CNC Machining NC: How Numerical Control Actually Cuts Metal

• 1
  What CNC Machining NC Means on the Shop Floor
• 2
  Where CNC Machining NC Fits and Where It Does Not
• 3
  Why the Fixture Decides the Tolerance
• 4
  How We Prove the Numbers After the Cut
• 5
  Material Choice Changes the Cut
• 6
  Choosing the Right Setup for the Part

NC stands for numerical control. A program made of coordinates tells the machine where the cutting edge should be, and the machine moves the slide to that point. Early NC machines read punched tape. Modern CNC adds a computer that reads the same coordinate data, applies cutter compensation, and adjusts feed and speed while the tool is in the cut. The letters still describe the same idea: the tool path comes from numbers, not from a handwheel.

On a three-axis mill, the table moves in X and Y and the spindle moves in Z. A CNC machining NC program for that machine is mostly a list of straight moves and arcs, plus tool changes and spindle commands. The operator still chooses the fixture, the tool, and the depth of cut. The program decides position and feed rate. That split matters when a part goes wrong, because the fix may sit in the CAM file or in the vise.

The control reads feedback from encoders on each axis. It compares the commanded position with the measured position many times per second and corrects the difference. That closed loop is why a machine can hold ±0.005 mm on a good day and still drift when the shop warms up in the afternoon. Thermal growth moves the ball screw, and the control has no idea the screw got longer.

Feed and speed come from the material, not from habit. Aluminum 6061 cuts at high surface speed with a two or three flute carbide tool. Titanium TC4 and Inconel want lower surface speed, more coolant, and a rigid setup. Push either one past its range and the tool wears on the flank instead of the tip, which shows up as a size drift across the run.

CNC machining NC suits parts with defined geometry, moderate volume, and tolerances that need to hold across every unit. Brackets, housings, manifolds, heat sinks, bone plates, and engine components all fit. The process cuts metal, plastic, and composite stock, so material choice rarely blocks a design. GreatLight runs aluminum 6061 and 7075, stainless 303 and 17-4PH, tool steel, brass C36000, and titanium TC4 on the same floor.

It is a poor fit when the part is a thin shell with large unsupported spans. A 0.8 mm wall on a 200 mm panel will move when the vise releases it, no matter how slow the finishing pass runs. It is also a poor fit for a part with internal channels that no tool can reach. Deep cross-drilled holes, blind undercuts, and closed cavities need a different process or a redesigned feature.

Volume changes the answer too. One prototype and ten thousand parts can come off the same machine, but the cost curve differs. Setup and programming spread over the run. At low volume the setup dominates. At high volume a casting or a die-cast tool often wins on unit cost, and machining moves to the critical faces only. GreatLight runs both, so we can tell you where the crossover sits for your part.

Surface finish is a boundary of its own. As-machined faces land at Ra 1.6–3.2 μm. A careful finishing pass reaches Ra 0.8–1.6 μm, and lapping or polishing can reach Ra 0.2–0.8 μm. If the drawing calls for a mirror finish on a large area, the cost is in the hand work, not in the spindle time. Ask whether the whole face needs it or only a seal land.

A machine that holds ±0.005 mm on a test block will not hold it on a part that rings like a bell. The workpiece and the fixture form one elastic system. Clamp a thin flange at four points and the middle bows. Cut it flat, release the clamps, and it springs back. The control did its job. The setup did not.

Five-axis work reduces the number of setups. A part that would need four sides on a three-axis machine can often be finished in two setups on a five-axis center, which removes the re-datum error each time the part is flipped. That is the main reason to pay for five axes on a complex housing. The rotary table on our machines is Ø400 mm, which sets a practical limit on part swing.

Datums carry across a process. If the first setup is built off a rough casting face, every later dimension inherits that error. We face the datum in the first operation, then build the rest from it. Leave a datum note on the drawing, and mark the faces you care about. A model with no datum callout forces the programmer to guess, and guesses are where size creep starts.

Stock removal order also matters. Roughing removes most of the material and releases residual stress from rolled plate. If you finish in the same setup, the part can bow overnight. A rough, a stress-relief pause, and a finish pass costs more but holds size on a long thin part. For a part with a 4:1 length-to-width ratio, it is usually worth it.

A tolerance on a drawing is a claim until someone measures it. We check raw material certificates on arrival, monitor size during the run, and inspect 100% of parts before shipment. Reports are available on request. For a first article, the report covers every dimension the drawing controls, not just the tight ones.

CMM inspection is the usual route for position and profile callouts. For a bore or a shaft, a micrometer or a bore gauge reads faster and is just as honest. The instrument has to match the tolerance. A caliper with 0.02 mm resolution cannot sign off a ±0.005 mm feature, and a report built that way is not evidence.

In-process checks catch drift before the run ends. A tool that has cut 300 parts will not hold the same size as a fresh one. We measure at set intervals and comp the tool when the trend shows up. That is how a 10,000 part run stays inside tolerance without sorting at the end.

The qualification rate across our floor is 99.99%. That number comes from inspection data, not from a marketing slide. When a part falls outside, we tell you what happened and what we changed. A supplier who reports zero problems on a hard part is not measuring the hard part.

Aluminum is the easy case. 6061-T6 machines fast, holds a good finish, and takes anodizing well. 7075 is stronger but gummier, so it needs sharper tools and more coolant. Thin walls in 7075 distort more than in 6061 because the strength that helps the part also fights the cutter.

Stainless 303 is free-machining and behaves well. 304 and 316 work-harden if the tool rubs instead of cuts, so keep the feed up and never dwell. 17-4PH in the H900 condition is hard enough to shorten tool life and is usually machined in the annealed state, then aged. If your drawing calls for H900, say so on the purchase order.

Titanium TC4 and Inconel sit at the other end. Low thermal conductivity pushes heat into the cutting edge. Tool life is short, cycle time is long, and the setup has to be stiff. A 0.5 mm depth of cut that works in aluminum will chatter in Inconel. These materials are worth the cost when the part sees heat or load, and hard to justify when it does not.

Finishing follows the material. Anodizing in clear, color, or hardcoat suits aluminum. Electroless nickel, zinc, silver, and gold plating cover steel and copper alloys. Powder coating and black oxide handle larger steel parts. Bead blasting, tumbling, brushing, and polishing set the cosmetic surface, and laser marking handles part numbers down to 1.5 mm character height.