CNC Turned Brass Dome Cap
Brass dome caps turned from C36000, with a dome radius held to ±0.05 mm, a centre bore concentric to the seat within 0.02 mm, a seat flat to 0.02 mm and a polished face at Ra 0.4 µm. Weights are matched within 3 g across a set.
- Dome Radius Within 0.05 mm
- Bore Concentric to 0.02 mm
- Seat Flat to 0.02 mm
- Polished Without Flats
*Send the seat diameter and the bore function. If the bore is a vent the dome is machined thinner at the top, and a cap turned to a uniform wall will be 15 % heavier and out of balance on a rotating part.
Brass Dome Cap Overview
A polished dome is the hardest surface to machine without showing a mark: the radius has to be continuous, the seat has to be flat, and the bore has to sit on the same axis or the cap rocks on its seat.
The cap is turned from bar, the dome radius is generated with a radius tool and blended by hand, then the seat is faced and the bore is drilled from the seat side so the two are concentric before polishing.
C36000 Free-Cutting Brass
C36000 takes a radius tool cleanly and polishes to a mirror without pitting, which is why it is used for visible caps. It also machines fast enough that a polished part is still economic at low volume.
C37700 Forging Brass or Copper
C37700 is chosen when the cap is hot forged close to shape and only the seat and bore are machined, and copper when the cap is also a heat spreader or a decorative element that will be lacquered.
For a cap above 100 mm diameter, the blank is cut from plate rather than bar so the dome does not carry the bar stress into the polished surface and open a line after buffing.
Brass Dome Cap Specifications
Turning and polishing data for dome caps, with the radius, seat and bore values that decide whether the cap sits flat and looks right.
| Specification | Machining Data | Specification | Machining Data |
|---|---|---|---|
| Product | Brass Dome Cap / Dome Cover | Machining Process | CNC Turning, Radius Milling, Polishing |
| Material | C36000 Free-Cutting Brass | Envelope | Ø 30 – 150 mm |
| Dome Radius | R 40 – 300 mm, ±0.05 mm | Dome Height | 8 – 45 mm |
| Centre Bore | Ø 3 – 30 mm, H8 | Bore Concentricity | 0.02 mm to the Seat |
| Seat Diameter | h8, ±0.03 mm | Seat Flatness | 0.02 mm |
| Wall Thickness | 3 – 10 mm | Surface Finish | Ra 0.4 µm Polished, Ra 1.6 µm Seat |
| Edge | R0.5 Rolled Edge or Square | Weight | Matched Within 3 g in a Set |
| Customization | Threaded Skirt, Cross Holes, Engraving, Nickel Plating | Lead Time | 4 – 6 Days Sample, 10 – 14 Days Batch |
| Inspection | Radius Gauge, Bore Air Gauge, Seat Flatness, Visual Polish | Packing | Soft Wrap per Cap, Foam Tray, Boxed |
Key Features of Our Dome Caps
Radius blended in one pass, seat faced after the dome, polished by hand.
Dome Radius in One Continuous Pass
The radius is generated with a radius tool and blended in one pass, which is what stops a facet showing up once the surface is buffed to a mirror.
Seat Faced After the Dome
The seat is faced last so it stays flat to 0.02 mm and square to the bore, and the cap sits on its face instead of rocking on the dome edge.
Bore Cut From the Seat Side
The bore is drilled and reamed with the cap held on its seat, so concentricity stays at 0.02 mm and the bore does not lean.
Hand Polished, Then Lacquered
Polishing is done by hand with a soft wheel to avoid flats, and the lacquer is applied within the hour so the brass does not start to tarnish under the coating.
Cap that has to look right and sit flat?
Surface Finishes for Brass Caps
The dome is decorative, the seat is a fit and the bore is functional, so finishing is selective.
| Finish | Main Purpose | Finish | Main Purpose |
|---|---|---|---|
| Polished Lacquer | Keeps the brass colour and stops tarnish | Nickel Plating | Silver finish for marine hardware |
| Masked Bore | Bore stays at H8 after plating | Vapour Blasting | Satin finish before lacquer |
| Ultrasonic Clean | Removes polishing compound | Degrease | Before Plating |
| Laser Marking | Engraving on the dome, shallow | Plating Build | 5 – 8 µm, Threads Masked |
| Soft Wrap | Prevents scratches in transit | Seat Protection | Taped for Shipping |
| Drying | Filtered Air, No Water Marks | Hand Polish | Final Pass on the Dome |
*Nickel plating on a polished dome shows every polishing mark, so the dome is buffed to a higher finish than the final look needs before plating. The bore and seat are masked.
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Machining Process for Brass Caps
The dome is turned as a radius and blended in one pass, so the polish does not reveal a facet where the tool changed direction.
01. Seat and Bore Review
Check the seat diameter, the bore function and the wall, then decide whether the dome needs a uniform or a tapered wall.
02. Turn the Blank and Seat
Face the seat, turn the outside diameter and cut the seat register, keeping enough stock for the dome radius.
03. Generate the Dome Radius
Cut the radius with a radius tool in one continuous pass, then blend the edge by hand before the seat is finished.
04. Face the Seat and Bore
Face the seat flat to 0.02 mm and drill and ream the bore from the seat side, checking concentricity with a pin.
05. Polish, Clean, Lacquer
Hand polish with a soft wheel, ultrasonic clean, then lacquer or plate with the bore and seat masked.
Send the seat diameter and the bore function, and we will quote a dome cap with the radius blended and the seat faced flat.
Brass Dome Cap FAQs
The radius is cut with a radius tool in one continuous pass and then blended by hand before polishing. A dome cut in steps and blended afterwards shows rings under a mirror polish.
The seat is faced flat to 0.02 mm after the dome is cut, so the cap sits on a true ring. A seat faced before the dome moves when the radius tool relieves the stress.
Yes, an internal or external thread is turned in the same set-up as the seat, so the thread and the seat are concentric within 0.02 mm and the cap screws down flat.
For indoor use, yes, for several years. Outdoors the lacquer breaks down at the edges within two seasons, so marine parts are usually nickel plated instead.
Within 3 g, which matters on a rotating assembly or on a set of caps that are handled as a group. Weight is recorded per cap and the set is boxed together.
H8 as standard and H7 on request, reamed from the seat side so it is concentric to 0.02 mm. A bore drilled from the dome side will lean by 0.05 mm over a 20 mm depth.
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