Process walkthrough

Watch the Fascinating CNC Machining Process in Action

This page walks through the fascinating CNC machining process the way it happens on our floor: setup, first cut, in-process checks, and finishing. Written for design engineers and buyers who want to judge whether a part can be machined to print, and where it usually goes wrong.

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Fascinating CNC machining process shown on a five-axis machining center

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Key takeaways

Setup decides everythingMost dimensional errors trace back to workholding and datum choice, not to the cutting tool.

Speeds and feeds are material-specific6061 aluminium runs at 300–500 m/min surface speed; 316L stainless sits near 120–180 m/min.

Check before you cut metalA dry run and first-article inspection catch most collisions and gouges before a part is scrapped.

Tolerance drives cost more than geometryMoving from ±0.05 mm to ±0.005 mm usually adds a finishing pass and extra inspection.

In-process checks beat final inspectionMeasure at rough, semi-finish, and finish so a drifting dimension is caught early.

Step 1

Read the Drawing Before the Machine Starts

Every job on our floor begins with the print, not the spindle. We check datum callouts, tolerance stack-up, and which features actually drive function. A bracket with one bored bearing seat and loose mounting holes is a different job from a housing with five tight bores on the same axis.

The first question is always the same: which surface locates the part in the fixture, and which surface locates it in the finished assembly? If those two are not the same, the setup will fight you. We flag that before quoting, not after the first article fails.

Material choice sets the rest of the plan. Aluminium 6061 and 7075 cut fast and hold ±0.005 mm without drama. Stainless 316L and 17-4PH move during roughing, so we leave 0.3–0.5 mm of stock and let the part cool before the finishing pass.

1
Datum checkConfirm the locating face in the fixture matches the assembly datum.
2
Tolerance stackIdentify the two or three dimensions that really matter.
3
Material behaviourKnow whether the alloy moves, work-hardens, or galls.

Step 2

Workholding and the First Setup

How the part is held decides how it comes out. A thin wall clamped at 40 bar will spring back when the vise opens, and the measured dimension will be wrong even though the cut was right. We use soft jaws machined to the part profile for anything under 3 mm wall thickness.

For five-axis work we prefer a single setup on a Ø400 mm rotary table with a zero-point system. One setup removes the re-datum error that shows up when a part moves between three vises. It also shortens the cycle, because the tool reaches five faces without an operator touching the door.

Roughing passes run at 2–4 mm radial depth with a 12 mm carbide end mill in aluminium. In stainless we drop to 0.8–1.5 mm radial and lower the feed, because the edge will not survive the same chip load. The goal at this stage is stock removal, not finish.

1
Thin wallsSoft jaws or vacuum chucks instead of hard vise jaws.
2
One setupUse the rotary table when three or more faces need work.
3
Leave stock0.3–0.5 mm for finishing, more if the alloy moves.

Step 3

Speeds, Feeds, and Cutting Parameters

Cutting data is not guesswork. Surface speed, chip load, and depth of cut set the tool life and the finish at the same time. In 6061 aluminium we run 300–500 m/min surface speed with a 0.05–0.10 mm chip load per tooth. In 316L stainless the same tool runs at 120–180 m/min and half the chip load.

Coolant matters as much as speed. Aluminium wants flood coolant or high-pressure air to clear chips from deep pockets. Titanium and Inconel want high-pressure through-tool coolant, because heat that stays in the cut will destroy the edge in minutes.

Finishing passes are slower on purpose. We take 0.15–0.25 mm radial and 0.1–0.2 mm axial to hold Ra 0.8–1.6 μm on a milled face. Push the finish pass harder and the surface tears, especially in 304 stainless and magnesium AZ31B.

1
Aluminium 6061300–500 m/min, 0.05–0.10 mm/tooth, flood coolant.
2
Stainless 316L120–180 m/min, half the chip load, high-pressure coolant.
3
Titanium TC440–70 m/min, through-tool coolant, sharp edges only.

Step 4

Inspection and Where the Process Usually Fails

Final inspection cannot fix a process that drifted. We measure at three points: after roughing, after semi-finishing, and before unclamping. A bearing bore that reads 0.01 mm over after roughing will not come back after finishing, no matter how careful the last pass is.

The most common failure is thermal, not mechanical. A spindle running for four hours heats the casting and the part grows with it. A 300 mm aluminium part can move 0.04 mm between the first and twentieth cut of the day. Letting the machine warm up and measuring at the same point in the cycle removes most of that.

The second failure is chatter from tool overhang. An end mill held 60 mm out of a holder will sing in stainless even at modest parameters. Shorten the overhang, use a shrink-fit holder, or reduce axial depth by half before touching the speed.

1
Thermal driftWarm up the spindle and measure at a fixed point in the cycle.
2
Tool overhangKeep it under 4× diameter, or reduce axial depth.
3
Chip recuttingDeep pockets need through-coolant or an air blast.

Follow along

Five Steps to Run the Process Correctly

Parameters are starting points for aluminium and stainless on a 12 mm carbide end mill. Adjust for tool holder and machine rigidity.

1
1. Load the model and check stockCompare the CAM stock model against the raw billet. Leave 0.3–0.5 mm on faces that will be finished. A stock model that is 0.2 mm undersized will scrap the part on the first pass.
2
2. Set the datum and probe the partTouch off X, Y, and Z on the locating face. Probe the actual billet position, not the nominal one. Billet tolerances of ±0.5 mm are normal and will shift every cut if you ignore them.
3
3. Rough with a constant chip loadAluminium: 2–4 mm radial, 8–15 mm axial, 300–500 m/min. Stainless: 0.8–1.5 mm radial, 4–8 mm axial, 120–180 m/min. Listen for chatter and reduce axial depth first.
4
4. Semi-finish and let the part settleTake 0.3 mm off the finishing faces, then pause. A 200 mm steel part can move 0.03 mm as it cools. Measure now, not after the finish pass.
5
5. Finish and inspect in processCut 0.15–0.25 mm radial at 0.1–0.2 mm axial for Ra 0.8–1.6 μm. Check the critical bore or slot on the machine with a probe or bore gauge before unclamping.
6
6. Deburr and documentBreak edges at 0.2–0.3 mm with a chamfer tool or by hand. Record the actual measured values, not the nominal ones, so the next run starts from real data.

Decision table

Which Setup Fits Which Part

Choose the machine and setup from the feature pattern, not from habit.

Part type	Setup choice	Typical tolerance	Watch out for
Flat plate, holes on one face	3-axis vise, one setup	±0.05 mm	Vise jaw lift on thin plate
Shaft with cross holes	4-axis or mill-turn	±0.02 mm	Runout from re-chucking
Housing, five faces	5-axis on rotary table	±0.01 mm	Tool reach inside deep pockets
Impeller or blade form	5-axis simultaneous	±0.005 mm	Cutter gouge on the leading edge
Long rail up to 4,000 mm	3-axis with bed fixture	±0.05 mm	Thermal growth over the length
Thin wall under 1 mm	Soft jaws or vacuum	±0.03 mm	Spring-back after unclamping

FAQs

Questions Engineers Ask

What tolerance can the process hold on aluminium?

We hold ±0.005 mm (±0.0002 in) on critical features in 6061 and 7075 when the setup is rigid and the part is measured on the machine.

Loose dimensions stay at ±0.05 mm, which keeps the price down. Tell us which features actually matter.

How long does it take to go from CAD file to first article?

Quotation and free DFM analysis come back within 12 hours. Production can start within 24 hours of approval.

Standard parts ship in 3–5 days. Complex five-axis geometry with finishing may take longer, and we say so at quote stage.

Do you machine titanium and Inconel?

Yes. We cut TA1, TA2, TC4 (Ti-6Al-4V), Inconel, and magnesium AZ31B and AZ91D on the five-axis centers.

These alloys cut at 40–70 m/min with through-tool coolant. Tool life is short, so we plan extra cutters into the quote.

How do you stop thin walls from springing back?

We machine soft jaws to the part profile, reduce clamping pressure, and leave 0.3–0.5 mm for a light finishing pass.

For walls under 1 mm we sometimes use a vacuum chuck instead of a vise.

Can you inspect and document every part?

Inspection is 100% before shipment: raw material check, in-process monitoring, and final inspection. Reports are available on request.

We work to ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. Uploads are secure and confidential, and an NDA is available on request.

What is the smallest order you accept?

There is no minimum order quantity. We run from one prototype to 10,000+ part runs.

Prototypes and production parts use the same machines, so the process you approve is the process that ships.