Ohio CNC machining guide

• 1
  What the spindle actually does to your part
• 2
  Why setup count matters more than spindle speed
• 3
  Tolerance callouts: what ±0.005 mm really costs
• 4
  Material choice and the machining consequences
• 5
  How to judge a machining supplier before the first cut
• 6
  When CNC machining is the wrong process
• 7
  Which machining route fits which part

A CNC machine does not shape metal by force. It removes it with a rotating edge that has to reach the feature from a direction the tool holder can physically enter. Everything else in machining follows from that constraint. The tool needs clearance behind the cutting edge, the holder needs clearance above it, and the part needs to sit rigidly enough that the cutting force does not push it away from the tool.

Rigidity is where most tolerance problems start. A long end mill cutting a deep pocket deflects under load, and the deflection changes as the tool moves deeper. The same program on a short, stubby cutter holds size; on a long one it may drift. This is why a part that looks simple on a drawing can be hard to hold and a part that looks complicated can be routine.

Heat is the second force at work. Aluminium 6061 conducts heat away quickly and cuts clean at high spindle speeds. Titanium TC4 (Ti-6Al-4V) does the opposite: heat stays at the cutting edge, which shortens tool life and can move the part as it expands. Stainless 316 work-hardens if the cutter rubs instead of cuts. The material list decides the cutting parameters before any geometry is considered.

• 1
  Tool access firstIf the holder cannot enter, the feature has to be re-set or redesigned.
• 2
  Rigidity sets the floorLong reach and thin walls both reduce achievable tolerance.
• 3
  Material sets the speedAluminium, stainless and titanium need different parameters.

Every time a part is unclamped and turned, a small error is added. That error comes from the fixture, the datum, and the operator's measurement of the new zero. Two setups usually hold a few hundredths of a millimetre. Five setups stack the error five times. This is the real reason a five-axis machine changes a quote: it lets more faces be cut in one setup.

A 3-axis machine cuts from one direction, so features on five faces of a cube need five setups. A 4-axis mill adds rotation around one axis, which handles parts like shafts and housings where features repeat around a bore. A simultaneous 5-axis center tilts the tool as it cuts, so it can reach undercuts and angled faces without releasing the part. The part stays at one datum from first cut to last.

Setup count also affects lead time, not just tolerance. Each setup needs its own fixture, its own first-article check, and its own queue slot. On a prototype run of one to ten parts, fixture design can take longer than cutting. On a 10,000-part run, the fixture cost spreads out and a dedicated workholding pays for itself.

There is a limit to what more axes can fix. Five-axis cutting is slower per unit of material removed because the tool is often tilted away from its stiffest direction. If a part is a flat plate with holes, a 3-axis machine will make it faster and cheaper. More axes are worth paying for when the geometry genuinely needs them, not as a default.

• 1
  One setup, one datumFewer re-clamps mean less stacked error.
• 2
  3-axis for flat workPlates and simple pockets do not need rotation.
• 3
  5-axis for reachUndercuts and angled faces without re-fixturing.

A tolerance is a cost statement. General machining holds around ±0.1 mm without fuss. Tightening to ±0.05 mm is routine on a good machine. Going to ±0.005 mm (equivalent to ±0.0002 in) is achievable, but it changes the process: temperature-controlled inspection, more frequent in-process checks, and sometimes a finishing pass that exists only to control size.

The critical question is not how tight the tolerance is, but how many features carry it. A single bore at ±0.005 mm is a controlled operation. Twenty holes at ±0.005 mm across a 400 mm plate is a different job, because the tolerance now depends on the machine's positioning accuracy over its full travel, not just on the cutter.

Surface finish follows the same logic. A Ra 1.6–3.2 μm as-machined finish is what a normal end mill leaves. Ra 0.8–1.6 μm needs a finer stepover or a finishing tool. Ra 0.2–0.8 μm usually means a separate finishing operation, sometimes on a different machine. Each step down doubles the time on that surface.

A practical check: does the drawing tolerance match the function? A cover plate with a sealing groove needs a controlled finish on the groove, not on the whole face. Marking the functional surfaces separately from the cosmetic ones lets the shop spend time where it changes performance.

• 1
  Count the tight featuresOne tight bore is easy; twenty is a positioning problem.
• 2
  Finish is per surfaceSpecify Ra only where it does work.
• 3
  ±0.005 mm needs checksTight size means in-process measurement.

Material selection is usually made for the part's function, then handed to the machine shop. That order is fine, but the shop should feed back on machinability before the design is frozen. Aluminium 6061-T6 machines cleanly and holds thin walls well. Aluminium 7075 is stronger but more prone to distortion when a lot of material is removed from one side.

Stainless 303 is the free-machining grade and cuts well. Stainless 316 and 316L are tougher, gummier, and work-harden if the feed is too light. Titanium TC4 (Ti-6Al-4V) needs low cutting speed, high coolant pressure, and sharp tooling; it is machinable but the cycle time is several times that of aluminium for the same shape.

Plastics behave differently again. POM and PA hold tolerance well but move with temperature. PEEK is dimensionally stable and expensive, so it is used where the part must survive heat or chemicals. Carbon fibre composite is abrasive and wears tooling fast, which shows up as a tooling cost line rather than a cycle time line.

For parts that need plating, anodizing or powder coating, the finish adds a dimensional layer. Hardcoat anodizing builds on the surface by tens of microns and can change a press fit. Tell the shop which surfaces are masked and which are coated before the final size is signed off.

• 1
  6061-T6 is the defaultGood finish, stable thin walls, wide availability.
• 2
  316 work-hardensKeep the feed up; do not let the cutter rub.
• 3
  Coating changes sizeAnodizing adds material; allow for it in the fit.

A supplier's machine list tells you what shapes are possible. It does not tell you whether the process is controlled. Ask for the inspection method, not the inspection promise. A shop that checks raw material on arrival, monitors size during cutting, and inspects before shipment is doing three different jobs. A shop that only inspects at the end cannot catch a drift that started on the first part.

Certification matters when the part matters. ISO 9001:2015 covers a general quality system. IATF 16949:2016 is the automotive standard and covers traceability and change control. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which is relevant when your drawings are confidential. Match the certificate to the industry, not to the marketing page.

Lead time claims need to be separated. Quotation and DFM feedback, production start, and shipping are three different clocks. A useful question is what happens if a dimension comes back out of tolerance on the first article: does the shop re-cut, rework, or re-quote? The answer tells you more than the lead time number.

For a first order, send a part that is representative but not your hardest one. You learn how the shop communicates about tolerance questions, how fast they return a DFM note, and whether the delivered part matches the drawing. That information is cheaper to gather on a simple part than on a complex one.

• 1
  Ask about in-process checksEnd-only inspection misses early drift.
• 2
  Match the certificateIATF for automotive, ISO 13485 for medical.
• 3
  Test with a real partA representative job reveals more than a sample.

CNC machining removes material from a solid block. When a part is mostly hollow, the block you pay for is mostly turned into chips. A die-cast or vacuum-cast housing can be cheaper at volume because the material is placed rather than removed. Machining then finishes the critical bores and faces.

Sheet metal fabrication wins for parts with constant wall thickness and bends. If the design is a bracket with formed flanges, cutting and bending a flat blank is faster than milling the same shape from plate. Machining is better when the wall thickness changes or when a pocket needs a controlled floor.

3D printing is the right route for a shape that cannot be reached by a cutter, or for a fit check before a metal run. It is not a substitute for a functional metal part under load. The useful pattern is to print the geometry, confirm the fit, then machine the final part in the specified alloy.

The decision usually comes down to quantity and geometry together. One complex part: machine it. Ten thousand simple parts: cast or form it, then machine the interfaces. The shop's job is to say which side of that line your part is on, and to say it before the tooling is ordered.

• 1
  Hollow shapesCasting places material; machining removes it.
• 2
  Constant wall and bendsSheet metal is faster than milling.
• 3
  Fit checksPrint first, machine the functional part.