Machining basics: start here

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  Key takeaways
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  What machining basics start with: cutting, not shaping
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  Cutting tool geometry and what it limits
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  Fixturing and how many setups a part needs
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  3-axis vs 5-axis: what actually changes
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  Where machining stops being the right answer
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  Tolerance, finish and how they are verified
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  Choosing a process by part and quantity

Machining basics start with one idea. A rotating or stationary cutting tool contacts a solid block and shears material away as chips. Nothing is cast, pressed or added. The finished part is the negative space left behind, which is why the process holds tight tolerances without a dedicated mold.

The tool edge does the work. Carbide inserts and solid end mills have a defined rake angle, and each tooth takes a chip of a set thickness. Feed per tooth, spindle speed and depth of cut decide whether that chip forms cleanly or rubs and work-hardens the surface. Get the chip load too low and the tool skates. Too high and the edge chips.

Heat is the constraint. Most cutting energy turns into heat at the shear zone, and the chip carries a large share of it away. Flood coolant, through-tool coolant or air blast all exist to control that balance. On titanium and stainless, poor chip evacuation is the usual cause of burnt edges and short tool life.

This is why material choice matters more than machine brand. Aluminium 6061 cuts at high surface speed with generous depth. 17-4PH stainless and Ti-6Al-4V need lower speeds, stiffer setups and more frequent tool changes. The same drawing can cost three times more in one alloy than another.

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  Chip loadFeed per tooth; too light rubs, too heavy breaks edges.
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  RigidityTool overhang and fixture stiffness set achievable finish.
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  CoolantChip evacuation matters as much as cooling.

Every cutter has a minimum internal radius. A Ø6 mm end mill cannot cut a sharper corner than its own radius, so a pocket drawn with R0.5 corners will force a smaller tool, slower feed and more passes. Designers who round internal corners to at least one third of the pocket depth usually cut cycle time without losing function.

Pocket depth also matters. A rule of thumb is that depth should stay under four times the tool diameter for a rigid cut. Deeper than that and the tool deflects, chatter appears on the wall, and the operator must reduce feed. Long-reach tooling exists, but it trades away finish and accuracy.

Threads, holes and slots each have their own limits. A Ø2 mm hole is fine in aluminium at 3,000 rpm and above. The same hole in Inconel needs peck drilling, rigid coolant and patience. Thread milling beats tapping on hard alloys and large diameters because it controls chip evacuation better.

Undercuts and side-entry features need a different approach. A T-slot cutter or a lollipop tool can reach beneath an overhang, but the tool is weak and the pass is slow. When a drawing has many undercuts, it is worth asking whether the part can be split into two pieces and assembled.

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  Internal radiusKeep corners at least one third of pocket depth.
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  Depth-to-diameterStay under 4:1 for a stable cut.
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  UndercutsOften cheaper to split the part than to reach them.

A setup is the operation of clamping a part in a known position. Every extra setup adds labor, adds alignment error, and adds the risk of a mark on a finished surface. Three setups and a careful operator will beat five setups and a rushed one, even on the same machine.

Soft jaws, vises and vacuum plates handle most prismatic work. For thin walls and rings, a custom fixture or a sacrificial tab holds the part without distorting it. Machining a thin wall with too much clamp pressure is a common cause of out-of-round bores and bowed faces.

Datum choice drives everything downstream. Pick a face and two edges that will be machined early, and dimension the drawing from those. When a drawing gives coordinates from three different datums, the programmer must reconcile them, and the risk of a mismatch rises.

On a 5-axis machine, one setup can reach five faces plus compound angles. The tradeoff is that the part must be rigidly held in a single orientation, and the programmer must think about tool reach across a wider work envelope. On a 3-axis machine, the same part may need four or five setups.

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  DatumPick one primary datum and dimension from it.
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  Thin wallsUse tabs or soft jaws to avoid clamp distortion.
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  Setup countEach re-fixture adds error, not only time.

A 3-axis mill moves X, Y and Z. The tool always points straight down, so the part must be re-fixtured to reach a new face. That is fine for plates, housings and brackets where most features lie on one or two planes. It is the cheapest and fastest route for simple geometry.

A 5-axis machine adds two rotary axes, so the tool can approach from an angle. This reaches compound faces, deep pockets and sculpted surfaces without re-clamping. For parts with many angled features, one 5-axis setup can replace four or five 3-axis setups, and the accuracy improves because the part never moves.

The cost picture is not automatic. Programming a 5-axis toolpath takes longer and needs simulation to catch collisions. Short cycle parts with simple geometry are often cheaper on a 3-axis machine. The 5-axis route wins when setup reduction or reach is the real constraint.

At GreatLight we run 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The right machine depends on the part, not on a preference for more axes. A mill-turn center, for example, finishes a turned shaft with cross-drilled holes in one cycle.

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  3-axisOne face per setup; lowest programming cost.
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  5-axisReaches compound angles; fewer setups, more programming.
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  Mill-turnTurned and milled features in one cycle.

Machining is wasteful by design. Material removal can take 60 to 80 percent of the stock as chips on a complex part, and that cost scales with the volume removed. When a part is mostly a box with a few holes, a casting or a welded assembly often wins on unit cost.

Very thin walls are another boundary. Below about 0.5 mm, deflection from cutting force becomes hard to control, and the part may need stress relief or a support fixture. Some geometries can be machined, but the yield drops and the price reflects it.

Internal channels and lattice structures are difficult for a rotating cutter to reach. Additive manufacturing builds those features directly. The usual compromise is a hybrid: print the internal geometry, then machine the critical mating faces to tolerance.

Volume is the last factor. Tooling for casting or forging costs thousands, so it only pays above a certain quantity. Machining carries no tooling charge and no minimum order quantity, which makes it the default from one prototype to a few thousand parts.

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  Material wasteChips can be most of the stock on complex parts.
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  Thin wallsUnder 0.5 mm, deflection and yield become the issue.
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  VolumeMachining has no tooling cost; casting needs volume to pay off.

A tolerance is a promise about size, and it only holds if the machine, the tool and the fixture can all support it. A ±0.005 mm callout on a thin, unsupported wall is unenforceable. On a rigid feature measured with the right gauge, it is routine work.

Surface finish is measured in Ra. As-machined faces typically sit at Ra 1.6–3.2 μm. A fine finish at Ra 0.2–0.8 μm needs a lighter finishing pass, a sharp tool and often a different insert. Asking for a fine finish across a whole part adds cost; asking for it only on sealing faces does not.

Verification matters as much as the cut. Calipers and micrometers cover most dimensions, but a coordinate measuring machine is the right tool for true position, profile and angular callouts. GreatLight inspects 100 percent of parts before shipment, with raw material checks, in-process monitoring and a final inspection, and reports are available on request.

We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. For regulated work, those certificates mean the process is documented, not that any drawing can be met. The drawing still has to be manufacturable.

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  ToleranceOnly valid if machine, tool and fixture support it.
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  FinishSpecify Ra only where function requires it.
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  Inspection100 percent before shipment; reports on request.