Machined Forged Discs & Flange Blanks: Finish, Tolerances and RFQ Checklist

Quick answer: A machined forged disc — often called a flange blank or simply a machined disc — is a forged round whose faces have been turned and surface-ground to a bright, flat, finished condition. The radial "sunburst" pattern on the disc in our yard is the grinding mark left by a rotary surface grinder, and it is the visual signature of a flat, parallel, ready-to-finish blank. It is the starting point for blind flanges, weld-neck flange blanks, gear blanks and machined components. Specify diameter × thickness, grade, face flatness and surface finish (Ra, or "sunburst-ground"), and any drilling or boring you want completed.

What a machined forged disc is

Start with a forged round billet or disc, then machine both faces (and usually the OD) on a lathe and finish at least one face on a rotary surface grinder. The result is a disc with:

• Two flat, parallel faces.
• A known, defect-free surface (scale and the forged skin are gone).
• A bright, low-Ra finish suitable for sealing or further machining.

Why machine it instead of leaving it as-forged

For a flange or sealed component, the machined blank saves the buyer a rough-machining operation and removes the defective surface layer so inspection is meaningful.

The "sunburst" finish, explained

The concentric/radial pattern is produced by a rotary surface grinder (or a polishing head) spinning against the slowly rotating disc. It is:

• Mostly cosmetic on the visible face, but it also confirms the face was ground flat.
• Quantified by Ra (roughness average) — typical machined discs run Ra 0.8–3.2 µm; a finer "mirror" lap goes lower.
• Not a defect — the rainbow iridescence is thin-film interference on freshly ground metal, normal on bright stainless and nickel alloys.

If you need a specific Ra or a fully polished face, state it; "sunburst-ground" alone describes the look, not a number.

Typical sizes we supply

The disc in the photo is a large, heavy blank — the kind used for a big blind or weld-neck flange.

Grades supplied as machined discs

For an annular part rather than a solid disc, the route is a rolled forged ring instead.

Flatness, finish and tolerance to put on the RFQ

From blank to finished flange

A machined disc becomes a flange by:

1. Drilling the bolt-hole circle (and any vent/tap holes).
2. Boring a centre hole for a weld-neck or lap-joint flange.
3. Welding a neck (weld-neck type) or leaving it blind.
4. Inspecting to the flange standard (e.g. ASME B16.5 / EN 1092).

If you only need the blank, we stop at step 0; if you need the finished flange, we drill and bore it. See our cast vs forged comparison for why wrought blanks are preferred for pressure service.

Machined-disc RFQ checklist

1. Diameter × thickness (mm).
2. Grade / standard (e.g. ASTM A182 F316, A105).
3. Face finish: sunburst-ground, specific Ra, or mirror.
4. Flatness and parallelism requirements.
5. Machining: plain blank, drilled bolt circle, or bored centre.
6. MTC and any UT per mill-test reading; weight planning via the weight calculator.

FAQ

Q: Is the sunburst pattern a defect? A: No. It is the normal mark of a rotary surface grind and actually shows the face was ground flat. The iridescent sheen is thin-film interference on bright metal.

Q: Disc blank or forged ring — which do I need? A: Solid faced part (blind flange, gear blank, valve body) → machined disc. Hollow annular part (ring, collar, hollow cylinder) → rolled forged ring.

Q: Can you drill the bolt holes for me? A: Yes — supply the flange standard and size (or a bolt-circle drawing) and we deliver drilled and bored, not just the blank.

Q: What Ra can I get? A: Standard machined discs are around Ra 0.8–3.2 µm; specify a lower Ra or a mirror lap if the sealing face demands it.

Q: Does machining remove the forged benefits? A: No. Machining only skins the surface; the forged grain flow and internal soundness remain. It is the same wrought blank, just brought to a finished, inspectable condition.