CNC Machined Aluminum Circular Positioning Base
CNC machined aluminium circular positioning bases for vision fixtures, rotary stations and robot tooling, with a turned register held to h7, a bolt circle positioned within ±0.02 mm and both faces flat to 0.02 mm. Machined in 6061-T6 and anodized red or black for station identification.
- Register Turned to 0.02 mm
- Bolt Circle Within 0.02 mm
- Flat Seat on Both Faces
- Red or Black Anodizing
*Tell us which face is the datum and whether the register is a press fit or a locating clearance. A register that is 0.01 mm tight will still seat, but it will not come off again without a puller.
Circular Positioning Base Overview
A positioning base is a reference surface, so its job is repeatability rather than strength. Two fixtures swapped on the same base have to land within a few hundredths of a millimetre, and that depends on the register diameter, the flatness of the seat and the position of the bolt circle relative to the register.
The disc is turned first to establish the register and the seating face on one axis, then the bolt circle and any slots are milled from the same centre. Because the register is turned rather than milled, it stays round within 0.015 mm, which is what keeps a fixture from rocking on the base.
6061-T6 Aluminium Disc
6061-T6 is used for fixture bases because it is stable after machining, takes an even dye and is light enough to swap by hand. It holds a turned register without springing, provided the disc is faced on both sides first.
7075-T6 or Cast Tooling Plate
7075-T6 is chosen for bases that carry a heavy fixture or see repeated clamping loads. Cast tooling plate is the alternative when flatness matters more than strength, because it is stress-relieved and moves less after machining.
If the base is machined from rolled plate, both faces are skimmed to release internal stress before the register is turned; skipping that step is what makes a base go concave after anodizing.
Circular Positioning Base Specifications
Turning and milling data for circular fixture bases, with the register, bolt circle and flatness values that set repeatability.
| Specification | Machining Data | Specification | Machining Data |
|---|---|---|---|
| Product | Circular Positioning Base / Fixture Pad | Machining Process | CNC Turning, Then Mill Bolt Circle and Slots |
| Material | 6061-T6 Aluminium Plate or Disc | Register Diameter Tolerance | h7 (0 / −0.021 on 30 mm) |
| Diameter Range | 40 – 300 mm | Bolt Circle Position | ±0.02 mm |
| Thickness | 8 – 40 mm | Face Flatness | 0.02 mm Across the Register Face |
| Register Height | 2 – 10 mm | Parallelism of Faces | 0.02 mm |
| Bolt Pattern | 3 to 12 Holes, M4 to M10, Slotted on Request | Concentricity Register to OD | 0.015 mm |
| Surface Finish | Ra 1.6 µm Register, Ra 3.2 µm Body | Anodizing | Type II 15 – 20 µm, Red or Black |
| Customization | Diameter, Register Step, Slot Pattern, Engraving | Lead Time | 6 – 8 Days (Sample), 12 – 16 Days (Batch) |
| Inspection | CMM Bolt Circle, Flatness on Granite, Register Gauge | Packing | Foam Interleaved, Bagged, Carton |
Key Features of Our Positioning Bases
Turned for roundness, milled for position, anodized for identification.
Turned Register, Not Milled
The register is turned on the same axis as the seating face, so it stays round within 0.015 mm and the fixture cannot rock on a high spot.
Bolt Circle From the Same Centre
Bolt holes are milled from the register centre rather than from the outside edge, so the pattern stays concentric even when the OD is machined to a loose tolerance.
Both Faces Stress-Relieved
Plate is skimmed on both sides before the register is cut, which releases rolled-in stress and keeps the base flat after anodizing.
Station Marking Built In
Station numbers or a bolt circle index are laser marked into the anodized face so an operator can identify the base without a label that peels off.
Need a base that repeats to a few hundredths?
Surface Finishes for Positioning Bases
Anodizing changes the register by a few microns, so any fit that matters is either masked or cut oversize.
| Finish | Main Purpose | Finish | Main Purpose |
|---|---|---|---|
| Type II Anodizing | Colour coding for fixture families and general wear | Hard Anodizing | Wear face where parts slide across the base |
| Bead Blasting | Uniform matte so the dyed colour reads even | Laser Marking | Station number and bolt circle on the top face |
| Chromate Conversion | Conductive path when the base is grounded | Masked Anodizing | Keeps the register bare where a press fit is required |
*Registers that are press fits are masked before anodizing or machined 0.02 mm oversize and finished after dye. Unmasked registers lose about 0.015 mm of clearance on a 30 mm diameter.
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Machining Process for Positioning Bases
The register, the bolt circle and the seating face are cut from one datum, so a fixture dropped on the base lands in the same place every time.
01. Interface Review
Check the register diameter, the fixture interface and the bolt pattern against the drawing, and confirm whether the register is a locating fit or a press fit.
02. Disc Preparation
Cut the disc oversize, face both sides to release stress, then re-face the datum side and mark it, since the register and bolt circle are both cut from that face.
03. Turn the Register
Turn the OD and the register step in one set-up, holding the register to h7 and checking roundness with a micrometer at three points.
04. Mill the Bolt Circle
Clamp on the register face, mill the bolt holes and slots from the centre, then deburr and break the edges by 0.3 mm.
05. Anodize, Inspect and Pack
Mask the register where required, anodize to colour, check the bolt circle on a CMM and flatness on granite, then pack with foam between parts.
Send the fixture interface drawing and the station layout, and we will quote the base with the register fit and finish already specified.
Circular Positioning Base FAQs
Within 0.015 mm concentricity between the register and the turned axis, and within ±0.02 mm for the bolt circle. A fixture located on the register and bolted through the circle will repeat inside 0.03 mm over repeated swaps.
Mask it if the fixture is a press fit, because anodizing adds 7 to 10 µm per surface and tightens the fit. For a locating clearance the coating can stay, since 0.01 mm does not affect location.
Yes, and slots are milled in the same set-up as the bolt circle so their centre lines stay radial. Slots let one base accept two fixture variants but cost some repeatability, so they are only used where adjustment is needed.
Yes, if both faces are skimmed before the register is turned. Rolled plate carries internal stress, and a base machined from one side only will bow by 0.05 mm or more once the top face is removed.
From 40 mm to 300 mm diameter, with thickness from 8 to 40 mm. Above 300 mm the part is milled rather than turned, because the register can be interpolated more accurately than it can be swung on a lathe.
Parts are interleaved with foam, bagged individually and packed so the register faces do not touch. Scuffed anodizing cannot be touched up, so packing is treated as part of the finish specification.
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