CNC Prototype Savannah: How Machined Prototypes Are Actually Made

• 1
  What a CNC Prototype Actually Is
• 2
  How 3-Axis, 4-Axis and 5-Axis Setups Change the Part
• 3
  Tolerance, Surface Finish and What They Cost
• 4
  DFM Checks That Decide Whether a Prototype Machines Cleanly
• 5
  Material and Finish Choices for Functional Prototypes
• 6
  When a Machined Prototype Is the Wrong Choice
• 7
  Matching the Prototype Method to the Question You Are Asking

A CNC prototype is a subtractive part. A rotating cutter removes material from a solid block, plate or bar until the geometry matches the CAD model. Nothing is molded, sintered or deposited. That matters for prototyping because the part you hold is made of the same alloy, at close to the same grain structure and heat treatment, as the eventual production part.

The process starts with a 3D model and a stock size. CAM software plans toolpaths, picks cutters and sets stepover and stepdown. A machinist then dials in workholding, datums and offsets. For a cnc prototype savannah teams can evaluate, the output is not a rendering. It is a physical part that can bolt onto the next assembly, survive a load test or drop into a fixture.

This differs from 3D printing and vacuum casting. Those methods are faster for complex organic shapes and cheaper for one or two units, but the material properties are approximations. Printed plastic usually sits below the strength of the molded resin it mimics. A machined 6061-T6 or 17-4PH part does not approximate anything. Its modulus, hardness and fatigue behavior are the real values.

The trade-off is geometry freedom. A cutter must physically reach every surface it cuts. Deep pockets, internal channels and undercuts can be made, but they raise cost, add setups and sometimes force a design change. The rest of this article covers where those limits sit and how to work within them.

• 1
  Subtractive, not additiveMaterial is removed from solid stock, so properties match the alloy.
• 2
  Same alloy as productionTest the real modulus, hardness and thread strength.
• 3
  Reachability rules geometryIf a cutter cannot reach it, the design or the setup must change.

Axis count decides how many faces you can cut without re-fixturing. A 3-axis machine moves the tool in X, Y and Z while the part stays still. That is fine for plates, brackets, housings with open pockets and any part where the features sit on one or two perpendicular faces. Our 27 three-axis machines handle most flat work at low cost.

A 4-axis mill adds rotation about one axis, usually A. The part turns while the cutter works, so you can cut around a cylinder, drill radial holes or machine a shaft in one setup. Our 12 four-axis mills are commonly used for cnc prototype work on manifolds, couplings and cylindrical housings where concentricity matters.

A 5-axis center adds a second rotary axis, so the tool can approach the part from nearly any direction. This is where complex prototypes become practical. A single setup can machine five faces, angled holes, blended fillets and contoured pockets. Our 16 simultaneous 5-axis machining centers cut parts up to 4,000 mm, with a Ø400 mm rotary table for round work.

More axes is not automatically better. Each extra axis adds programming time and machine cost. A part that can be made in two 3-axis setups with a simple flip is usually cheaper to prototype than the same part run on a 5-axis center. The right question is whether the geometry truly needs the extra angle, or whether a small design change removes it.

• 1
  3-axisPlates, brackets, open pockets. Lowest cost per part.
• 2
  4-axisShafts, couplings, radial holes. One rotary setup.
• 3
  5-axisAngled faces, blended contours, five sides in one setup.
• 4
  Do not over-specTwo 3-axis setups often beat one 5-axis setup on cost.

Tolerance is the allowed deviation from the nominal dimension. We hold ±0.005 mm (±0.0002 in) on critical features when the drawing calls for it. That number is not free. It requires stable stock, temperature control, sharp tooling, light finishing passes and inspection with the right instrument. Blanket-tolerancing every dimension to ±0.005 mm raises cost for no functional gain.

A better approach is to tolerance only what the assembly needs. Identify the mating surfaces, the bores that hold bearings, the faces that set parallelism, and the holes that locate pins. Leave the rest at general tolerance. On most prototypes, fewer than 20 percent of dimensions actually control fit or function.

Surface finish follows the same logic. As-machined finish sits around Ra 1.6–3.2 μm and is usually fine for internal brackets. Visible covers and sliding surfaces often need Ra 0.8–1.6 μm, which we reach with finer stepover and a finishing cutter. Optical and sealing surfaces may need Ra 0.2–0.8 μm plus lapping or polishing. Each step down adds time.

Material choice interacts with all of this. Aluminium 6061-T6 cuts cleanly and holds tight tolerance well. Stainless 316L and 17-4PH work-harden, so light passes and rigid setups matter more. Titanium Ti-6Al-4V and Inconel move when machined and need stress relief or a second finishing cut. For a cnc prototype savannah engineers plan to test under load, that behavior is part of the data.

• 1
  Tolerance only what mattersMating surfaces, bores and locating holes. Not every dimension.
• 2
  As-machinedRa 1.6–3.2 μm. Fine for hidden or non-contact faces.
• 3
  Fine finishRa 0.8–1.6 μm for visible and sliding surfaces.
• 4
  Hard alloys moveTitanium and Inconel may need an extra finishing pass.

Design for manufacturability on a machined prototype comes down to four things: can the cutter reach the feature, is the part rigid enough to hold, can it be held without crushing a finished surface, and can it be measured. A model that fails any of these will need a design change, a custom fixture or an EDM operation, and all three add time.

Pocket depth is the most common problem. A cutter needs length to reach the floor, and a long cutter deflects. As a rule, pocket depth should stay under about four times the cutter diameter for reliable finish and tolerance. A 6 mm wide pocket that is 40 mm deep will chatter, and the walls will not come out straight.

Small internal radii are the second problem. The cutter leaves a radius equal to its own radius in every internal corner. If the model shows a sharp internal corner, the machinist must either leave the corner round or cut it with EDM. Both are workable, but neither matches the model. Set internal corner radii at least one third of the pocket depth where possible.

Thin walls are the third. A wall under about 0.8 mm in aluminium or 0.5 mm in steel will deflect under cutting force and may vibrate. If the prototype needs a thin wall for weight or flex, say so on the drawing. The machinist can then take lighter passes, add support material or plan a stress-relief step before finishing.

We run a DFM review on every quote request and return feedback within 12 hours. That review flags deep pockets, sharp internal corners, thin walls, unreachable faces and tolerances that will not repeat. Fixing these on screen costs nothing. Fixing them after the first part is cut costs a setup.

• 1
  Pocket depthKeep depth under about 4× cutter diameter.
• 2
  Internal cornersRadius at least one third of pocket depth.
• 3
  Thin wallsBelow 0.8 mm in aluminium, deflection is likely.
• 4
  Free DFMFeedback on the model within 12 hours of quote request.

Material should match the intended production process. If the final part will be machined from 7075-T6, prototype in 7075-T6. If it will be die cast from ADC12, prototype in ADC12 or in a close wrought equivalent, then note the difference. Prototyping in a softer alloy to save machining time usually hides the problem you were trying to find.

We machine aluminium 6061, 2024, 5052, 5083, 6063, 6082 and 7075; stainless 303, 304, 316, 316L, 17-4PH and 440C; steels including 1018, 1045, 4130, 4140 and 4340; copper and brass grades such as C110 and C36000; titanium TA2 and Ti-6Al-4V; and engineering plastics including POM, PEEK, PC and carbon fibre composite.

Finishing can be functional or cosmetic, and the two have different rules. Anodizing adds a hard oxide layer that changes the dimension by a few micrometres per surface, so bores and threads that mate should be masked. Hardcoat anodizing is thicker and matters more. Electroless nickel adds a uniform layer and is often used on aluminium to give wear resistance and a conductive surface.

Cosmetic finishes such as bead blasting, brushing, polishing and powder coating change the look but not the fit, unless masking is specified. Laser marking needs a minimum character height of 1.5 mm to stay legible. For prototypes shown to investors or customers, a consistent finish across a small batch often matters more than the specific finish selected.

• 1
  Match the production alloyPrototype in the material the final part will use.
• 2
  Anodizing shifts sizeMask bores and mating threads before coating.
• 3
  Laser markingMinimum character height 1.5 mm.
• 4
  Batch consistencyOne finish across the set reads better than mixed finishes.

Machining loses to other processes in three situations. The first is very complex internal geometry, such as conformal cooling channels, lattice structures or organic ribs generated by topology optimization. A cutter cannot reach inside a sealed curved channel. Additive manufacturing can, so print those parts and machine the interfaces around them.

The second is high volume at low unit cost. If the part will be injection molded or die cast in the tens of thousands, a machined prototype is a validation step, not a production method. Machining a hundred units is possible and we do it, but the economics shift toward tooling once amortization makes sense.

The third is soft, elastomeric or very low-stiffness parts. Silicone gaskets, rubber seals and living hinges do not machine well. Vacuum casting or 3D printing with a rubber-like resin gives a more representative part. Machining can still make the mold for those processes, which is often the better use of the machine.

There is also a timing question. If a decision is blocked on a part arriving next week, a machined part from stock material is realistic. Production can start within 24 hours of a released model and parts ship in 3–5 days. If the design is still changing daily, printing a set first and machining the final revision can save money on setups.

• 1
  Internal channelsPrint them, then machine the mating faces.
• 2
  Very high volumeMachining validates the design; tooling makes the parts.
• 3
  Soft elastomersVacuum casting or rubber-like printing fits better.
• 4
  Still changing dailyPrint the iterations, machine the frozen revision.