Guide
Should a Custom Hex Standoff Be Made From Hex Bar or Turned Round Bar With Milled Flats?
A custom hex standoff is a hexagonal spacer, usually male-female or threaded at both ends, that holds a board or panel at a set distance from its mounting surface. It is made either by turning the ends from hex bar, whose flats come with the bar, or by turning round bar and milling the flats; the across-flats tolerance on the drawing, compared with what the bar standard allows, is what decides between them.
By Saola Manufacturing engineering team · Published · Updated
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What actually decides hex bar versus round bar with milled flats?
The across-flats tolerance decides it first. If the drawing allows what the bar delivers, hex bar is the cheaper route, and if the drawing is tighter than the bar class or the corners must be sharp, the flats have to be milled.
Hex bar arrives with its flats already formed to the tolerance class of the bar standard, so the shop only turns the ends, drills and threads. Round bar has to be turned and then milled, which adds a setup and cycle time but lets the flat dimension be held by the machine instead of by the mill that made the bar. The trade is therefore material and process cost against control of the flat dimension.
Three secondary questions follow: whether the size is a stocked hex, whether sharp corners matter, and whether the flats must be located to the thread or bore with a position tolerance. The sections below take each in turn.
What across-flats tolerance does hex bar come in?
It depends on the material standard and the tolerance class ordered, and the numeric limits sit in the standard text, not in the supplier’s catalog summary. Look up the current edition for the grade you buy before deciding that hex bar is tight enough.
For brass, ASTM B16/B16M covers free-cutting rod, bar and shapes in alloys UNS C36000 and C36010, intended for high-speed screw machining, and the active edition is B16/B16M-24. For stainless steel, ASTM A484/A484M is the general-requirements specification for bars, shapes and forgings, listed with A484/A484M-24B as the active version when the ASTM page was read, and it applies under product specifications such as A276/A276M. For carbon and alloy steel, ASTM A29/A29M notes that dimensional tolerances for cold-finished bars are found in Specification A108.
Ambica Steels (2026) names ISO tolerance classes for hex bar: h11 as the standard commercial tolerance, h10 for bars feeding automatic screw machines or CNC bar feeders, and h9 for components fitted with minimal clearance. The page does not print the class limits, so read them from the standard or ask the mill or distributor for the certificate that states the standard and class supplied.
| Drawing requirement | Hex bar is enough when | Round bar with milled flats when |
|---|---|---|
| Across-flats tolerance | It is at or looser than the class of the bar ordered | It is tighter than the bar class, so the mill must set the flats |
| Corner condition | Normal bar corners are acceptable | Sharp, fully formed corners or a defined corner chamfer |
| Flat position to thread or bore | Not controlled, or loose | Controlled by a position tolerance to the thread axis |
| Size availability | The across-flats size is a stocked hex | Non-standard size, so bar is turned from a larger round |
| Material and finish | Material is stocked as hex in the grade needed | Hex is not stocked in the grade, or surface must be uniform after machining |
| Volume and cost | Steady quantities and material cost matter most | Volume is low enough that the extra setup is acceptable |
Why can corners and bar size be a problem when milling flats from round bar?
A regular hexagon is wider across the corners than across the flats by a fixed ratio, so a round bar must be at least that diameter to leave sharp corners. If the bar is smaller, the corners are partly round, and if it is larger, more material is cut away.
The across-corners distance of a regular hexagon is about 1.155 times the across-flats distance. A round bar equal to that diameter gives full corners after the flats are milled, and a bar between the across-flats and across-corners diameters leaves cylindrical remnants at the corners that look like a worn hex. Both can be acceptable, but only if the drawing says which one is wanted.
That is also why a drawing should state the corner condition. Hex bar has its own corner shape from the rolling or drawing process, and a milled hex has the corner of the cutter path; the two are not identical. If a mating wrench socket or an enclosure pocket depends on the corners, say so and give a tolerance on the across-corners dimension instead of leaving it open.
How should a hex standoff drawing be written so the supplier can choose?
Dimension the across-flats size with a tolerance, state whether corners must be sharp, and leave the bar form open unless a requirement forces it. A drawing that fixes the route makes the part cost more than the function needs.
Call out the across-flats dimension with a tolerance that you actually need, and avoid copying a catalog value without checking it. Add a position or symmetry tolerance for the flats to the thread axis only if the assembly depends on it, since a controlled position nearly always points to milled flats. Give the length, the thread at each end with its tolerance class, and an edge break on the ends so the part does not leave burrs on the board.
Then add a note such as hex bar acceptable if the across-flats tolerance is met, or milled flats required, depending on your assembly. Tolerance symbols are defined in ASME Y14.5-2018, and the general rules for choosing tolerances are in the cost of tight tolerances guide.
What changes at 10,000 to 20,000 standoffs a year?
At that volume the cheaper material and the removed milling step usually favor hex bar, as long as the across-flats size is stocked and the tolerance is met. Round bar with milled flats is justified by a requirement, not by habit.
A steady annual quantity lets a shop stock the hex bar size and run it in long campaigns on a bar-fed lathe, which is the setting where hex bar is most economical. An extra milling operation per part adds cycle time across the whole run, so it has to be paid for by a requirement such as a tight across-flats tolerance, sharp corners, or a non-stocked size.
Material choice is a separate decision and not covered here. Whatever the material, ask the supplier to quote both routes if the drawing leaves the choice open, and to say which bar standard and class each quote assumes. The comparison of bar condition for steel parts is in the hot-rolled vs cold-rolled steel parts guide.
What should the RFQ ask a supplier about hex standoffs?
Ask which bar form and standard each quote uses, what across-flats tolerance and corner condition it assumes, and how the flats will be inspected. Those three answers tell you whether two quotes are for the same part.
Ask also whether the flats were measured across flats and across corners at first article, and whether the thread and the hex were checked relative to each other when that relationship is on the drawing. State the annual quantity and the release pattern so the supplier can decide whether to stock the bar. The full package is in the guide to preparing a machining RFQ.
Saola makes standoffs and similar bar-fed parts by CNC turning and milling at its lead CNC plant; see the CNC turning capability page and the CNC milling capability page. The bar materials the lead plant runs today are listed on the turning page, so name the grade and the bar form, hex or round, in the RFQ and the quote will confirm the route. Any plating or anodizing on the finished standoff is arranged through a partner plant that Saola manages.
Frequently asked
Is hex bar always cheaper than milling flats?
Usually when the size is stocked and the tolerance is met, because the milling step is removed. It is not cheaper when the size is non-standard or when the tolerance forces the shop to mill anyway.
Can I get tighter across-flats tolerance than the bar gives?
Yes, by milling the flats on round bar, where the machine sets the dimension. Expect an added operation, and state the tolerance only if the assembly needs it.
Do round-bar hex standoffs look different from hex-bar ones?
They can. A milled hex from a bar smaller than the across-corners diameter has partly round corners, while hex bar has the corner form made by the mill. Specify the corner condition if appearance or fit matters.
Which standard governs the bar I am buying?
It depends on the material: ASTM B16/B16M for free-cutting brass, ASTM A484/A484M with A276/A276M for stainless, and A29/A29M with A108 for carbon steel. Ask the supplier to state the standard and class on the quote and certificate.
Does this guide cover standoff material choice?
No. Material, such as aluminum, brass, steel or plastic, is a separate decision. This guide only covers the bar form and the flats.
Sources
- ASTM B16/B16M, Free-Cutting Brass Rod, Bar and Shapes for Use in Screw Machines, ASTM International, 2015 edition page; active edition B16/B16M-24
- ASTM A484/A484M, General Requirements for Stainless Steel Bars, Billets, and Forgings, ASTM International, 2023 edition page; active edition A484/A484M-24B
- ASTM A29/A29M, General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought, ASTM International, 2020 edition page
- Stainless steel hex bar standards and tolerance classes, Ambica Steels, August 2026