Don Ts for CNC Machine: Dos and Don'ts That Matter

• 1
  What the Don Ts for CNC Machine Work Really Protect
• 2
  Setup Dos and Don'ts: Zero, Workholding, and First Cut
• 3
  Feeds and Speeds: Where Don'ts Cost You Tools
• 4
  Tool Life and Tool Choice: When a Don't Becomes Expensive
• 5
  Daily Habits That Keep Accuracy Inside ±0.005 mm
• 6
  Dos and Don'ts at a Glance

Every rule in a machine shop traces back to one of four things: cutting force, heat, vibration, or chip evacuation. A do that does not touch any of those four is usually a preference, not a rule. A don't that does not touch them is often superstition passed down from an old shift.

The don ts for cnc machine work that actually matter are the ones that keep force, heat, and vibration inside a band the machine and tool can survive. Everything else is secondary.

Take clamping. A vise bolted to a 750 × 1,150 × 550 mm table can hold a block rigid enough for a 16 mm end mill at full radial engagement. The same vise will chatter on a thin-walled housing if you clamp across the open side. The rule is not 'always use a vise.' The rule is 'match the fixture to the direction of cut.'

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  Force pathClamp so the cutting force pushes toward a solid stop, not into air.
• 2
  Heat pathGive chips a place to go before they recut and heat the edge.
• 3
  Vibration pathShorten overhang and support the part near the cut.

Set the work offset on the same surface you will inspect. If the print calls a bore from datum B, touch off datum B. Touching a raw stock face and hoping the saw cut is parallel is how you lose 0.05 mm before the first chip.

Warm the spindle before a tight-tolerance run. A cold spindle grows 10–20 μm as it reaches thermal steady state. On a ±0.005 mm job, that growth alone can put the first ten parts out of tolerance and the last ten in. Run a warm-up cycle for 15–20 minutes.

Do not trust a single edge finder on a part with a 4,000 mm maximum processing size. Use a probe or an indicator on long parts, and re-check the offset after the first part. Thermal drift over a long cycle is real.

Do not start a program with the rapids at 100%. Dry-run the first article at 25% feed and rapid override, one hand on the feed hold. Air-cut the first pass if the setup is new. It costs two minutes and saves a fixture.

• 1
  Touch off the datumUse the inspection surface, not a saw-cut face.
• 2
  Warm up 15–20 minSpindle growth of 10–20 μm moves tight parts.
• 3
  Air-cut new setupsCheck clearance before the first real cut.

The most common don't is running a tool too slow with a light chip. Rubbing instead of cutting work-hardens stainless and burns the edge. In 304 stainless, a 12 mm carbide end mill wants a chip load around 0.05–0.10 mm per tooth, not 0.01 mm. The second most common is running too fast on aluminum with a deep axial cut and no coolant. Aluminum conducts heat into the tool faster than steel, and a 6061 block will weld chips to a flute in seconds.

Use the tool maker's starting numbers, then adjust by sound and chip shape. A good chip is a comma or a 6, not dust. Dust means you are rubbing. Long stringy chips on steel mean you are too slow on feed or too high on speed.

Do not change two variables at once. If you raise speed and feed together, you will not know which one fixed the finish. Change feed first on steel, speed first on aluminum, and log what you did.

Do not run a finishing pass with the same parameters as a roughing pass. Roughing removes volume. Finishing controls Ra. A Ra 0.8–1.6 μm finish usually needs a smaller radial engagement and a sharper, newer edge, not more speed.

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  0.05–0.10 mm per toothStarting chip load for 12 mm carbide in 304.
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  Read the chipDust means rubbing. A 6 means cutting.
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  One change at a timeLog it so the next setup benefits.

Do not run a tool past its wear land to save a tool change. A 0.2 mm flank wear land on a finishing tool will push a bore 0.02–0.04 mm over size and leave a torn finish. The tool change costs less than the rework.

Do use the right geometry for the material. A tool with a sharp, polished flute for aluminum will chip on 17-4PH. A tool with a strong edge for steel will rub and gall on 6061. Keep them separate and label the holders.

Do not assume a coated tool is always better. TiAlN coatings help on steel and stainless at higher temperatures. On aluminum, an uncoated polished tool often cuts cleaner because the coating is softer than the work material and can smear.

Do not re-use a tool after it has chipped on a roughing pass. The missing corner will show up as a witness mark on the finish pass. Set it aside for roughing only or scrap it.

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  Change at 0.2 mm wearFinishing tools drift out of size past that.
• 2
  Match geometry to materialAluminum tools and steel tools are not interchangeable.
• 3
  Retire chipped toolsA missing corner marks the finish pass.

Do check the way lube and coolant every shift. A dry way will show up as a step in the Z axis after a few hundred cycles. Low coolant concentration will rust the table and grow bacteria that clog the nozzles.

Do not ignore a small spindle noise or a new vibration. A bearing on the way out will show as a Ra 3.2 μm finish on a cut that used to run Ra 0.8–1.6 μm. Catch it at the noise, not at the scrap report.

Do clean the taper and the tool holder before every load. A chip on the taper is 5–10 μm of runout before you cut. That is more than the whole tolerance on some jobs.

Do not leave a part clamped overnight under load if it is thin. Stress relief moves the part and the next morning's measurement will not match. Unclamp, let it sit, and re-measure.

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  Lube and coolantCheck every shift, not every week.
• 2
  Listen to the spindleNew noise means a finish change is coming.
• 3
  Clean the taperA chip is 5–10 μm of runout.