CNC Turned Aluminum Eight-Lobe Flange Plate
Eight-lobe flange plates turned and milled from 6061-T6, with a H7 pilot bore round to 0.015 mm, eight lobes indexed to ±0.05 mm, a bolt circle within 0.05 mm of the bore axis and faces flat to 0.02 mm.
- Pilot Bore H7, 0.015 mm Roundness
- Eight Lobes Indexed to 0.05 mm
- Face Flat to 0.02 mm
- Bolt Circle Within 0.05 mm
*Send the mating spigot and the bolt pattern. The lobe index is set from the pilot bore, and a plate indexed from the outside edge will be 0.1 mm out at the last lobe.
Eight-Lobe Flange Plate Overview
A scalloped flange is a clocking part: the lobes have to be equally spaced, the bolt circle has to be concentric to the pilot bore and the face has to be flat, or the joint opens at one lobe when the bolts are torqued.
The plate is turned to thickness and bored first, then moved to the mill where the eight lobes are cut with the bore as the rotary datum before the bolt circle is drilled from the same centre.
6061-T6 Aluminium Plate
6061-T6 is used for flange plates because the lobes remove a lot of section and the remaining rim still has to carry bolt preload. It also anodizes evenly on the scalloped edges.
6082-T6 or 5083
6082 is used where the plate is welded to a duct or a housing, and 5083 where the flange sees salt water and the anodized surface has to resist pitting at the lobe edges.
Above 250 mm diameter the plate is turned from cast or rolled plate and stress relieved before the lobes are cut, otherwise the rim goes out of flat by 0.05 mm after machining.
Eight-Lobe Flange Plate Specifications
Turning and milling data for scalloped flanges, with the bore, lobe index and bolt circle values that keep the plate interchangeable.
| Specification | Machining Data | Specification | Machining Data |
|---|---|---|---|
| Product | Eight-Lobe Flange Plate / Scalloped Flange | Machining Process | CNC Turning, Milling, Bolt Circle Drilling |
| Material | 6061-T6 Aluminium Plate | Envelope | Ø 90 – 320 mm |
| Pilot Bore | Ø 20 – 160 mm, H7 | Bore Roundness | 0.015 mm |
| Flange Thickness | 8 – 25 mm | Face Flatness | 0.02 mm |
| Lobe Count | 6 – 12 Lobes, Equal Index | Lobe Index | ±0.05 mm |
| Bolt Circle | 4 – 12 Holes, Position ±0.05 mm | Counterbore | Depth ±0.05 mm |
| Concentricity | Bolt Circle to Bore 0.03 mm | Surface Finish | Ra 1.6 µm Faces, Ra 0.8 µm Bore |
| Customization | Spigot, Keyway, Lightening Pockets, O-Ring Groove | Lead Time | 5 – 7 Days Sample, 12 – 16 Days Batch |
| Inspection | Air Gauge Bore, CMM Bolt Circle and Lobe Index, Granite Flatness | Packing | Foam Cradle, Bore Plug, Boxed |
Key Features of Our Flange Plates
Lobes and bolt circle from the bore, faces flat before coating.
Lobes Indexed From the Pilot Bore
Each lobe is cut with the bore as the rotary datum, so the index stays within 0.05 mm and any of the eight positions lines up with the mating duct.
Bolt Circle From the Same Centre
The bolt holes are drilled from the bore axis rather than the outside diameter, so the plate does not fight the bolts when it is fitted.
Faces Flat to 0.02 mm
Both faces are skimmed after the lobes are cut, because scalloping the rim releases stress and dishes the plate by 0.03 mm if the faces are finished first.
Bore Masked Before Coating
The pilot bore is plugged for anodizing and air gauged again afterwards, so the spigot still goes in with the fit it was machined to.
Flange that has to clock into eight positions?
Surface Finishes for Flange Plates
The pilot bore and the mounting face are fits, the scalloped edge is visible and the bolt holes must stay clear.
| Finish | Main Purpose | Finish | Main Purpose |
|---|---|---|---|
| Type II Anodizing | Corrosion protection and colour | Hard Anodizing | Wear resistance on the spigot |
| Masked Pilot Bore | Bore stays at H7 after coating | Bead Blasting | Even matte before dye |
| Laser Marking | Bolt pattern and balance position | Masked Flange Face | Keeps the mounting face flat and conductive |
| Anodizing Build | 10 – 15 µm, Sealed | Vapour Blasting | Satin Finish Between Lobes |
| Bore Plug | Removed After Coating | Face Protection | Taped for Shipping |
| Assembly Oil | Light Film in the Bore | Balance Check | Static Balance Within 10 g·mm |
*Anodizing closes the pilot bore by 20 µm and builds unevenly on a scalloped edge. The bore is plugged and the mounting face is taped, otherwise the plate sits 0.01 mm proud and the joint leaks.
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Machining Process for Flange Plates
The lobes and the bolt circle are cut from the pilot bore, so the flange indexes the same way whichever of the eight positions it is turned to.
01. Spigot and Pattern Review
Check the mating spigot, the bolt pattern and the number of lobes, then set the index and the bolt circle from the bore.
02. Turn Faces and Bore
Turn both faces, the outside diameter and the pilot bore in one set-up, leaving 0.5 mm on the faces for a final skim.
03. Cut the Lobes
Mount on the bore, index each lobe from the same datum and mill the scallops, checking the index with a pin before unclamping.
04. Drill the Bolt Circle
Drill and counterbore the bolt circle from the bore axis, then skim both faces to remove the burr left by the lobes.
05. Inspect, Mask, Anodize
CMM the bolt circle and lobe index, air gauge the bore, plug the bore, anodize, then re-gauge and protect the faces.
Send the spigot drawing and the bolt pattern, and we will quote a flange plate with the lobes and the bolt circle cut from the pilot bore.
Eight-Lobe Flange Plate FAQs
0.05 mm between lobe centres, cut with the bore as the rotary datum. An error of 0.1 mm means the plate only fits in one of the eight positions, which defeats the point of a scalloped flange.
Yes, a spigot from 5 to 25 mm deep is turned in the same set-up as the bore, so the spigot and the bore are concentric within 0.02 mm and the plate self-centres.
Because cutting the lobes releases stress in the rim and dishes the plate by about 0.03 mm. Skimming after the lobes is what keeps the face flat to 0.02 mm.
It adds 10 to 15 µm per surface, so the bore is plugged and re-gauged after coating. The mounting face is taped so the joint stays flat and conductive.
Yes, statically to 10 g·mm by milling a pocket on the heavy side, which matters on a fan or a rotating flange above 1,000 rpm.
320 mm from one plate. Above that the flange is turned as a ring and welded to a hub, which keeps the face flat but adds a weld that has to be stress relieved.
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