Basic Knowledge of CNC Steel Processing

Hardness is the first variable to pin down. Low-carbon grades around 150–180 HB cut cleanly with sharp carbide and plenty of coolant. Pre-hardened 4140 at 28–32 HRC is a different job. The same cutter, the same feed, and the same speed will burn an edge in minutes.

Heat leaves the cut more slowly than it does in aluminium. Most of it goes into the tool and the chip, not the workpiece. That is why steel parts are usually run at lower surface speed, with rigid setups and flood coolant rather than air blast. Light passes chatter. Heavy passes load the spindle. The window in between is narrower than most people expect.

Chip control matters as much as speed. Mild steel produces long stringy chips that wrap around the tool and scratch a finished face. A 1045 or 4140 cut with a proper insert geometry breaks the chip into short curls that clear the flutes. Get this wrong and you spend more time stopping the machine than running it.

• 1
  Low-carbon steelFree-cutting, low tool wear, good for brackets, plates, fixtures.
• 2
  Medium-carbon steelNeeds correct feeds and inserts; responds well to heat treatment.
• 3
  Alloy steelHigher strength, tighter process control, slower cutting speeds.
• 4
  Tool steelUsually machined annealed, then hardened and ground afterward.

Start from the function, not from a familiar grade. A part that only holds position can run in 1018 or A36. A shaft that carries torque and sees fatigue cycles usually wants 4140 or 4340. Anything that must resist wear at a contact face belongs in tool steel or a hardened stainless such as 440C.

Machinability is the trade-off. 1018 cuts fast and cheap. 4140 at the same hardness cuts about 30–40 percent slower and eats inserts faster. 316 stainless is worse again: it work-hardens under a dull tool, so a light pass with a worn edge hardens the surface and the next pass fights it. 303 stainless is the free-machining option when corrosion resistance matters more than weldability.

We machine 1018, 1045, 4130, 4140, 4340, A36 and tool steel, plus 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH stainless. If the drawing does not state a hardness range, we will ask. A 4140 part specified at 28–32 HRC and the same part specified as annealed are two different quotes.

Steel parts rarely start as a near-net blank. We saw bar or plate to size, face it, and rough the main geometry with a stock allowance. Roughing removes most of the volume and lets the part move before anyone cares about dimensions. On a thin plate or a long shaft, stress relief between roughing and finishing is often the difference between a flat part and a banana.

Finishing follows the datum strategy set at the start. Holes are drilled, then bored or reamed if the tolerance is tight. Threads are milled or tapped depending on depth and position. Slots, pockets and radii are cut in an order that keeps the part rigid for as long as possible. On parts with features on several faces, we either run multiple setups on a three-axis machine or cut them in one cycle on a four- or five-axis center.

Inspection is not a final step bolted onto the end. We check incoming material, monitor dimensions during the run, and inspect before shipment. Reports are available on request. Final tolerance on steel parts is normally ±0.005 mm where the geometry allows it, and surface finish runs from Ra 0.2–0.8 μm on a fine-ground or polished face to Ra 1.6–3.2 μm as machined.

Steel magnifies every setup error. Each time you unclamp a part and turn it, you introduce a new datum and a new chance to miss alignment. Multi-face parts in steel are where that cost adds up. Cutting five faces in one setup removes most of it, because the relationship between features is set by the machine, not by a fixture.

The gain is largest on parts with angled faces, contoured pockets, or holes that do not sit perpendicular to any natural face. It is smaller on a simple prismatic block that two setups handle fine. Five-axis is not automatically better. It is better when the alternative is four fixtures and a stack of tolerances.

We run 16 simultaneous five-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, with a maximum processing size of 4,000 mm. For long shafts and structural steel parts, the large travel is 4,000 × 400 × 150 mm. A Ø400 mm rotary table handles round work that needs indexing. The right machine is chosen by geometry and volume, not by what happens to be free.

If a steel part needs to be hard, that decision belongs early in the process. Harden after machining and the part will move. Harden before machining and you are cutting 50 HRC stock with ceramic or CBN, which is possible but slow and expensive. The usual route is to machine in the annealed or pre-hardened state, leave grinding stock on critical faces, then harden and finish by grinding.

Corrosion protection is the other reason to finish a steel part. Black oxide gives a thin, dimensionaly stable coating that suits tooling and fixtures. Electroless nickel adds uniform coverage including inside bores, which matters when a plated bore must still fit a shaft. Zinc plating is common on structural and automotive brackets. For stainless parts, bead blasting or brushing is often enough.

Cosmetic finishing is a separate question. Polishing to a mirror face on steel takes time and is rarely needed unless the part is a mold surface or a visible component. Laser marking works down to a minimum character height of 1.5 mm, which is fine for part numbers and traceability codes but not for dense data matrices on small faces.