6061-T6511 Bar Tolerances: Straightness, Twist, and What Extruded Bar Actually Arrives Like
By Kanan Jarrus
Extruded 6061-T6511 bar arrives with mill-finish surfaces and commercial dimensional, straightness and twist allowances set by the Aluminum Association, not with the flatness and squareness people often expect from something sold as bar stock. That expectation gap causes more disputes than any other aspect of buying bar. This post covers what the tolerances actually are, which one catches people out, when it is worth paying for precision or manifold-quality material, and how much stock to leave so the part cleans up.
In Short
- →Commercial extruded 6061-T6511 bar is supplied to Aluminum Association standard tolerances, published in Aluminum Standards and Data. It is not ground, not guaranteed flat, and not guaranteed straight to a machining standard.
- →The three allowances that actually bite are dimensional tolerance, straightness, and twist. Twist is the one people forget and the one that ruins long parts.
- →Precision or manifold quality extruded bar exists for this: roughly half the commercial twist and straightness allowance, tightened dimensional tolerance held to the plus side, and an elevated minimum ultimate around 44 ksi.
- →Typical manifold-quality dimensional tolerance runs from about +0.012 / -0.000 at 1 to 1.5 inches up to about +0.054 / -0.000 at 8 inches.
- →Standard mill stock length is 12 feet. Sawn cut length is a separate tolerance from the section tolerance, and you should ask for both.
- →If you need four square faces and a controlled thickness, you are machining it anyway. Buy with facing stock, or buy plate cut to size instead.
The Three Allowances That Matter
Section tolerance is the one buyers ask about. The other two are the ones that scrap parts.
| Allowance | What it controls | When it hurts |
|---|---|---|
| Dimensional | Thickness, width, diameter across the section | Tight-fitting parts, stack-ups, thin walls |
| Straightness | Bow along the length of the bar | Long parts, anything held in a fixture or vise |
| Twist | Rotation of the section along the length | Long rectangular sections, the classic surprise |
| Cut length | Sawn end-to-end dimension | Parts machined to a shoulder rather than faced both ends |
Commercial vs Precision vs Manifold Quality
There are effectively three grades of extruded bar and the price difference tracks the tolerance difference.
- Manifold quality was developed for hydraulic and pneumatic manifolds, where a block gets drilled from six faces and any twist or bow turns into misaligned cross-drillings.
- Holding the dimensional tolerance all on the plus side is the useful part: the bar is never undersize, so you always have material to clean up to nominal.
- Typical manifold dimensional tolerances run about +0.012 / -0.000 at 1.000 to 1.499 inches, +0.024 / -0.000 at 2.000 to 3.999, and about +0.054 / -0.000 at 8.000 inches.
- It costs more. Specify it when twist or flatness genuinely drives the part, and do not pay for it on a bracket you are going to face anyway.
| Grade | Dimensional | Straightness and twist | Mechanical minimum |
|---|---|---|---|
| Commercial extruded | Aluminum Association standard | Standard allowance | 38 ksi ultimate |
| Precision tolerance | Tightened | Improved | 38 ksi ultimate |
| Manifold quality | About half commercial, held to the plus side | About half commercial twist and straightness | About 44 ksi ultimate |
How Much Stock to Leave
The practical answer, which is what you actually came for.
| Situation | Leave per machined face | Why |
|---|---|---|
| Commercial extruded, short part | 0.060 in | Covers dimensional allowance plus a cleanup pass |
| Commercial extruded, long or thin part | 0.125 in or more | Straightness and twist consume the rest |
| Manifold or precision quality | 0.030 to 0.060 in | Tolerance is tighter and biased positive |
| Sawn length, faced both ends | 0.060 in total | Cut tolerance, not section tolerance |
| Sawn length used as-is | Do not | A saw cut is not a finished face |
Standard Stock and Cut Lengths
- Standard mill stock length for extruded 6061-T6511 bar is 12 feet. Distributors buy and rack it that way.
- You pay for the whole stick unless the supplier cuts it. If you need 30 inches, buying 12 feet and racking the remainder is a real cost in both money and floor space.
- Cut length tolerance is a separate specification from the section tolerance. Ask for the number on your length, not just on your diameter or thickness.
- Long bar frequently exceeds parcel carrier length limits before it exceeds weight limits, so cutting to length often changes the shipping mode and lowers the landed cost.
- We cut to your length and give you a committed ship date at quote time.
When Bar Is the Wrong Purchase
Three cases where a buyer asking for bar should be buying something else, and we would rather say so.
- You need a dimensionally controlled rectangular blank. Plate is rolled to tighter thickness tolerance than extruded bar, carries the higher 42,000 psi ultimate minimum in T651, and we cut it to any rectangle.
- Your section is not a stocked bar size. A rectangle cut from plate weighs exactly the same and does not force you up to the next catalogue section.
- The part is short and fat rather than long and thin. Bar economics work when you need length. For a stubby block, plate is usually cheaper and more accurate.
- Bar is the right buy when the section is standard, you need real length, and mill finish is acceptable. That is a common case and bar wins on price there.
Extruded bar is a commercial product with commercial allowances, and treating it as precision stock is what produces the disappointment. Ask for dimensional, straightness, twist and cut-length tolerances in writing on your section, leave stock accordingly, pay for manifold quality only when twist genuinely drives the part, and buy plate cut to size when what you actually want is a controlled rectangular blank.
Frequently Asked Questions
How straight is extruded aluminum bar?
Commercial extruded bar is supplied to Aluminum Association standard straightness allowances published in Aluminum Standards and Data, which are looser than most machinists expect. Precision and manifold-quality extruded bar hold roughly half the commercial straightness and twist allowance and are the right choice when it matters.
What is manifold quality aluminum bar?
A precision extruded product developed for hydraulic and pneumatic manifolds. It carries about half the commercial twist and straightness allowance, dimensional tolerance tightened and held entirely on the plus side so the bar is never undersize, and an elevated minimum ultimate strength of around 44 ksi.
What tolerance is 6061-T6511 bar held to?
Commercial extruded bar follows Aluminum Association standard tolerances for dimension, straightness and twist. Manifold-quality material typically runs about +0.012 / -0.000 at 1 to 1.5 inches, widening to about +0.054 / -0.000 at 8 inches. Always ask for the figure on your specific section.
How much material should I leave on extruded bar for machining?
About 0.060 inch per machined face on a short commercial extruded part, and 0.125 inch or more on long or thin sections where straightness and twist consume the rest. Precision or manifold material lets you come down to roughly 0.030 to 0.060 inch.
How long is a standard stick of aluminum bar?
12 feet is the standard mill stock length for extruded 6061-T6511 bar. You pay for the full stick unless the supplier cuts it, and long bar often exceeds parcel length limits, so cutting to length frequently lowers the landed cost as well as the material cost.
Bar, cut to length
Ask us for the tolerance on your section.
6061-T6511 round and flat bar cut to your length, with mill certs on every order and a committed ship date before you commit.