CNC process guide

How to Produce and Treat Parts Subject to Deformation in CNC Parts

Thin walls, tall ribs and low-rigidity pockets move when you cut them. This guide shows the order of operations, cutting parameters and clamp strategy we use to hold size on parts that want to spring. Written for engineers and buyers who need to judge whether a design can hold ±0.005 mm.

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Thin-wall millingStress reliefClamp sequenceIn-process checks

5-axis CNC machining of a part prone to deformation in CNC parts

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Key takeaways

Rough, relieve, finishLeave 0.3–0.5 mm on walls, stress-relieve, then take the last cuts cold.

Clamp on the strong sideSupport under the wall, not across the opening. Vise pressure deforms more than cutting force.

Symmetrical toolpaths winCut both sides of a rib in the same pass so the material relaxes evenly.

Measure before releaseCheck the wall while the part is still clamped. Once the vise opens, the reading changes.

Ask for the DFM noteA 12-hour DFM review flags thin floors and sharp internal corners before metal is cut.

What actually moves

What causes deformation in CNC parts

Deformation in CNC parts comes from three sources, and they stack. Cutting force pushes the wall away from the tool. Heat from the same cut expands the material locally. Then the clamping force that held the blank flat releases and the part springs back to whatever shape the residual stress wants. A 1 mm aluminium wall is soft enough that all three act at once.

Residual stress is the one engineers underestimate. Rolled plate and extruded bar carry internal stress from the mill. When you remove 60 percent of the material on one side, that balance breaks and the part bows. The bow often shows up hours after machining, not during the cut, so the inspection table tells a different story than the machine.

Geometry sets the limit. A wall with a height-to-thickness ratio above 10:1 is hard to hold without support. Below 5:1, normal practice is usually enough. Long unsupported floors, deep narrow pockets and thin flanges around a bore all fall into the difficult group.

The material matters too. Aluminium 6061 and 7075 move differently after heat treat. Titanium TC4 (Ti-6Al-4V) and Inconel hold heat at the cutting edge and warp from thermal load, while unfilled POM and PA move with humidity and coolant absorption. Pick the strategy after you pick the material, not before.

1
Cutting forceRadial load bends a thin wall away from the tool, then it springs back undersize.
2
Cutting heatLocal expansion of 20–40 °C shifts a 200 mm feature by tens of microns.
3
Residual stressReleased when material is removed unevenly; the part moves after unclamping.
4
Clamp loadA vise closing on a thin section flattens it before the first cut.

Process order

Set the process order before the first cut

The sequence decides the result more than any single parameter. Rough the part on all faces, leaving 0.3–0.5 mm on every finished wall and floor. Do not finish one face while the rest is still solid stock. An even allowance keeps the stress balance roughly symmetrical through the roughing stage.

Then stress-relieve. For aluminium, a low-temperature thermal cycle between roughing and finishing removes most of the locked-in stress. For steel and titanium, the same logic applies with a different cycle, and for tight workpieces we send the part out for a controlled anneal. This is a real cost, so use it where the drawing demands it, not on every bracket.

Finish with light, fast passes. High spindle speed and a small radial depth of cut (5–10 percent of tool diameter) reduce the force on the wall. A Ø10 mm carbide end mill running 8,000–12,000 rpm with 0.5–1.0 mm axial depth is a workable starting point in aluminium, then tune from the chip and the sound.

Plan the datum so it survives. If the fixture face is machined away in the last operation, every measurement you made before that point is void. Machine the datum first, protect it, and measure from it consistently.

1
Rough evenSame allowance on both sides of a wall keeps bending predictable.
2
Relieve between stagesStress relief after roughing, not before.
3
Finish lightSmall radial engagement, high rpm, sharp tool.
4
Protect the datumA datum that gets cut away cannot anchor inspection.

Tooling and coolant

Tool choice, coolant and heat control

Use the shortest, stiffest tool that reaches the feature. A long reach tool deflects under the same load, and deflection on a thin wall shows up as taper. If the pocket is deep, step down with a stub tool first and only reach in with the long tool for the final 0.2 mm.

Sharp edges matter. A worn corner radius rubs instead of cutting, and rubbing is heat. Change inserts on a schedule rather than on failure when the wall is under 2 mm. Coated carbide with a polished flute works well in aluminium; for stainless 17-4PH and titanium TC4, use a coated grade with a positive rake and keep the cut continuous.

Coolant does two jobs here: it removes heat from the part, not just the tool. Flood coolant at high pressure reaches the cutting zone in a deep pocket. Through-tool coolant is better where the pocket is narrow. Air blast alone is usually not enough on a 1 mm wall because the thermal cycle keeps reversing.

Never let the part sit hot and clamped. If a finishing pass raises the part temperature, let it return to room temperature before the final measurement. A 30 °C shift on a 300 mm aluminium part is roughly 0.2 mm of length change, which is far outside ±0.005 mm.

1
Shortest toolReduce overhang to cut deflection and taper.
2
Fresh edgesRubbing from a dull tool adds heat and load.
3
Flood or through-toolCool the part, not only the chip.
4
Cool before measuringThermal expansion dwarfs the tolerance while the part is warm.

Fixturing

Fixture and clamp strategy for thin walls

Clamp on the thick section and support under the thin one. A wall with a solid rib behind it will not move; the same wall spanning an open pocket will. Where the part allows it, add a temporary sacrificial rib or leave a web that gets removed in a later operation.

Distribute the clamp load. Three or four light contacts beat one heavy vise jaw. On thin plates, vacuum fixturing or a low-melt holding method spreads the load across the whole face instead of concentrating it at two points.

Cut the clamp pressure down for the finishing pass. If the part is held at 20 bar for roughing, drop it for finishing and accept a lighter grip. The risk is chatter, so pair the lighter clamp with a smaller radial depth of cut and a higher spindle speed.

Watch for the release. When the vise opens, the part relaxes. Measure a witness feature before and after release on the first article. If the reading shifts more than the tolerance, the fixture, not the toolpath, is the problem.

1
Support under the wallA gap under the feature lets the wall vibrate and bend.
2
Spread the loadSeveral light contacts instead of one hard clamp point.
3
Lower pressure for finishingPair it with a smaller radial cut to avoid chatter.
4
Check the release shiftCompare before and after unclamping on the first part.

Shop floor sequence

7 steps to produce a low-deformation part

Use this order on thin-wall aluminium, stainless and titanium work.

1
1. Review the drawing for weak geometryFlag walls under 2 mm, height-to-thickness ratios above 10:1, thin floors and sharp internal corners. Ask for a DFM note before quoting. Most deformation problems are visible on paper.
2
2. Pick the stock with the right conditionChoose pre-stress-relieved plate where the budget allows. For 6061 and 7075, check the temper. Avoid cutting a thin wall out of a heavily rolled plate that has never been annealed.
3
3. Rough with an even allowanceLeave 0.3–0.5 mm on all finished surfaces. Use a larger tool at moderate speed and a 40–60 percent radial stepover. Rough fast, but keep the allowance symmetrical on both sides of each wall.
4
4. Stress-relieve before finishingRun the thermal cycle suited to the material, or send the part out for controlled relief. Let it cool fully to room temperature before it goes back on the machine.
5
5. Re-fixture with support under the wallsMachine the datum first and keep it. Add a temporary rib or web if the part allows it. Set clamp pressure light, and use vacuum or a low-melt fixture for very thin plates.
6
6. Finish with light, fast passesKeep radial engagement at 5–10 percent of tool diameter, raise spindle speed, and use flood or through-tool coolant. Take the last 0.1 mm at a steady feed rather than a slow rubbing pass.
7
7. Measure warm, then cool, then measure againCheck the critical features while clamped, cool the part to room temperature, then repeat the measurement. Report both readings. A part that only passes warm will not pass at the customer.

Judgment table

Which parts need which treatment

Match the geometry and material to the treatment before you commit to a process plan.

Part condition	Risk level	Treatment	Typical check
Wall ≥ 5 mm, short	Low	Standard rough and finish	Caliper or bore gauge
Wall 2–3 mm, ratio 5:1–10:1	Medium	Even allowance, light finish cuts	Micrometer on wall
Wall < 2 mm, ratio above 10:1	High	Relief plus support fixture	CMM before and after release
Thin floor over a pocket	High	Sacrificial web, remove last	Dial indicator on floor
Aluminium 6061 / 7075	Medium	Low-temperature relief after roughing	Flatness on surface plate
Titanium TC4 or Inconel	High	Coolant through tool, low radial cut	CMM with thermal soak
Unfilled POM or PA	Medium	Dry or mist cut, control humidity	Measure at 20 °C, 50% RH

The short answer

Control deformation in CNC parts with sequence, not with a slower feed: even roughing allowance, stress relief, supported fixturing, light finishing cuts. If the geometry is too weak for that, change the geometry or accept a looser tolerance.

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FAQs

Common questions about deformation in CNC parts

How much material should I leave for the finishing pass on a thin wall?

Leave 0.3–0.5 mm after roughing on aluminium and steel. On titanium and Inconel, use the higher end because the roughing pass leaves more subsurface stress.

Then take the finished size in two light passes rather than one heavy one. A single 0.5 mm radial cut on a 1 mm wall will bend the wall and the tool will follow the bend.

Can stress relief be skipped if the tolerance is loose?

Yes, when the wall is thicker than 5 mm and the tolerance is wider than ±0.05 mm. The released stress is small compared with the allowance.

Skip it on a thin wall with a tight tolerance and the part will often move after it leaves the machine. That is the expensive failure mode because the problem appears at the customer.

Does 5-axis machining reduce deformation?

It helps because you can finish several faces in one setup. Fewer re-clamping steps means fewer chances to bend the part between operations.

It does not remove cutting force or residual stress. A 5-axis machine with a heavy radial cut will still push a thin wall. The toolpath and the allowance do most of the work.

What surface finish can we expect on a flexible part?

On a stable wall, Ra 0.8–1.6 μm is routine and Ra 0.2–0.8 μm is possible with a fine finishing pass.

On a wall that vibrates, finish drops fast. Chatter leaves a pattern that no polishing removes without changing the size. Support and light cuts come before finish targets.

How do you inspect a part that moves after unclamping?

Measure the critical features twice: once on the fixture and once after the part has soaked at room temperature. Report the released state, because that is the state the customer receives.

For high-risk geometry we use CMM checks with the part free of clamping and a thermal soak first. Reports are available on request.

What should I send with the RFQ?

A 3D model, a 2D drawing with tolerances and datums, the material and temper, and the surface finish callouts. Note which features are critical. That lets us flag the deformation risk in the DFM review.

Uploads stay secure and confidential, and an NDA is available on request.