Guide
Why Do Thin-Wall Aluminum Parts Distort After Machining, and What Should the Drawing Allow?
Thin-wall aluminum parts are machined components, such as housings, hub shells and stem bodies with deep pockets and slender ribs, whose walls are flexible enough to move under cutting load, clamping load or released internal stress. They can leave the machine on size and come out of the vise bowed, twisted or out of round.
By Saola Manufacturing engineering team · Published · Updated
Short answer: the movement comes from three loads, the cutting forces, the clamping forces and the residual stress locked into the stock, and it cannot be removed by tighter tolerances alone. A drawing should say when and how flatness and roundness are measured, leave finish stock on the thin features, and the first article should be checked after the part has been unclamped and left to settle.
This guide covers aluminum parts that move because of machining. Distortion from hardening is in the guide on heat treatment distortion allowance, and distortion from welding is in weld distortion on machined weldments.
On this page
Why do thin-wall aluminum parts move after machining?
Three things load a thin wall: the tool pushes it, the fixture squeezes it and the stock itself contains stress that is released as metal is removed. Each can leave the part out of shape after unclamping.
A peer-reviewed review of thin-wall machining of light alloys states that both clamping and cutting forces produce elastic deformation, and that induced residual stresses may also modify the final geometry of the part during the process (Del Sol et al., Materials, 2019). It separates static deflection, which appears when cutting forces act on a part of low stiffness and creates a dimensional error, from chatter, which occurs when the system’s natural frequency is excited and leaves marks that raise roughness. For the buyer the distinction matters: static deflection leaves a wall too thick or tapered on the machine side, chatter shows up as surface marks, and residual stress shows up after the part is released.
For large skin panels, the same review notes that workholdings and fixtures do not usually ensure the tolerances of the final part on their own. A fixture that holds the part rigidly can itself be a source of error, because a part clamped flat can spring back when the clamps open.
How is this different from heat-treat or weld distortion?
The cause is mechanical, not thermal. The part moves because material was removed and loads were released, so the fix is in the machining plan, not in the furnace or the weld sequence.
Heat-treat distortion comes from phase change and quenching and is handled by finish stock after hardening; weld distortion comes from shrinkage along a weld and is handled by sequence, fixturing and stress relief. Machining distortion in aluminum happens in the cutting and holding steps and in the stock’s starting stress state. For steel parts the question of stress relief before machining is covered in stress relieving steel before machining; for aluminum the equivalent lever is the temper the stock is bought in.
Temper designations make this concrete: the 51 in T651 means the stock was stress relieved by stretching after solution heat treatment, as the guide on 6061-T6 vs T651 explains. A drawing that names the temper tells the supplier which stock to buy, but it does not eliminate residual stress, so it is a risk reducer, not a guarantee.
How should the drawing call out flatness and roundness on a thin wall?
State the condition of measurement: free after unclamping, or restrained in a defined gauge. A bare flatness number leaves the supplier and the buyer measuring different things.
ASME Y14.5, the dimensioning and tolerancing standard in its 2018 edition reaffirmed in 2024, sets the symbols and rules for stating geometric tolerances on a drawing (ASME Y14.5, 2018 (R2024)). The overview page does not mention free state or non-rigid parts, so check the current edition for how it treats a part that is flexible when free. For the buyer the practical point is to say in a drawing note whether flatness, roundness and position apply with the part free, or with it held in a named fixture or gauge, and to say when the check is made.
The related guides on flatness versus parallelism and checking fixtures cover how those callouts are written and gauged. For a thin wall, add a statement of the datum scheme, because a flexible part will sit differently on different datum features.
What should the supplier do in the machining plan?
Rough, release, then finish: remove most of the metal, let the part relax, and take a light final cut on the thin features with balanced loads and support.
The review reports that choosing parameters to reduce the induced residual stress lowers deflection, that one study used quasi-symmetric machining to reduce deformation produced by residual stresses, and that machining results are considerably more accurate when the compensation is made at the last layer (Del Sol et al., Materials, 2019). It also discusses vacuum fixtures, noting they are common for thin parts but can put a tensile stress on the part that affects deformation, and flexible supports that follow the tool. Those findings translate to four supplier-side habits.
- Leave finish stock on the thin features. The wall is brought to final size only in the last pass, after the part has been released from roughing stresses.
- Balance the cuts. Remove material evenly from both sides of a wall rather than finishing one face completely first.
- Support the wall. Use soft jaws, a sacrificial backing, or a fixture that supports the thin region. This is common shop practice rather than a finding of the review, and it should be shown on the process plan.
- Re-clamp lightly for the final cut. Release the part after roughing, re-seat it with light clamping and finish. Heavy clamping at the final step can bow a wall, which follows from the review’s point that clamping forces produce elastic deformation.
The review also describes measuring deformation before the finishing stage and adjusting the cutting depth, which is how a shop that has measured a first article can compensate in production.
| Cause | What you see | Drawing or RFQ lever |
|---|---|---|
| Cutting force (static deflection) | Wall tapered or too thick on the tool side | Finish stock, light final cut, supplier’s process plan |
| Chatter | Marks on the wall, rough finish | Surface-finish note, support, ask for the cutting strategy |
| Clamping force | Flat in the fixture, bowed when released | State free-state or restrained measurement; light clamping at the final cut |
| Residual stress in stock | Part moves hours or days after release | Name the temper (for example T651); roughing before finishing; first article after rest |
How should the first article be checked for thin-wall parts?
Measure it twice: once right after unclamping and once after the part has rested, and record both on the report. The second number is the one that matters to assembly.
A part can be measured on the machine, again on the CMM and again after a rest, and the three readings differ when stress is being released. Ask the supplier to report the flatness or roundness at those stages for the first article and to state the support used on the CMM. If the rested reading drifts out of limit, the cure is in the machining plan, not in more inspection. The guide on first article inspection for an overseas supplier lists what the report should contain.
If the part is anodized, measure again after finishing, because the finishing step adds a layer and handling and can add distortion. The guide on dimensional change from anodizing covers the layer; whether a given racking method moves a thin wall is a question for the finishing partner.
How is a thin-wall aluminum part handled on a Saola order?
Aluminum in the grades listed on the capability pages is lead-plant CNC work, and the first article can be reported at the stages above. Anodizing, if called for, is done through a partner finishing plant that Saola manages.
The machining runs on CNC milling and CNC turning. The milling page describes milling mainly on castings rather than parts cut entirely from billet, so a thin-wall part machined from plate or bar is confirmed at drawing review; ask for the process plan to show the roughing, release and finishing steps for each thin feature. Send the drawing with the measurement condition for flatness and roundness, the stock temper if you have a preference, and the cosmetic requirement, and the quote will say how the part is supported and checked.
Frequently asked
Can a tighter flatness tolerance fix a part that bows after machining?
No. A tighter tolerance only means more parts fail. The movement comes from cutting, clamping and stock stress, so the fix is in the machining plan, the measurement condition and the finish stock.
Does buying T651 plate prevent distortion?
It lowers the risk because the temper designation means the stock was stress relieved by stretching, but it does not remove stress entirely. Machining still releases what is left, so roughing before finishing is still needed.
Should flatness be measured free or clamped?
That is the buyer’s call and it must be on the drawing. Say which applies, and which fixture or gauge is used if restrained, so the supplier and the buyer measure the same thing.
When does the part stop moving?
There is no fixed rule. Ask the supplier to report the part at several stages on the first article, and agree on the stage that counts for acceptance.
Sources
- Thin-Wall Machining of Light Alloys: A Review of Models and Industrial Approaches, Del Sol, Rivero, López de Lacalle, Gamez, Materials, 2019 (full text read via Europe PMC)
- ASME Y14.5, Dimensioning and Tolerancing, ASME, 2018 (R2024)