Savannah CNC machining: how the process actually works

• 1
  What Savannah CNC machining actually does to metal
• 2
  3-axis, 4-axis, and 5-axis: what each setup can reach
• 3
  Where the process stops being the right answer
• 4
  Reading a tolerance callout without over-specifying
• 5
  How material choice changes the cut
• 6
  Surface finish: what the number means on the shop floor
• 7
  Choosing the right setup for the part

Savannah CNC machining is subtractive. A rotating cutter removes material from a solid block, bar, or casting until the remaining shape matches the CAD model. Nothing is formed or welded, so the mechanical properties of the stock carry straight into the finished part. That is the main reason machined parts are chosen for load-bearing brackets, housings, and manifolds.

The cutting edge does the work through controlled shear. Each tooth of the tool takes a chip of a defined thickness, and the machine moves the tool along a programmed path at a set feed and speed. Get the chip load right and the material peels off cleanly. Get it wrong and you get chatter, work hardening, or a burnt edge.

Heat is the constraint that shapes every decision. Aluminum 6061 conducts heat away fast, so it tolerates high surface speed and deep cuts. Titanium TC4 and Inconel 718 hold heat at the edge, so the same parameters burn the tool in seconds. Speeds and feeds are chosen per material, not per machine.

The practical result is a process that holds ±0.005 mm on a well-set-up feature and Ra 0.2–0.8 μm on a turned or fine-bored surface. Those numbers come from machine rigidity, tool condition, and how the part is held. They are not automatic on every geometry.

A 3-axis machine moves the tool in X, Y, and Z only. The part stays still. This is the cheapest and fastest way to cut flat plates, pockets, and holes that open onto one face. If a part can be reached from a single direction, it should be quoted on a 3-axis machine.

A 4-axis machine adds a rotary table, usually Ø400 mm. The part indexes between faces, so four sides of a block can be cut without re-fixturing. Re-fixturing is where most dimensional error creeps in, so each avoided setup is a direct accuracy gain.

A 5-axis machine moves the tool and the part at the same time. Undercuts, sculpted surfaces, and angled holes are cut in one continuous pass. We run 16 simultaneous 5-axis machining centers for exactly this class of work.

The trade-off is programming time and setup cost. Five-axis work needs a competent CAM programmer, a verified post-processor, and a machine that is calibrated regularly. For a simple plate, that overhead buys nothing.

Deep pockets are the classic limit. A pocket deeper than about three times the cutter diameter forces a long, thin tool. Long tools deflect, and deflection shows up as taper in the wall or a floor that is not flat. If your design calls for a 4 mm cutter at 40 mm depth, expect to pay for multiple passes with a smaller stepover and still accept a looser floor tolerance.

Sharp internal corners are impossible to machine. A rotating cutter always leaves a radius equal to its own radius. A drawing that calls for a true 90° internal corner will be quoted with a note about the corner radius, or it will be EDM work instead.

Thin walls are another boundary. Below roughly 0.8 mm, wall thickness starts to move under cutting force. The part can spring away from the tool, and the finished wall will not be uniform. Sometimes a support fixture solves it. Sometimes it does not.

Very hard materials change the calculation too. Above roughly 45 HRC, carbide tooling wears quickly and the process becomes slow and expensive. Hardened tool steel beyond that range is usually ground, not milled.

A tolerance is a cost driver, not a safety blanket. Every tight callout adds inspection time, slower feeds, and often an extra finishing pass. Engineers who specify ±0.005 mm across an entire drawing usually get a higher quote and no functional benefit.

Apply tight tolerance only where the part interfaces with something else. A bearing bore, a dowel hole, or a sealing face needs control. A clearance edge on a cover plate does not. Mark the critical dimensions and leave the rest at general tolerance.

Material matters here as well. Aluminum 6061 and 7075 hold tight tolerances well because they cut cleanly and move little. Stainless 316 and 17-4PH work-harden, so a finishing pass that takes too small a chip can actually raise the surface hardness and wear the tool.

Thermal drift is the quiet one. A part machined in a warm shop and measured in a cool inspection room can shift by a few microns over 100 mm. For long parts, we check dimensions after the part has stabilized rather than straight off the machine.

Aluminum is the default for prototypes and most enclosures. Grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12 all machine differently. 6061 is the general-purpose choice. 7075 gives higher strength but is less forgiving on thin walls. 2024 machines well and is common in aerospace work.

Stainless 303 and 304 cut cleanly enough for most housings. 316L is chosen for corrosion resistance in medical and marine parts, and it is gummier under the tool. 17-4PH can be machined in the solution-treated state and then aged, which avoids cutting hard material.

Steel grades 1018, 1045, 4130, 4140, 4340, and A36 cover most structural work. 4140 and 4340 are usually supplied pre-hardened, so the cutting parameters drop and the cycle time rises. A36 is soft and cheap, and it is rarely used for anything dimensionally critical.

Titanium TA1, TA2, and TC4 (Ti-6Al-4V) plus Inconel sit at the difficult end. They hold heat, work-harden, and demand rigid setups. They are machinable, but the cost per cubic centimeter removed is several times that of aluminum.

Ra is the arithmetic mean roughness of the surface profile. Ra 1.6–3.2 μm is a normal as-machined finish straight off a milling cutter. Ra 0.8–1.6 μm needs a finer stepover or a finishing pass with a sharp tool. Ra 0.2–0.8 μm usually means turning, fine boring, or a polishing step.

The number is not uniform across a part. A turned OD and a milled pocket floor on the same component will not read the same Ra even if the drawing states one value. Specify finish per surface if it matters.

Anodizing, plating, and powder coating change dimensions. Hardcoat anodizing can add 25–50 μm per surface depending on the process. If a coated surface has a tolerance, that build-up has to be in the drawing or the part will not fit after finishing.

Laser marking needs at least 1.5 mm character height to stay legible after anodizing or powder coating. Smaller text fills in and becomes unreadable, which is a common cause of rejected first articles.