be 25–50 μm thick, which matters if a bore has a tight tolerance. Tell the shop about the finish before the dimensions are finalized, not after.

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  6061-T6Good all-round choice. Stable, fast to cut, easy to finish.
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  304 stainlessWork-hardens. Use sharp tools and light cuts.
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  Ti-6Al-4VSlow speeds, short tool life. Budget more cycle time.