What Kind of Raw Material for CNC Machine Work?

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  Machinability Is a Set of Numbers, Not a Label
• 2
  Aluminum, Steel, and Stainless: Where Each One Fits
• 3
  Copper, Brass, Titanium, and Engineering Plastics
• 4
  Bar, Plate, or Billet: Stock Form Changes the Plan
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  How Material Choice Drives Cost and Lead Time
• 6
  When the Wrong Material Shows Up in the Part
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  A Practical Method for Choosing the Raw Material
• 8
  Material Families for CNC Machining at a Glance

Every material for CNC machine work gets judged on four numbers before anyone programs a toolpath: hardness, thermal conductivity, chip behavior, and work-hardening rate. Hardness sets the floor on cutting speed. Aluminum 6061 at roughly 95 HB runs fast. 17-4PH in the H900 condition runs slow because the same insert that shears soft metal rubs instead of cutting.

Thermal conductivity decides where the heat goes. Aluminum and copper pull heat into the chip and the fixture, so the tool stays cool. Titanium and stainless push heat back into the cutting edge, which is why Ti-6Al-4V eats inserts and demands flood coolant.

Chip behavior controls surface finish. Free-machining brass C36000 breaks chips short and leaves a clean wall. 304 stainless galls and smears unless you keep the feed per tooth high enough to stay under the work-hardened skin.

Work-hardening is the trap most new engineers hit. Cut too light on austenitic stainless and the surface gets harder with every pass. The next pass cuts hardened metal, not the original stock.

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  HardnessSets maximum cutting speed and tool grade
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  ConductivityDecides whether heat leaves with the chip
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  Chip formShort chips mean stable finish and no recutting
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  Work-hardeningLight passes on 304 and 316 make the next cut worse

Aluminum is the default for housings, brackets, and heat sinks. 6061-T6 is the workhorse: weldable, corrosion resistant, and stable enough to hold ±0.005 mm on a well-fixtured 5-axis setup. 7075 gives roughly twice the yield strength of 6061 and machines almost as easily, but it is not weldable and it costs more. 2024 has better fatigue life and worse corrosion resistance, so it usually gets anodized or painted.

Carbon steel covers shafts, plates, and wear parts. 1018 and 1045 machine cleanly and take a good finish. 4140 and 4340 hold up under load and are common in automotive and industrial machinery. A36 is cheap plate stock with inconsistent hardness, so it is a poor choice when you need a tight tolerance across a large face.

Stainless earns its cost in wet, sterile, or corrosive service. 303 is the free-machining grade and the easiest of the family to run. 304 and 316 resist corrosion better but work-harden, so roughing passes need to stay aggressive. 17-4PH can be aged after machining to reach high strength while the part is still soft enough to cut.

Tool steel and Inconel sit at the far end. They are cut only when the service condition demands it: hot work, abrasion, or high-temperature strength.

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  Pick 6061-T6General housings, fixtures, prototypes, heat sinks
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  Pick 7075High-strength brackets where welding is not required
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  Pick 304 or 316Food, medical, marine, and wash-down environments
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  Pick 17-4PHStrength above stainless grades, aged after machining

Copper alloys are chosen for conductivity, not strength. C101 and C110 are near-pure copper for busbars and RF parts. C36000 brass machines faster than any steel on the floor and holds fine detail, which makes it a favorite for fittings and small connectors. Beryllium copper is reserved for spring contacts that need both conductivity and fatigue resistance.

Titanium is a weight problem solver. TC4 (Ti-6Al-4V) gives a high strength-to-weight ratio and excellent corrosion resistance, but it cuts at a fraction of the speed of aluminum and the chips are a fire risk in fine form. Magnesium AZ31B and AZ91D are even lighter and machine quickly, with the same chip hazard.

Plastics split into commodity and engineering grades. ABS, PC, and PMMA are cheap and fast, good for covers and prototype enclosures. POM holds tolerance well and slides without lubrication. PEEK survives high temperature and chemicals, costs many times more, and needs sharp tooling to avoid melting.

Carbon fiber reinforced plastic is abrasive. It wears carbide quickly and releases dust that needs extraction, so it is normally reserved for parts where stiffness per kilogram matters more than tool cost.

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  Conductivity partsC101, C110, C36000 for electrical and RF work
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  Weight-critical partsTC4 titanium, magnesium AZ31B or AZ91D
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  Sliding partsPOM for low friction and dimensional stability
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  High-temperature partsPEEK when service temperature rules out other plastics

The same alloy behaves differently depending on how it arrives. Extruded bar is dense and consistent, so it turns well and holds straightness. Rolled plate can carry internal stress that releases when you remove material, which bows a long part after the last pass.

Billet is usually the soundest choice for thin walls and tight flatness because it is forged or rolled with fewer internal voids. It costs more per kilogram and generates more chips, but it saves the second operation you would otherwise need to straighten a warped plate.

Castings and die-cast blanks are worth considering when the part has ribs or non-machined features. They reduce stock removal, but porosity can open up on a sealing face. If the part needs a pressure-tight surface, billet is the safer route.

For parts up to 4,000 mm, we machine from plate, bar, and billet on the same floor. The stock form is picked during DFM review, not after the quote.

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  Extruded barConsistent grain, good for turned parts
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  Rolled plateCheck stress relief before thin-wall work
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  BilletBest flatness and wall stability
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  Cast blanksLower stock removal, porosity risk on sealing faces

Material cost is only part of the picture. A cheap alloy that machines at half the speed of 6061 can cost more per finished part than the expensive one, because cycle time dominates on small runs. Titanium is the classic example: the stock is costly and the cutting time is long.

Tool life moves the number too. Inconel and carbon fiber reinforced plastic consume carbide at a rate that shows up in the quote. Soft plastics look cheap until you account for the slow feeds needed to avoid melting and the fixtures needed to hold a flexible part.

Availability matters for schedule. Common grades like 6061, 303, and 1018 are usually on the shelf. Exotic grades and special tempers may need to be ordered, which pushes the start date. We can begin production within 24 hours on stocked material.

Post-processing is part of the decision. Anodizing suits aluminum, passivation suits stainless, and black oxide suits steel. Choosing a material that cannot take the finish you need forces a redesign later.

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  Cycle time beats stock priceOn small runs, cutting speed often dominates total cost
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  Tool consumptionHard and abrasive materials add real cost per part
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  AvailabilityStocked grades start faster than special tempers
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  Finish compatibilityPick the alloy that takes the required coating

Material problems rarely appear at the machine. They appear in the field. A bracket made from the wrong temper bends at the bolt hole. A bushing made from the wrong plastic grows with temperature and seizes.

Warping after machining is usually a stock problem, not a machine problem. Rolled plate with residual stress releases it as material comes off. The fix is to specify stress-relieved stock or switch to billet, not to add more finishing passes.

Poor surface finish on stainless often traces back to feed rate. Too light a cut rides on the hardened layer and tears the surface. Increasing feed per tooth often improves the finish instead of ruining it, which surprises people running aluminum habits on 304.

Threads and thin walls fail for a different reason. Soft aluminum strips at high torque, and thin plastic walls deflect under clamping pressure. Both get solved at the design stage by specifying a stronger alloy or a thicker wall.

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  WarpingInternal stress in plate, released during cutting
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  Torn finish on stainlessFeed too light, cutting the hardened skin
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  Stripped threadsSoft alloy at high assembly torque
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  Creep in servicePlastic grade chosen below its temperature limit