How to Use a CNC Machine for Metal

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  Key takeaways
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  Read the Print and the Material First
• 3
  Set Up Workholding and Zero the Part
• 4
  Pick Tooling for the Alloy, Not the Catalog
• 5
  Set Feeds and Speeds, Then Watch the Cut
• 6
  Measure the First Article Before the Batch
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  Step by Step: Use a CNC Machine for Metal
• 8
  Cutting Parameters by Common Metal

Every metal job starts with two documents: the drawing and the material cert. Read the drawing for tolerances, datum callouts, surface finish notes, and any thread or bore depth that is easy to miss. Read the cert for alloy and temper, because 6061-T6 and 6061-O cut very differently even though the print says 6061.

Identify the tightest tolerance on the print and the feature it applies to. That single feature drives your machine choice, your workholding, and how many setups you can afford. If the tightest callout is ±0.005 mm on a bore, a three-axis machine with a good boring head can hold it. If it is a compound angle on an aerospace bracket, you are looking at a 5-axis setup.

Check the material form. Plate, bar, and casting all behave differently in the vise. Plate can warp after the first face cut releases internal stress. Castings may have hard skin or porosity. Bar stock is usually the most predictable, which is why prototypes often start as bar even when production will be a casting.

Write down the setup sequence before you touch the machine. How many faces need machining, which face is the datum, and where the part will be held on each op. Machinists who plan this on paper scrap fewer first articles than those who figure it out at the control.

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  Tightest toleranceFind it and let it set the machine class and setup count.
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  Material temperT6, T4, annealed, and cast stock all cut and move differently.
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  Stock formPlate warps, castings hide porosity, bar is the most stable starting point.

Workholding decides whether the rest of the job runs clean. For a rectangular part, a machine vise with soft jaws machined to the stock profile beats a standard vise every time. Soft jaws spread the clamping load and give you a repeatable stop, so reloading the second op lands in the same place.

For thin plates, clamp down onto parallels and support the underside across the full area. A plate held only at two edges will chatter and bow, and no feed rate fixes that. For round or irregular parts, a 3-jaw chuck, a fixture plate, or a vacuum table may be the better call depending on how much material you are removing.

Zero the part carefully. Touch off X, Y, and Z against a known datum, then verify with a dial indicator or a probe if the machine has one. A 0.05 mm error in your zero point becomes a 0.05 mm error on every feature, and it will not show up until inspection.

Check for clearance before the first rapid move. Tool holders, vises, and rotary tables all eat into the envelope. A single crash on a 5-axis machine can cost more than the part is worth. Run the first pass with rapid override down and single block on.

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  Soft jawsMachined to the stock profile for repeatable reloading.
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  Thin plateSupport the full underside or expect chatter and bow.
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  Verify zeroProbe or indicator, not just a touch-off, on tight jobs.

Aluminium is forgiving. A 2-flute or 3-flute carbide end mill with a high helix and polished flutes clears chips fast, and you can run surface speeds of 300-500 m/min on 6061. Keep air blast or mist on it; aluminium welds to the cutter when chips recut.

Stainless steel is the opposite. 304 and 316 work harden the moment the tool rubs instead of cuts, so use sharp geometry, a positive rake, and lower surface speed, roughly 60-120 m/min. Never let the cutter dwell in the cut. If you hear the spindle load climb while the feed stays flat, you are rubbing.

Titanium and Inconel need the same discipline with more caution. TC4 (Ti-6Al-4V) cuts around 30-60 m/min with generous coolant and light radial engagement. Inconel is worse: it holds heat, so the tool takes the thermal load. Expect shorter tool life and plan for it.

Pick the smallest tool that can reach the feature without excessive length. A long, skinny end mill deflects, and deflection shows up as taper in a wall or a bore that is out of round. If a feature needs a 6 mm cutter with 60 mm of reach, slow the feed and take lighter axial passes.

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  Aluminium2-3 flute high helix, 300-500 m/min, air or mist blast.
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  Stainless 304/316Sharp positive rake, 60-120 m/min, never dwell in the cut.
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  Titanium TC430-60 m/min, light radial engagement, flood coolant.

Start from the tool maker's recommended surface speed for the alloy and work backward to spindle rpm. Then set feed per tooth. For a 10 mm carbide end mill in 6061, a chip load around 0.05-0.10 mm per tooth is a reasonable starting range. For a 6 mm cutter in 316 stainless, drop to roughly 0.02-0.04 mm per tooth.

Roughing and finishing are different operations. Rough with a large axial depth and moderate radial engagement to move material, leaving 0.3-0.5 mm of stock for the finish pass. Finish with a small radial stepover and a consistent chip load so the surface comes out even. Changing the chip load mid-wall leaves visible marks.

Watch three things while the machine runs. Chip color tells you heat: silver or light straw is fine in steel, blue or black means too much heat. Spindle load tells you whether the cutter is cutting or rubbing. Sound tells you the same thing faster, a clean cut is steady, chatter or squeal means back off.

If the cut goes bad, stop the program before the tool breaks. Adjust one variable at a time: feed first, then speed, then depth. Changing three things at once teaches you nothing about what fixed it.

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  Chip load0.05-0.10 mm/tooth in aluminium, 0.02-0.04 mm/tooth in 316.
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  Rough vs finishLeave 0.3-0.5 mm stock, then finish with a light stepover.
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  Stop earlyKill the program before the tool breaks, not after.

Pull the first finished part and measure it against the print. Check the tightest tolerance first, then the datum-referenced dimensions, then the cosmetic features. Use the right instrument: calipers for general dimensions, micrometers for outside diameters, bore gauges for holes, and a height gauge or CMM for position callouts.

Record the numbers, do not just glance at them. A dimension that reads 0.01 mm off nominal is fine if the tolerance is ±0.05 mm, but it is a warning if the tolerance is ±0.01 mm. Trends matter more than single readings.

If the first article is out, fix the process rather than the part. Adjust the tool offset, the work offset, or the program. Hand-filing a part to print hides a setup problem that will repeat on part two.

For production runs, check at intervals rather than only at the start. Tool wear moves dimensions slowly, and a part that was good at 8:00 may be out by 14:00. In-process monitoring catches that before you have a bin of scrap.

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  Measure tightest firstThen datum dimensions, then cosmetic features.
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  Use the right toolMicrometers and bore gauges, not calipers, on tight callouts.
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  Fix the processAdjust offsets or code, never file the part to print.