Carbon Steel CNC Processing Guide

The base alloy is iron plus carbon, with manganese, sulfur and phosphorus left in from the melt. The carbon percentage is the single number that predicts how the material cuts. Low-carbon grades under 0.30% C stay soft and gummy. They form a built-up edge on the insert, which smears the surface and makes chip control hard.

Medium-carbon grades from 0.30% to 0.60% C cut cleanly with coated carbide at moderate speeds. This is the sweet spot for shafts, gears and brackets. High-carbon grades above 0.60% C hold hardness and wear resistance, but they fight the tool and generate more heat. You pay for that in cycle time.

Free-machining grades such as 12L14 add sulfur or lead. Chips break short, surface finish improves and tool life climbs. Use them for bushings, fittings and high-volume turned parts. Skip them for welded assemblies and for anything that will be heat treated to high hardness.

A36 is structural plate and bar stock for frames, base plates and weldments. It machines easily and welds well. It is not a precision alloy; do not expect tight tolerance stability across a long run without stress relief.

1018 is the default low-carbon choice for turned and milled parts that need a clean finish and case hardening. 1045 gives you more strength in the as-supplied condition and responds well to induction hardening. Both are common for hydraulic rods, spindles and fasteners.

4130, 4140 and 4340 are the chromoly family. Add chromium and molybdenum and you get deeper hardenability and better fatigue strength. 4140 pre-hardened at 28–32 HRC is the workhorse for mold bases and stressed brackets. 4340 is for highly loaded parts where toughness matters more than cost.

• 1
  A36Structural frames and weldments. Easy to cut, not for tight tolerance.
• 2
  1018General turned parts, case hardened pins and bushings.
• 3
  1045Shafts, rods, gears. Good strength without heat treatment.
• 4
  4140 / 4340High-load brackets, mold bases, aerospace fittings.

Rough machine, stress relieve, then finish machine. That sequence holds geometry on any part with thin walls, long bores or an asymmetric section. Skip the stress relief and the part will move after hardening, sometimes by 0.05 mm or more.

Case hardening on 1018 gives a hard skin over a tough core. It suits pins, gears and wear surfaces. Through hardening on 4140 or 4340 gives uniform hardness through the section, which is what you want for highly loaded parts.

Hardened parts above 45 HRC need either grinding or hard milling with CBN or ceramic tooling. Send us the final hardness and we will plan the operation sequence around it. Do not specify a hardness range without telling us which surfaces need it.

Our general machining tolerance is ±0.005 mm on critical features, with 100% inspection before shipment. That is achievable on stable geometry with the right fixturing. On long, thin or unsupported sections, expect the practical limit to loosen.

As-machined surfaces land around Ra 1.6–3.2 μm. Fine turning and milling reach Ra 0.8–1.6 μm, and lapping or fine boring can push to Ra 0.2–0.8 μm. If your drawing calls for Ra 0.4 μm across a large face, expect a second operation and a cost increase.

Threads, keyways and bores all benefit from being called out with their own tolerance. A single blanket tolerance across the whole drawing forces us to over-inspect and slows the quote. Separate the critical dimensions from the cosmetic ones.

Carbon steel is magnetic, so magnetic chucks work well for flat plates. For thin walls, use soft jaws or a custom fixture rather than clamping directly. Vibration is the main cause of poor finish and short tool life on steel.

Coated carbide covers most jobs. TiAlN and AlTiN coatings handle the heat at higher speeds. For hardened material above 45 HRC, switch to CBN or ceramic inserts. Keep the tool overhang short and the setup rigid.

Five-axis machining earns its cost on parts with angled faces, deep pockets or features on multiple sides. One setup removes the repositioning error you would stack up across three operations. On simple prismatic parts, 3-axis is faster and cheaper. We have 16 simultaneous 5-axis centers and 27 three-axis machines, so the choice follows the geometry, not the other way around.

Bare carbon steel rusts. Any part that ships or sits in a humid environment needs protection. Black oxide gives a thin, dimensional-stable finish that suits tooling and fasteners. It adds almost no thickness, so it will not close a tight bore.

Zinc plating is the common choice for outdoor brackets and hardware. Electroless nickel gives uniform coverage on complex shapes and better wear resistance. Powder coating covers large frames, but mask your mating surfaces and threaded holes.

Bead blasting and tumbling remove tool marks and deburr edges before plating. Laser marking works on steel for part numbers and traceability; keep character height at 1.5 mm minimum so the mark stays legible after coating.