CNC Machining vs Sheet Metal Fabrication for Aerospace Parts
Machine the part from billet when it needs varying thickness, precision bores, sealing faces or tight feature to feature tolerances, or when quantity is low and the alloy is cheap. Form, weld or rivet it from sheet when walls are thin and uniform, the part is large or boxy, quantity is higher, or the alloy is expensive to waste.
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CNC machining vs sheet metal side by side
Machining removes material from a solid block to make any shape a cutter can reach, at tight tolerance and high material waste. Sheet metal cuts and bends a flat blank of nearly constant thickness, then joins pieces by welding, rivets or hardware, at low waste and looser tolerance across bends. The table compares them on the factors that usually decide a part.
| Factor | Machined from plate, bar or billet | Formed sheet, welded or riveted |
|---|---|---|
| Geometry | Varying thickness, pockets, bosses, integral ribs, threaded holes, sealing faces, compound angles | Constant wall thickness, flanges, boxes, channels and shells; features near bends are restricted |
| Typical published tolerance | Plus or minus 0.005 in default at one quick turn shop, tighter on request with inspection cost | Plus or minus 1 degree on bend angles and 0.015 to 0.040 in across two or more bends at the same shop |
| Material utilization | Poor on hogged parts; machined titanium structure runs about 20:1 to 4:1 buy to fly | Good; blanks nest on a sheet and the part is close to the weight of the material bought |
| Weight efficiency | Material only where needed, no fasteners or laps | Thin walls are light, but laps, doublers, fasteners and weld land add mass |
| Tooling | Fixtures and CAM programs; no hard tooling | Standard brake tooling for simple bends; form blocks or dies for flanged and contoured parts |
| Inspection | CMM against a rigid part; every feature measurable | Free state vs restrained measurement, bend angles, flatness, plus weld or rivet inspection |
| Best quantity range | One to moderate lots; cost per part falls slowly with quantity | Prototype to high volume; cost falls faster once forming is set up |
Tolerance figures are Protolabs' published design guidelines for CNC milling and for sheet metal, read in October 2026, and represent one shop's standard capability, not an industry limit. The buy to fly range is from a 2016 TWI paper on titanium aerostructures. Process detail lives on the 5-axis CNC machining and sheet metal and aerospace welding pages and is not repeated here.
Geometry: the first filter
If the part needs a thickness change, a pocket or a precision bore, it is machined or has machined details; if it is a box, channel or shell of one thickness, it is a sheet metal candidate. Ask these questions of the model before anything else:
- Is wall thickness constant? Sheet metal gives one gauge per piece. Local reinforcement means a doubler, a weld pad or a machined insert.
- Are there threads, bores or sealing faces? Sheet can carry self-clinching nuts, studs and rivet nuts, but an O-ring groove, a bearing bore or a tapped boss in thick section is a machining feature.
- How close are features to bends? Protolabs warns that features within about 4 times material thickness of a bend risk deformation, and asks for flanges at least 4 times material thickness. Dense features near bends push a part toward machining or a hybrid.
- How deep is the part? A tall, thin box is cheap to bend and slow to hog. Protolabs limits milled depth to 2 in from either side of a part on its quick turn mills, which shows how quickly tall machined boxes leave the easy category.
- Is the outer shape contoured? Simple bends are brake work. Compound contours need form dies, stretch forming or hot forming, while machining handles 3D surfaces with no tooling but at long cycle time.
Tolerances and how they stack
Machining holds tighter tolerances between features because every feature is cut relative to the same setup; sheet metal tolerances grow with each bend between two features. Protolabs' sheet metal guidelines show the pattern clearly for material under 0.13 in thick:
| Measurement on a formed part | Published tolerance, under 0.13 in material | Over 0.13 in material |
|---|---|---|
| Hole to hole or edge to hole, same flat surface | 0.005 in | 0.015 in |
| Bend to edge | 0.010 in | 0.020 in |
| Bend to hole | 0.015 in | 0.025 in |
| Across 2 bends | 0.015 in | 0.025 in |
| Across 3 or more bends | 0.030 in | 0.040 in |
| Bend angle | plus or minus 1 degree | plus or minus 1 degree |
The same company states a default milling tolerance of plus or minus 0.005 in, or ISO 2768 medium or fine at its other facilities. Aerospace shops routinely hold tighter on critical features, at a price in setup and inspection.
Design moves that let a formed part meet a tight requirement:
- Put mating holes on the same flat face, or pierce them after forming, so no bend sits between them.
- Dimension from a formed datum that the assembly actually uses, not from the flat blank edge.
- Machine the few critical features after forming or welding, and leave the rest at sheet tolerance.
- For flexible parts, state the restraint condition. ASME Y14.5 provides a free state modifier and restrained condition notes, so a panel that flexes can be inspected the way it is installed.
Weight, stiffness and part count
A formed sheet part is usually the lightest way to make a thin wall, and a machined part is usually the lightest way to make a stiff, compact one with no joints. Joints carry weight that the stress model sometimes forgets: lap flanges, doublers, rivets or bolts, nutplates and weld land. A machined one-piece part with integral ribs replaces those with material placed only where the load path needs it.
The opposite case is a large thin shell. Hogging a 0.040 in wall out of a 3 in block wastes most of the block, takes hours and risks distortion as stress is released, while bending the same wall from sheet takes minutes. When stiffness matters on thin parts, flanges, beads and return bends add it cheaply.
Welded sheet adds one caution: the heat affected zone of heat treatable aluminum loses strength. The aluminum alloys page covers the 6061 weld zone numbers, and the material choice itself is covered in titanium vs aluminum.
Material utilization and cost by alloy
Material waste matters little in aluminum and decides the route in titanium and nickel alloys. TWI's 2016 paper on titanium aerostructures states the buy to fly ratio of machined titanium structural parts can be as poor as 20:1 and is rarely as good as 4:1, and reports a near net shape route that cut one part from 10.9:1 to 3.7:1. At 20:1, nineteen pounds of expensive alloy become chips for every pound that flies, and every pound is cut slowly.
- Aluminum: material is a modest share of a machined part's price, so hogging is often the cheapest route below production volumes.
- Stainless and steels: waste costs more and cutting is slower; formed and welded routes win more often on brackets and frames.
- Titanium and nickel alloys: utilization and spindle time dominate. Sheet, near net forgings, or a formed part with a machined pad are worth a quote comparison. Titanium sheet is hard to form at room temperature, so formed titanium parts are usually hot formed; ask whether a source has that capability.
Certified remnants and drops can make a small machined part cheaper and faster; the cert must still match the drawing spec and condition.
Tooling, lead time and quantity
Machining has almost no hard tooling, so it wins on first parts and design churn; sheet metal has low tooling on brake-formed parts and higher tooling on formed contours, then wins as quantity grows.
- Machining setup: CAM programming, a fixture or soft jaws and first article prove-out. A drawing change is mostly a program change.
- Brake forming setup: a flat pattern, standard punches and dies, and a bend sequence. Protolabs lists standard inside radii of 0.030, 0.060, 0.090 and 0.120 in and recommends one radius for all bends on a part, which keeps tooling changes down.
- Formed contours: flanged holes, joggles, beads and curved flanges need form blocks or dies. These pay back over many parts and are a sunk cost if the design changes.
- Joining and finish: welding adds procedure qualification, fixtures and often post-weld machining; riveted assemblies add drilling templates. Each outside process adds queue time.
Lead time follows the longest chain, not the cutting time. A machined part waiting on thick titanium plate, or a weldment waiting on a qualified procedure, an NDT lab and a finisher, will both run long. We do not quote lead times before a qualified shop reviews the package; the lead times and delivery page explains the drivers.
Inspection and first article differences
A machined part is a rigid object a CMM can measure feature by feature; a fabricated part adds process records for forming, welding and hardware that must be inspected and kept.
- Machined: dimensional inspection of every characteristic for the AS9102 first article, then sampling. Distortion after release from the fixture is the usual surprise on thin parts.
- Formed: bend angles, flange lengths, flatness and the restraint condition. A change of brake tooling or sheet lot can affect formed dimensions.
- Welded: visual inspection of every weld plus penetrant or radiography by class. AWS D17.1 sorts aerospace fusion welds into Class A, B or C by the consequence of failure, and the class sets the inspection. Weld maps, procedures and welder qualifications become part of the record.
- Riveted and inserted hardware: rivet head and edge distance checks, and push-out or torque-out samples for self-clinching hardware. NASM45938, which superseded MIL-N-45938, covers self-clinching nuts when the program calls for NAS parts.
Our inspection and metrology page covers measurement methods, and the AS9102 first article guide covers the forms.
Redesign options between the two
Many parts are cheapest as a hybrid, so ask suppliers for an alternate route before the drawing is released. Options we see work:
- Formed part plus machined pad. Bend the bracket from sheet and rivet or bolt a small machined block where the precision bore or tapped hole lives.
- Weld then machine. Fabricate a frame from plate and tube with stock left on interfaces, stress relieve where the procedure calls for it, then machine the mounting faces in one setup.
- One-piece machined replacement for a riveted assembly. Where three or four formed pieces and twenty fasteners make one fitting, a machined part removes the fasteners, the drilling and the joint inspection, often paying for the extra material in aluminum.
- Formed enclosure instead of a hogged box. Bend the walls, add self-clinching hardware for covers, and machine only a connector panel or heat sink face.
- Bent tube instead of welded profiles. For GSE handles, guards and frames, bent tube removes welds and fixtures.
- Thicken to remove a bend tolerance problem. If a formed part cannot meet a position tolerance across bends, a machined angle may cost less than the scrap.
Our engineering and DFM support can review the drawing for these options before you commit.
Example decisions: brackets, enclosures and GSE frames
| Part | Usual route | When to switch |
|---|---|---|
| Harness or avionics bracket, low load, one thickness | Formed 5052 or 6061 aluminum sheet with self-clinching hardware | Machine it if it needs a precision bore, a thickness change or the quantity is one or two and no flat pattern exists |
| Highly loaded structural fitting or lug | Machined from plate or bar, or a forging at volume | Rarely sheet; consider a near net forging when titanium waste becomes the main cost |
| Multi-piece riveted angle bracket | Machined one-piece in aluminum | Keep formed if the parts are thin, the quantity is high and the joints are already qualified |
| Electronics enclosure, medium size | Formed and welded or riveted sheet with a machined connector panel | Hog from billet for EMI-tight lids, sealing grooves, integral heat sinks or very low quantity |
| Small sealed housing with O-ring groove | Machined | Formed only if the seal can move to a separate machined flange |
| GSE frame, cart or stand | Welded tube and plate to AWS D1.1, D1.2 or D17.1 as the program requires, with machined interface plates | Bolted extrusion if it must be reconfigured; fully machined only for small precision fixtures |
| Handling fixture that mates to flight hardware | Weldment, stress relieved, then machined at interfaces | Machined from plate if small enough to fit a mill and flatness is tight across the whole part |
For GSE specifics, see ground support equipment and the GSE cost guide.
Callout examples for the drawing or PO
State the route only where it matters, then state the requirements the route must meet, so suppliers can offer an alternate without breaking the design intent.
- Machined bracket: MATL: AL ALLOY 6061-T651 PLATE PER AMS 4027. DIMENSIONS APPLY AFTER FINISH. Add the finish spec and the general tolerance block.
- Formed bracket: MATL: AL ALLOY 5052-H32 SHEET, thickness and spec as the drawing defines. INSIDE BEND RADIUS 0.060 PLUS OR MINUS a stated tolerance. FLAT PATTERN FOR REFERENCE ONLY. Hardware by NASM45938 part number where required.
- Weldment: WELD PER AWS D17.1, CLASS stated per the weld map. MACHINE INDICATED SURFACES AFTER WELDING. State NDT method and sampling.
- Flexible formed panel: add a restrained condition note per ASME Y14.5 stating how the panel is held for inspection.
- Route open to alternates: a PO note such as ALTERNATE MANUFACTURING ROUTES MAY BE PROPOSED FOR BUYER APPROVAL. ALL DRAWING REQUIREMENTS APPLY.
For buying either route, see the buyer guides for aerospace CNC machining and aerospace sheet metal fabrication, and the RFQ checklist.
Get both routes quoted on one request
Send the drawing through a machining quote request and tell us if a formed or hybrid alternate is acceptable. We can ask qualified machine shops and fabricators to quote in parallel, flow down the same material, weld, inspection and finish requirements to both, and give you one quote and one PO for the route you pick, with the paperwork checked before shipment. Proprietary or export controlled drawings should follow our confidentiality and controlled programs steps. Back to the processes hub.
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Questions
When should a bracket be machined instead of bent from sheet?
Machine it when it needs a thickness change, a precision bore, tapped bosses, a sealing face or tolerances between features on different faces tighter than a formed part can hold. Protolabs publishes 0.015 to 0.040 in across two or more bends and plus or minus 1 degree on bend angles. Bend it when the wall is one thickness, the features sit on flat faces and quantity is more than a handful.
Is a machined part always more expensive than a sheet metal part?
No. In aluminum at low quantity a machined part often wins because it needs no flat pattern, form tooling, weld procedure or joint inspection, and the material is relatively cheap. The balance shifts toward sheet metal as quantity grows, as walls get thinner and taller, and in expensive alloys where a machined part turns most of its stock into chips.
What tolerances can sheet metal hold compared with CNC machining?
Published figures from one quick turn shop give plus or minus 0.005 in for machined features by default. For sheet metal under 0.13 in thick it gives 0.005 in between holes on one flat face, 0.015 in across two bends, 0.030 in across three or more bends, and plus or minus 1 degree on bend angles. Aerospace shops hold tighter on critical features at added cost.
What is buy to fly ratio and why does it matter here?
It is the weight of material bought divided by the weight of the finished part. A 2016 TWI paper on titanium aerostructures states machined titanium structure can be as poor as 20:1 and is rarely better than 4:1. Formed sheet is usually close to 1:1 plus the nest scrap, so material utilization strongly favors forming or near net shapes in titanium and nickel alloys.
Should a GSE frame be welded or machined?
Usually welded from tube and plate, then machined at the interfaces that mate to flight hardware or fixtures. Weld it to the code the program names, such as AWS D1.1 for steel, D1.2 for aluminum or D17.1 where flight hardware interfaces require it, and state whether flatness applies before or after welding. Small precision fixtures that fit a mill are often machined from plate.
Can a supplier change my part from machined to formed?
Only with your approval. The drawing defines the part, and a route change can affect strength, weight, tolerances and the first article. Many buyers add a PO note allowing alternate routes to be proposed for approval, which lets a fabricator or machinist offer a cheaper route without changing requirements. Any approved change should be reflected on the drawing and in a new or partial AS9102 first article.
Related
Sources
- Protolabs, sheet metal fabrication design guidelines: tolerances, flange length, bend radii, hole rules (read October 2026)
- Protolabs, CNC milling design guidelines: default tolerance, depth and size limits (read October 2026)
- TWI, Linear Friction Welding of Ti 6Al 4V for Aerostructure Applications (2016), buy to fly ratios
- ANSI Blog: AWS D17.1:2024 Fusion Welding for Aerospace Applications
- AWS: D1.2/D1.2M:2026 Structural Welding Code, Aluminum
- ASME: Y14.5 Dimensioning and Tolerancing
- PennEngineering: self-clinching nut conforming to NASM45938/4
- ATI, Ti-6Al-4V ELI technical data sheet: forming at elevated temperature
- SAE International, AS9102C Aerospace First Article Inspection Requirement
- SAE Mobilus, AMS4027 6061 T6 sheet and T651 plate
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