Fresno CNC Machining: What Actually Drives Part Cost

• 1
  How CNC machining removes material, and why that sets cost
• 2
  Fresno CNC machining tolerance: what ±0.005 mm really costs
• 3
  Geometry rules: pockets, walls and holes in Fresno CNC machining
• 4
  Material choice and its boundary conditions
• 5
  What a Fresno CNC machining quote should tell you
• 6
  Which process fits your part

CNC machining is subtractive. A rotating cutter follows a toolpath and shears material away, so every finished surface is the trace of a tool that had to reach that spot without hitting anything else. That single fact explains most cost and most design trouble. A pocket is cheap because the cutter can drop straight in. A deep rib beside a tall wall is expensive because the tool must be long and thin.

A long thin tool deflects. Push it too hard and it chatters, leaving marks and drifting out of tolerance. So the shop slows the feed, takes lighter passes and adds semi-finishing cuts. Each of those adds spindle time. The part has not changed. The number of times the tool must travel the same surface has.

Five-axis machining changes the equation because the table or the spindle tilts. The cutter can approach a face from an angle instead of straight down, which shortens the tool and stiffens the setup. Undercuts and blended curves that would need three separate fixtures on a three-axis machine often come off in one setup. Fewer setups means fewer datums to stack, so position error shrinks as well.

That is the trade: you pay for programming and machine time, and you buy back accuracy and setup count. For a simple plate with drilled holes, a three-axis machine is the right answer. For a part with compound angles on five faces, it is not.

• 1
  Short tool, stiff setupReach matters more than diameter for surface quality.
• 2
  Setup count drives position errorEvery refixture adds a datum stack.
• 3
  Toolpath length drives cycle timeLight passes cost more spindle hours.

A tolerance is not a wish, it is a process statement. ±0.005 mm is achievable on a rigid setup in aluminum or brass, measured at 20 °C on a climate-controlled CMM. It is not achievable on a 300 mm long thin wall in titanium without spending a lot of cycle time on spring passes and thermal control.

The practical rule: tighten tolerance only on the features that mate. A bolt hole pattern needs position control. A cosmetic outer face usually does not. When a drawing puts a blanket ±0.005 mm across every dimension, the shop must inspect and hold every one of them, and the price reflects that.

Material stiffness sets the floor. Aluminum 6061 and 7075 cut cleanly and hold size well. Stainless 316L work-hardens, so the cutter must stay engaged and the feed cannot drop too low. Titanium TC4 (Ti-6Al-4V) conducts heat poorly, so the heat goes into the tool edge instead of the chip, and tool wear shows up as taper in a deep bore.

Surface finish and tolerance are linked but not the same. Ra 0.8–1.6 μm is a normal machined finish. Getting to Ra 0.2–0.8 μm takes a finer stepover or a finishing pass with a small nose radius, and that time is billed.

• 1
  Call out mating featuresTighten only where fit matters.
• 2
  Watch long, thin wallsDeflection grows with the cube of length.
• 3
  Finish is a separate specRa and size tolerance are priced apart.

A pocket needs a corner radius. If the drawing shows a sharp internal corner, someone has to cut it with an EDM or leave it round. Size the radius to at least one third of the pocket depth and the shop can use a normal end mill instead of a long, fragile one.

Depth-to-diameter ratio governs everything. A 6 mm cutter can reach about 18 mm comfortably in aluminum before chatter risk climbs. Beyond that, expect a smaller stepdown, more passes and a longer cycle. Bore depth over four times the diameter is where reaming or a boring head usually becomes cheaper than trying to mill it.

Thin walls move. Roughing pulls material away and releases residual stress from the plate, so the part bends. If a wall is thinner than 1 mm over 50 mm of length, expect the shop to rough, stress-relieve and finish in a second operation. That is a real cost line, not padding.

Holes have their own rules. Standard drill point angles leave a cone at the bottom, so a flat-bottom hole needs an end mill. Threads under M2 or 0-80 break taps easily in stainless, so the shop may thread-mill instead, which is slower but more reliable. Laser marking is limited to a minimum character height of 1.5 mm, so tiny engraved text is not an option.

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  Corner radius ≥ one third of depthKeeps the cutter rigid.
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  Stay under 3× diameter for millingPast that, ream or bore.
• 3
  Flat-bottom holes need a millDrills leave a cone.

Aluminum covers most brackets, housings and heat sinks. 6061-T6 is the general-purpose grade; 7075 gives higher strength but machines with more spring in thin sections; 2024 has better fatigue behavior and poorer corrosion resistance, so it usually gets anodized. ADC12 is a die-casting alloy, not a billet grade, so do not specify it for machined parts.

Stainless 303 machines freely because of its sulfur content, which is why it is common for shafts and fittings. 304 and 316L are tougher and gummier, and 316L is the usual pick for medical and marine exposure. 17-4PH (SUS630) can be aged to high strength after machining, but that heat treatment moves dimensions, so leave stock and finish after aging.

Titanium and Inconel sit at the far end. TC4 is strong and light, used in aerospace brackets and implants. Inconel keeps strength at high temperature but destroys tooling quickly, so cycle times are long and cost per part is high. Neither is a default choice; they are chosen for a temperature or strength requirement that aluminum cannot meet.

Plastics behave differently. POM holds tolerance well and is easy to machine. PEEK is expensive and abrasive, and it needs sharp tooling. Carbon fiber reinforced plastic wears cutters fast and can delaminate at the exit, so support the back side and use a compression cutter.

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  Match alloy to exposure316L for corrosion, 303 for machinability.
• 2
  Leave stock for aging17-4PH moves in heat treatment.
• 3
  Titanium is a requirement, not a preferenceUse it only when aluminum fails.

A useful quote states the process, the machine class and the inspection plan, not just a number. If the quote lists a three-axis mill for a part with compound angles on five faces, the shop either misunderstood the drawing or plans to refixture it several times. Both are worth a question.

Check whether DFM feedback came back with the price. Good shops flag a sharp internal corner or an impossible thread before cutting metal. GreatLight returns a quotation and free DFM analysis within 12 hours, and production can start within 24 hours once the drawing and material are fixed.

Ask about inspection. A 100% inspection before shipment, with raw material check, in-process monitoring and a final report on request, is the baseline for anything going into an assembly. For regulated work, confirm the quality system: ISO 9001:2015, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security.

Finally, check the size envelope. A 4,000 mm maximum processing size covers large frames, but a 4,000 × 400 × 150 mm travel suits long thin parts, while 750 × 1,150 × 550 mm suits boxy housings. Sending a part that does not fit the quoted machine wastes a week.

• 1
  Match machine to geometryThree-axis for plates, five-axis for compound angles.
• 2
  Ask for DFM notesThey are cheaper than a rework loop.
• 3
  Confirm the size envelopeTravel limits vary by machine class.