CNC Machining Rochester: How Metal Cutting Actually Works
This page explains what happens inside a CNC machine, where the process stops being accurate, and how to read a drawing before you send it out. Written for design engineers and buyers around Rochester who specify metal and plastic parts and need to judge a quote, not just accept one.
What CNC machining Rochester shops actually control
A CNC machine does one thing: it moves a cutting tool along a path defined by coordinates. Every other capability comes from how tightly that path is held. The controller reads G-code, drives ball screws, and closes the loop with encoders or glass scales on each axis. When the scale reports a position, the controller corrects the next command before the cut finishes.
The loop speed sets the ceiling. A machine that closes its position loop at 1 kHz can correct a 0.01 mm error before the tool travels 0.5 mm at a typical feed rate. A machine closing at 100 Hz cannot. This is why two machines with the same spindle and the same travels can hold very different tolerances on the same part.
Rigidity matters more than raw servo speed on hard materials. Titanium and Inconel push back. If the frame flexes under cutting force, the tool deflects, the wall goes thin on one side, and the finish turns patchy. Cast iron bases, linear guides under preload, and short tool holders all reduce that deflection.
Thermal drift is the slow error nobody sees on the first part. A spindle that has run for two hours is longer than a cold one. Shops that hold ±0.005 mm either warm up spindles before the first cut or measure and compensate. Ask how the shop handles warm-up if your part has tight bores.
Why 5-axis changes the setup, not just the shape
Three-axis machining needs one setup per face. Each time the part comes off the vise and goes back on, the datum shifts. A few thousandths here, a few there. On a part with six faces and two tight bores, the stack of setup errors can eat the whole tolerance budget before the first chip is cut.
Five-axis work keeps the part in one fixture and tilts the tool. A Ø400 mm rotary table plus a trunnion lets the spindle reach the back of a pocket and the side of a boss without re-clamping. Positional tolerance between features stops depending on how carefully an operator tapped the part against a stop.
The gain is not unlimited. Simultaneous 5-axis motion is slower than 3+2 because the controller has to coordinate five axes through every block. For a flat plate with holes on one face, 3-axis is faster and cheaper. For an impeller or a hydraulic manifold with intersecting ports, 5-axis is the only path that holds position.
Tool access is the other limit. A long reach tool that clears a deep cavity will chatter. When the reach-to-diameter ratio passes about 4:1 on aluminum or 2:1 on steel, you need to slow down, step over less, or redesign the feature so a shorter tool reaches it.
Material behavior sets the real cutting limits
Aluminum 6061 cuts clean at 3,000–8,000 rpm with carbide and plenty of coolant. It also moves. Thin walls on a 6061 housing will spring back after the vise opens, so roughing passes should leave 0.3–0.5 mm and a finishing pass should take it off with light radial engagement. Skip that and the bore measures round in the machine and oval on the bench.
Stainless 304 work-hardens. A tool that rubs instead of cutting raises the surface hardness and dulls the next pass. Feed rates below 0.05 mm per tooth on 304 are a warning sign. 303 machines better, but it is not weldable. 17-4PH in the H900 condition holds ±0.005 mm well but needs sharp tooling and a rigid setup.
Titanium Ti-6Al-4V conducts heat poorly, so the cutting edge stays hot. Roughing at 40–60 m/min with high-pressure coolant keeps the edge alive. Inconel pushes that lower again, often 25–35 m/min. Both materials punish light finishing passes that dwell in the cut.
Plastics are the opposite problem. POM and PEEK cut fast but hold heat, and a part with a 2 mm wall can warp after machining as internal stress releases. ABS and PC scratch easily, so soft jaws and air blast instead of flood coolant protect the finish. Carbon fiber needs diamond or coated tooling and dust extraction.
How tolerance and finish are verified before shipment
A tolerance callout is only real if someone measures it. Calipers read to 0.02 mm at best and depend on the operator's thumb. For anything at or below ±0.02 mm, the measurement needs a micrometer, a bore gauge, or a CMM with a stated uncertainty smaller than the tolerance.
Temperature affects the reading. A 100 mm aluminum part measured at 30 °C reads about 0.023 mm larger than the same part at 20 °C. Shops working to ±0.005 mm either control the inspection room temperature or correct the number. This is a normal part of the process, not a special request.
Surface finish is specified as Ra. As-machined aluminum usually lands around Ra 1.6–3.2 μm. A finishing pass with a wiper insert or a smaller stepover gets to Ra 0.8–1.6 μm. Below Ra 0.8 μm you are usually looking at a secondary operation, not a different cutting pass.
Drawing callouts that stack tightly in one area are worth reviewing. If a bore, its shoulder face, and a perpendicular hole all carry ±0.005 mm, the shop has to hold all three at once. Sometimes relaxing one callout to ±0.02 mm cuts cost with no functional loss. A DFM review catches this before the first cut.
When each machining approach fits
Use the part geometry and tolerance callout to pick the process, not the other way around.
| Approach | Best for | Watch out for | Typical tolerance |
|---|---|---|---|
| 3-axis milling | Flat plates, single-face pockets | Multiple setups shift datums | ±0.02 mm |
| 4-axis milling | Shafts, parts with holes around a cylinder | Indexing error on tight bolt circles | ±0.01 mm |
| 5-axis simultaneous | Impellers, manifolds, contoured surfaces | Slower cycle than 3+2 positional | ±0.005 mm |
| Mill-turn | Turned parts with milled flats or slots | Long parts need steady support | ±0.01 mm |
| Wire EDM | Hardened steel, sharp internal corners | Slow, limited to through features | ±0.005 mm |
The trade-off in one line
If your part has one machined face and a loose tolerance, 3-axis is the right call and 5-axis only adds cost. If it has intersecting features on multiple faces and a ±0.005 mm callout, 5-axis in one setup is the only way to hold position without stacking setup errors.
Common questions
How tight a tolerance can CNC machining actually hold?
For most aluminum and stainless parts, ±0.005 mm is achievable on critical features with the right machine, tooling, and inspection. That is not the same as holding ±0.005 mm across every dimension on a large part.
Tolerance is per feature. A bore can be held tighter than a slot on the same part. If a drawing applies one tight tolerance globally, a DFM review usually finds places where it can be relaxed without affecting function.
When does 5-axis machining stop being worth the cost?
When the part has features on one face, or when the tolerance is loose enough that setup variation does not matter. A flat bracket with four holes does not need five axes.
The cost shows up in cycle time. Simultaneous 5-axis motion is slower than 3-axis for the same path length because five axes have to stay coordinated. Use it for geometry or for position, not by default.
Why does my part measure correctly in the machine but not on the bench?
This usually means residual stress or clamping force. Aluminum releases internal stress as material is removed, and the part moves when the vise opens. Roughing with stock left on, then a light finishing pass, reduces the shift.
Temperature is the other cause. A part measured warm reads larger than the same part at 20 °C. For tight work, let the part cool before final inspection.
What surface finish can I expect from a single machining pass?
As-machined aluminum typically lands between Ra 1.6 and Ra 3.2 μm. A finishing pass with a wiper insert or reduced stepover gets to Ra 0.8–1.6 μm.
Below Ra 0.8 μm generally means a secondary operation such as polishing or bead blasting, not just a different cutting parameter. Specify the function of the surface, not only the number.
Do I need to send 3D CAD, or will a 2D drawing work?
Both help. The 3D model defines the geometry and lets the shop run toolpath simulation. The 2D drawing carries tolerances, datum callouts, and finish requirements that a model alone does not express.
If only a model exists, the shop has to assume general tolerances. That is workable for prototypes but risky for production parts with functional fits.
How do I know the parts were inspected before they shipped?
Ask for the inspection report with the shipment. A standard report lists the measured dimensions against the drawing callouts, plus the instrument used.
For parts at ±0.005 mm, the report should name the CMM or gauge and its uncertainty. A report that only says 'inspected' is not a measurement record.
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