CNC Precision Metals: How Alloys Behave Under the Tool

• 1
  What CNC precision metals actually means
• 2
  Why aluminium, stainless and titanium machine differently
• 3
  When a part needs 5-axis CNC precision metals work
• 4
  Where the last 0.01 mm goes
• 5
  Deciding between 5-axis, mill-turn and 3-axis
• 6
  Why tighter tolerance is not always better value
• 7
  Choosing the machining route by part feature
• 8
  Machining behavior by alloy group

Precision metals is a shop-floor term, not a metallurgical one. It describes the group of alloys that hold a tight tolerance after cutting, keep their shape when the fixture comes off, and do not move again three weeks later on the customer's bench. Aluminium, stainless, tool steel, titanium, copper and their heat-treated variants all qualify. What separates them is not hardness. It is how predictable each one is when the tool pushes through it.

Two numbers describe the result. One is dimensional: ±0.005 mm on a controlled feature. The other is surface: Ra 0.8–1.6 μm as a normal machined finish, down to Ra 0.2–0.8 μm when a sealing face or bearing bore needs it. Both are achievable on the materials listed below, but not on every geometry, and not at the same cost.

The part that surprises most engineers is where the error comes from. The machine is rarely the weak link. Thermal growth, workholding deflection, tool wear and residual stress in the stock account for most of what you measure on a CMM. A 40 °C spindle and a cold casting will disagree with each other no matter how good the ballscrews are.

GreatLight has run these alloys since 2011 across three wholly-owned plants, 7,600 m² of floor space and 127 high-precision CNC machines. That history is useful mostly because it shows which combinations of material and feature reliably land inside tolerance, and which ones need a conversation before quoting.

Aluminium is the forgiving one. Grades 6061 and 6061-T6 cut fast, hold a sharp edge and move little after machining. The trap is thin walls: a 0.8 mm aluminium rib will deflect under cutting force, spring back, and measure oversize on one side and undersize on the other. Light passes at higher spindle speed fix it. Other common grades we run include 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 for cast blanks.

Stainless is where surface finish becomes the real specification. Grades 303 and 304 gummy up on the tool and work-harden if the feed is too light. Grade 316L behaves better for medical and marine parts, and 17-4PH (SUS630) holds tolerance after heat treatment better than the austenitic grades. If a drawing calls for Ra 0.8 μm on a 316L bore, that is a boring operation with a rigid bar, not an end mill.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the other end. They conduct heat poorly, so the cutting edge takes the temperature. Tool life drops, spindle time rises, and the cost per part follows. Magnesium AZ31B and AZ91D cut easily but need chip control, because fine magnesium swarf is a fire risk.

Copper and brass fall in between. C101 and C110 are soft and sticky; C36000 free-cutting brass is one of the easiest metals on any machine. Beryllium copper machines well but needs dust control. Steel grades 1018, 1045, 4130, 4140, 4340 and A36 cover most structural and shaft work.

Three-axis machining cuts from one direction, then the operator re-fixtures for the next face. Every re-fixture adds a datum shift. If a part has four or more angled faces, deep pockets on two sides, or a bore that must stay concentric to a face cut in a different setup, the stack of small errors becomes the tolerance. That is the point where 5-axis pays for itself.

Simultaneous 5-axis moves the tool and the table at the same time, so the cutter can approach an undercut or a compound angle without releasing the part. One setup, one datum. Our 16 simultaneous 5-axis centers cover travels of 750 × 1,150 × 550 mm, 600 × 600 × 600 mm and 500 × 500 × 450 mm, with a Ø400 mm rotary table on the smaller platforms. The largest platform reaches 4,000 × 400 × 150 mm.

Not every part belongs there. A flat bracket with two holes is cheaper on a 3-axis machine, and a round shaft with a thread is faster on a mill-turn center. We run 27 three-axis machines and 16 mill-turn centers for exactly those jobs. Putting simple work on a 5-axis center raises the hourly rate without improving the result.

The boundary is usually feature count, not part size. A 300 mm housing with one angled port may still be a 3-axis job with a sine plate. A 60 mm medical component with a curved slot, two cross-holes and a sealing face is a 5-axis job, even though it fits in your hand.

A tolerance on a drawing is a budget, and the machine only spends part of it. Thermal compensation systems on our German and Japanese 5-axis centers handle spindle growth and ballscrew expansion. High-frequency spindles up to 40,000 rpm keep cutting force low on small tools, which matters when you are milling a 0.5 mm slot in 7075. Vibration damping on the casting handles the rest.

What remains is the part itself. A 4140 blank that was normalized will move after the first roughing pass removes 3 mm of stock. A 6061 extrusion with rolled-in stress will bow when you cut one side away. The standard fix is a roughing pass, a stress-relief pause, then a finishing pass. It costs one extra setup and it is the difference between a part that measures well in the shop and a part that measures well at the customer.

Inspection closes the loop. We check raw material on arrival, monitor dimensions in process, and inspect 100% of parts before shipment, with reports on request. For a first article, ask for the CMM report on the features that carry function, not on every dimension. Ten meaningful numbers tell you more than two hundred nominal ones.

Across production runs we hold a 99.99% qualification rate. That number is a result, not a promise about your specific part. Complex geometry, exotic alloys and very tight tolerances each move the risk, which is why we prefer to review the drawing before quoting.

A part should go on the machine that finishes it in the fewest setups, not the newest one. That single rule resolves most routing arguments. Fewer setups mean fewer datums, and datums are where tolerance disappears.

Mill-turn centers handle parts that are mostly rotational but carry milled features: a shaft with a flat, a cross-hole, or a keyway. Turning and milling in one cycle eliminates the concentricity error you get by moving the part to a mill. With 16 mill-turn centers, we route most hydraulic and drivetrain work there.

Three-axis machining is still the right answer for prismatic parts. It is faster to program, faster to run, and cheaper per hour. Twenty-seven three-axis machines exist for that reason. If your part has one dominant face and no undercuts, do not pay for rotary motion you will not use.

Five-axis simultaneous machining is for geometry that cannot be reached otherwise: compound-angle bores, deep pockets with drafted walls, impellers, and parts where two features must stay in a tight relationship across several faces. It is also the right tool when a single datum must survive the whole operation, which is common in aerospace and medical work.

Tolerance is a cost curve, not a quality badge. Going from ±0.05 mm to ±0.02 mm on a simple aluminium bracket usually adds little. Going from ±0.02 mm to ±0.005 mm on the same bracket can double the price, because it forces slower passes, temperature control, and a metrology step that the part did not need.

The useful question is what the tolerance is for. A bolt clearance hole at ±0.1 mm works fine. A bearing bore that sets shaft runout needs ±0.005 mm. A cosmetic edge needs no tolerance at all, just a finish. When a drawing puts ±0.005 mm on every dimension, we ask which ones carry function and quote the rest at a normal band.

Material choice moves cost more than tolerance in many cases. Switching a housing from 304 stainless to 6061-T6 can cut cycle time significantly, and anodizing gives a comparable surface for indoor use. Switching from Inconel to 17-4PH where the temperature allows it does the same. Engineers who bring two alloy options to the quote usually get a better answer.

Volume changes the picture again. There is no minimum order quantity here, so a single prototype and a 10,000-part run both get quoted. At low volume the setup dominates; at high volume the cycle time dominates. A design change that saves 30 seconds per part matters far more at 10,000 pieces than at ten.