Custom Flanges, Machined to Your Print

Which flange styles can be made to your print?

A flange is a protruding rim or lip that adds strength, and flanges are often attached with bolts on a bolt circle (source 1): holes spaced evenly on an imaginary circle, written as the hole count and the circle diameter, such as 4x130 mm (source 2). Custom machined flange parts come in the styles below; the table gives how each one joins and what its print needs beyond the outline.

What should a flange print call out?

The bolt pattern. Give the bolt circle diameter, the hole count and the hole size, a position tolerance on the holes to your datums (source 7), and whether each hole is clearance or tapped. Position, perpendicularity and runout callouts are quoted from the print. The first question below works out how much position tolerance a clearance hole can carry.

The turned features. The faces, the bore, the pilot and any face groove are usually turned: a nearly stationary tool cuts the rotating part, and a groove can be cut into the face itself (source 8). The bolt holes are usually drilled on a mill (source 9). Mark the pilot and the bore as critical diameters with their own limits. A STEP file with no drawing is quoted to a ±0.005 in general tolerance, or the one your notes name, so a pilot that has to center a motor needs a drawing or a note; what file to send covers that rule.

The sealing face. An O-ring in a face groove is squeezed top and bottom, like a gasket, and should sit against the low-pressure side of the groove so pressure holds it where it already is. Apple Rubber’s guide puts the groove’s outside diameter at or just under the O-ring’s mean OD for internal pressure, and the inside diameter at or just over its mean ID for external pressure or vacuum (source 10). Put the O-ring size and the pressure direction on the print, or the groove dimensions, along with any flatness or finish callout on the face.

The weld joint. For a flange that is welded into an assembly, put the joint on the print: on a weld neck, the pipe or tube OD and ID at the hub and the bevel, the preparation ASME B16.25 covers for the buttwelding ends of piping components (source 11); on a slip-on, the OD the bore slides over. Weld-in bungs for the same assembly can go on the same quote, and machined parts for fabrication shops covers the rest of a weldment order.

The mating part. Name what the flange bolts to: a motor face, a gearbox, a tube end, a vacuum port or a pipe flange. If it has to match a standard pattern, name the standard and the size.

Can a custom flange bolt to an ASME B16.5 pipe flange?

It can take its bolt pattern from one. ASME B16.5 covers pressure-temperature ratings, materials, dimensions, tolerances, marking and testing for pipe flanges and flanged fittings from NPS 1/2 through NPS 24, in rating classes 150, 300, 400, 600, 900 and 1500, with class 2500 up to NPS 12 (source 12). Larger flanges are covered by ASME B16.47 (source 1). A custom flange can take its bolt circle, hole count and hole size from the matching B16.5 size and class, so it bolts up to that flange. It is not a rated B16.5 flange, and no rating is implied. The quote does not state a pressure rating.

No C of C or material certifications ship with parts by default. If your PO needs them, say so before the quote. Inspection reports on critical dimensions aren’t included by default. If you need one, ask when you place the order.

Which material and finish suit a flange?

6061 aluminum is one of the most common alloys for general-purpose use and is highly weldable by TIG or MIG, though after welding the metal next to the weld typically has 6061-T4 properties, a strength loss of around 40% (source 13). 7075 is used for highly stressed structural parts (source 14) but is prone to hot cracking in fusion welding (source 15), which makes it a poor choice for a flange that gets welded to tube.

In stainless, 303 is the free-machining grade. Carpenter does not recommend it for welding, because its high sulfur content may cause hot cracking, or for vessels containing gases or liquids under high pressures (source 16), so a flange that will be welded, or that holds gas or liquid under high pressure, should not be 303. 316 adds molybdenum, which gives it more resistance than 304 to localized attack by chlorides (source 17). 303 and 304 stainless and 316 stainless each have their own page.

Low-carbon (mild) steel, about 0.05-0.30% carbon, can be easily machined and welded, and a higher carbon content reduces weldability; pipes are among its typical uses (source 18). It is susceptible to rust, especially where there is moisture or salt, and is protected by a coating such as paint (source 18), so a steel flange either gets a finish on the print, such as powder coat, or ships as-machined to be painted with the weldment it goes into. Brass is commonly used where corrosion resistance and low friction are required, in plumbing, hose couplings and valves among other things, and is often used in fittings that must be non-sparking near flammable or explosive materials (source 19). Other alloys and plastics quoted on request. Name the grade on the print.

Vacuum flanges need their own choices. The free-machining 303 grade contains sulfur, which tends to outgas, and 304L is used for ultra-high vacuum systems. Aluminum alloys have low outgassing unless they contain higher proportions of zinc, and aluminum vacuum parts must not be anodized, because the oxide layer traps water vapor and then releases it; the zinc in brass may cause problems too (source 20). 7075, whose main alloying element is zinc at roughly 5.6-6.1% (source 14), is one of those higher-zinc alloys. Vacuum flanges are quoted to the print, and no leak rate is stated.

A finish called out on the print is quoted with the part. The finishes offered are anodize on aluminum, passivation on stainless and powder coating, or the flange ships as-machined. Anodize grows into and out from the surface by equal amounts, adding half the oxide thickness to each surface, and the anodic layer is insulative, which also makes welding more difficult (source 21). Mark a pilot, bore or O-ring groove that has to hold size so it is masked or allowed for, and keep anodize off a face that will be welded.

What comes back with the quote?

The quote gives the price, the material and the lead time, plus the free design review: notes on anything in the files likely to cost more or cause trouble, such as a pilot with no tolerance, a bolt hole that breaks into the O-ring groove, or a thin wall behind a counterbore. Ask for more than one quantity, such as 1, 25 and 100, and the quote shows each.

Lead time starts at order approval. Put your need date in the part description and the quote answers it. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date.

If a part doesn’t match the print, it’s remade or credited. Flanges can go on one quote with the fittings or manifold blocks they connect to, or with the shaft couplings on the same drive, and every other part type is on the parts index.

Sources

All accessed 27 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Flange": en.wikipedia.org/wiki/Flange (definition; bolt circles; uses of weld neck, slip-on, blind, threaded and socket weld; ASME B16.47 above 24 in).

2. Wikipedia, "Bolt circle": en.wikipedia.org/wiki/Bolt_circle (definition and notation).

3. Globe Scott Motors, "Motor Mounting Configurations Explained: B3, B5 and B14", 25 August 2026: globescott.net/blog/motor-mounting-configurations-explained/ (register spigot; through-holes and tapped holes).

4. Wikipedia, "Marman clamp": en.wikipedia.org/wiki/Marman_clamp (how the V-groove strap holds the flanges).

5. Wikipedia, "Vacuum flange": en.wikipedia.org/wiki/Vacuum_flange (KF, ISO-K, ISO-F and CF: joints, sizes and gaskets).

6. Wermac, "Flanges General: Flange Types": wermac.org (how each pipe flange style attaches).

7. Gene R. Cogorno, "Tolerancing Floating Fasteners", Technical Training Consultants, from Geometric Dimensioning and Tolerancing for Mechanical Design, 2nd ed., McGraw-Hill, 2011: ttc-cogorno.com/Newsletters/130218FloatingFastener.pdf (floating fastener formula and its 0.250 in example).

8. Wikipedia, "Turning": en.wikipedia.org/wiki/Turning (definition; face grooving).

9. Wikipedia, "Milling (machining)": en.wikipedia.org/wiki/Milling_(machining) (vertical mills drill).

10. Apple Rubber, "Seal Design Guide, Section 4: Seal Types & Gland Design": applerubber.com/src/pdf/section4-seal-types-and-gland-design.pdf (face seal groove sizing by pressure direction; static gland finish).

11. ASME B16.25-2022, "Buttwelding Ends": asme.org (scope: welding bevels).

12. ASME B16.5-2025, "Pipe Flanges and Flanged Fittings: NPS 1/2 Through NPS 24 Metric/Inch Standard": asme.org (scope, sizes and rating classes).

13. Wikipedia, "6061 aluminium alloy": en.wikipedia.org/wiki/6061_aluminium_alloy (general-purpose use; weldability; strength near the weld).

14. Wikipedia, "7075 aluminium alloy": en.wikipedia.org/wiki/7075_aluminium_alloy (zinc content; highly stressed parts).

15. M. Sokoluk, C. Cao, S. Pan and X. Li, "Nanoparticle-enabled phase control for arc welding of unweldable aluminum alloy 7075", Nature Communications, 2019: pmc.ncbi.nlm.nih.gov/articles/PMC6327098 (hot cracking in fusion welding).

16. Carpenter Technology, "CarTech 303 Stainless" technical datasheet, edition 29 May 2007: carpentertechnology.com/hubfs/7407324/Material%20Saftey%20Data%20Sheets/303.pdf (sulfur; welding; high-pressure vessels).

17. Wikipedia, "SAE 316L stainless steel": en.wikipedia.org/wiki/SAE_316L_stainless_steel (molybdenum and chloride resistance).

18. Wikipedia, "Carbon steel": en.wikipedia.org/wiki/Carbon_steel (mild steel carbon range; machining and welding; rust and coatings; typical uses).

19. Wikipedia, "Brass": en.wikipedia.org/wiki/Brass (uses in plumbing, hose couplings and valves; non-sparking fittings).

20. Wikipedia, "Materials for use in vacuum": en.wikipedia.org/wiki/Materials_for_use_in_vacuum (stainless grades, aluminum, anodizing and brass in vacuum).

21. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (growth per surface; insulative layer).

22. Tec-Ease, "The Fixed and Floating Fastener Formulas Are Not Just for Fasteners", GD&T Tips, January 2002: tec-ease.com/gdt-tips-view.php?q=110 (fixed fastener formula).

23. Wermac, "Flange Faces": wermac.org (raised and flat faces; cast iron joints; face finish).