Oklahoma CNC Processing Expert Guide

• 1
  What CNC processing actually removes
• 2
  Setup count: the first thing to check
• 3
  What ±0.005 mm means on the shop floor
• 4
  Material choice and the cutting window
• 5
  Finishing changes the last 0.02 mm
• 6
  Which machine setup does your part need?
• 7
  When CNC is the right process, and when it is not

CNC processing is subtractive. A cutter follows a toolpath and shears material away, so the finished part is whatever is left when the tool stops. That single fact drives every decision downstream. The part cannot be stronger than the stock it came from, it cannot hold a feature the tool cannot reach, and every surface that gets touched will carry some tool mark.

The three variables on the machine are cutting speed, feed rate and depth of cut. Speed is how fast the cutting edge moves past the material, feed is how far the tool advances per tooth, and depth is how much material one pass bites off. Push all three and you either break the tool or burn the surface. Back off and cycle time climbs. The workable window is narrower than most CAD models assume.

Soft materials let you run fast. Aluminum 6061 machines at high spindle speeds with generous feed, which is why it stays the cheapest metal to prototype in. Titanium and Inconel behave the opposite way: low surface speed, light depth of cut, and much more tool wear. The same geometry in Ti-6Al-4V can take several times the cutting time of 6061-T6.

Tool access sets the ceiling on geometry. A 3-axis mill cuts from one direction only. Any feature on the side of the part, or any undercut, forces the operator to stop, unclamp the part, rotate it and clamp again. Each of those repositions is a setup, and each setup adds alignment error and time. Cutting the number of setups is the single biggest lever on both cost and accuracy.

• 1
  Subtractive by natureThe tool can only remove what it can reach.
• 2
  Speed, feed, depthSet the window; nothing outside it machines well.
• 3
  Material sets the pace6061 is fast, Ti-6Al-4V is slow, Inconel slower.

A setup is one fixturing of the part on one machine, with a known zero point. A part machined from six sides needs at least six setups on a 3-axis machine, unless someone builds soft jaws or a custom fixture to hold it in odd orientations. Every extra setup means the operator touches the part again, and datum shift creeps in.

A simultaneous 5-axis center moves the tool and the part at the same time, so a compound surface or a set of angled holes can come off in one setup. That is the real advantage: not that the machine is faster, but that the part is handled once. Accuracy improves because there is no second re-clamp to introduce error, and the shop saves the labor of fixture building.

Not every part justifies 5-axis. A flat plate with holes on one face, a simple shaft, a housing open from one direction: these run well on 3-axis, faster and cheaper. Reach for 5-axis when the part has contoured surfaces, undercuts, deep pockets on more than one face, or features whose angles cannot be squared to a standard vise.

There is a middle ground. A 4-axis mill adds a rotary table, so a part can be indexed to a new face without being unclamped. Round parts with cross-drilled holes, or long parts with features on four sides, often fit here better than on either 3-axis or full 5-axis. The rotary table on our machines is Ø400 mm, which sets the practical size limit for that approach.

Setup count also decides whether a design is prototype-friendly. One-off brackets with 12 faces and no fixturing surfaces will cost more in setup than in cutting. If a part is meant for low-volume runs, design it so it can be held from one or two directions.

• 1
  3-axisOne direction per setup; best for prismatic parts.
• 2
  4-axisRotary indexing; good for parts with four-sided features.
• 3
  5-axisOne setup for compound geometry and angled holes.

Tolerance is not a wish, it is a cost. A general machining tolerance of ±0.1 mm holds on most features without special effort. Tightening to ±0.05 mm means slower passes and more in-process checks. At ±0.005 mm the process changes: temperature matters, the machine must be warmed up, and the part may need to be measured on a coordinate measuring machine rather than with calipers.

Thermal drift is the reason. Aluminum expands about 23 μm per meter per degree Celsius. A 300 mm part that warms by 5 °C during roughing grows roughly 0.03 mm before finishing even starts. Shops that hold tight tolerances let the part cool, take a skim pass, and measure in a controlled room. That is time, and time is money.

Tolerance should be assigned per feature, not per drawing. A mounting hole pattern needs a tight position tolerance because bolts must line up. A clearance pocket on the same part may be fine at ±0.2 mm. Marking every dimension ±0.01 mm is a common habit that adds cost without adding function.

Surface finish and tolerance travel together. Fine finishes come from small stepovers and sharp tooling, which also help dimensional control. A Ra 0.8–1.6 μm finish is a normal machined finish on aluminum and steel. Ra 0.2–0.8 μm requires slower finishing passes, fresh inserts and often a separate operation. As-machined Ra 1.6–3.2 μm is fine for brackets and internal parts no one sees.

• 1
  General±0.1 mm costs nothing extra on most features.
• 2
  Tight±0.005 mm needs warm-up, cooling and CMM checks.
• 3
  Per featureTighten only where function demands it.

Aluminum is the default for prototypes and most housings. 6061-T6 machines cleanly, welds, anodizes and holds moderate tolerances. 7075 is stronger but less corrosion resistant and harder on tooling. 2024 machines well but does not anodize as evenly. If the part is a fixture or a stressed bracket, 7075 usually makes sense. If it is a housing or a cover, 6061 does the job.

Stainless steel covers a wide range. 303 is the free-machining grade and the easiest to cut. 304 and 316 resist corrosion better but work-harden, so light passes and constant feed are mandatory; dwelling in the cut destroys the surface. 17-4PH can be machined then aged to a high strength. 316L is the medical and marine default.

Titanium and nickel alloys are where cost jumps. Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays in the cutting edge. Inconel is worse: it work-hardens fast and eats tooling. These materials are chosen for temperature, weight or corrosion reasons, never for cost. When they appear in a design, expect long cycle times and plan the finishing operation carefully.

Plastics behave differently again. POM and PA cut cleanly but move with temperature and moisture. PEEK is expensive and abrasive. ABS and PC are common for enclosures. With plastics, sharp tooling and high spindle speed matter more than depth of cut, and clamping pressure has to be low enough not to deform the part.

Magnesium AZ31B and AZ91D are light and machine fast, but chips are flammable, so the shop needs dedicated handling. Copper and brass cut easily and conduct heat away from the tool, which is why beryllium copper and C36000 show up in electrical and mold work.

• 1
  Aluminum6061-T6 for general parts, 7075 for stressed ones.
• 2
  Stainless303 easy, 304/316 work-harden, 17-4PH ages hard.
• 3
  ExoticsTi-6Al-4V and Inconel are slow and tool-hungry.
• 4
  PlasticsSharp tools, light clamping, watch thermal growth.

A machined surface is not the final surface. Anodizing builds an oxide layer, typically a few micrometers to tens of micrometers depending on type. Hardcoat anodizing is thicker and grows into the part, so a tight bore can close up. Plating adds metal. Powder coating adds tens of micrometers. If a dimension is critical and the part will be coated, tell the shop before the part is cut.

Bead blasting and tumbling change texture, not size, but they round edges. A sharp corner that measured correctly before blasting will show a small radius after. Brushing leaves directional lines and hides tool marks on visible faces. Polishing is a manual step and the most variable of the group.

Laser marking is a common final step. Minimum character height is 1.5 mm for legible marks, and the mark goes on a flat or gently curved surface. Deep engraving on a stressed part can act as a stress riser, so keep it shallow on load-bearing areas.

Finishing also affects lead time. Anodizing and plating are usually sent to a partner line, which adds a day or more. If a part is needed fast, consider leaving it as-machined and finishing later, or asking what finish is already in-house.

• 1
  AnodizingBuilds oxide; hardcoat can close tight bores.
• 2
  BlastingRounds edges; radius appears after the fact.
• 3
  Marking1.5 mm minimum character height.