TOOL
Radius dish
Gives the top and back their curve, consistently.
| Common radii | 15 ft (back), 25 ft (top), 28 ft |
|---|---|
| Diameter | 500 – 600 mm, to cover a full body |
| Material | MDF, plywood or machined polymer |
| Surface | Smooth, true, with a flat rim |
| Use | Sanding braces to radius and gluing up under a caul |
| Pairing | Usually bought as a matched top and back pair |
| Accuracy | The surface has to be a true spherical section |
|---|---|
| Abrasive | Self-adhesive paper, replaced before it dulls |
| Glue-up | Braces clamped or go-barred against the dish |
| Flat rim | Lets the dish sit flat on the bench |
| Cleaning | Vacuum dust out; grit trapped in the dish marks the plate |
| Price band | Moderate |
A radius dish is a shallow concave disc — typically 15 ft or 25 ft radius — that the braced plate is sanded in and glued up on. It puts a spherical curvature into the top and back, which is what gives an acoustic guitar its structural strength and its resistance to humidity changes.
Most builders own two: a 25 ft dish for the top and a 15 ft dish for the back. The dish is used both as a sanding form, with abrasive stuck to the surface, and as a caul during glue-up.
At this stage
Radius is built in at glue-up. Braces glued flat to a plate cannot be made to curve afterwards — the dish is where the shape comes from.
Suits these instruments
- Steel-string acoustic
- Classical
- Archtop
Works with
- Go-bar deck
- Bracing chisel
- Hide glue
- Spruce top
- Sanding in a dish generates a lot of fine dust. Extraction or a respirator, every time.
- A dish is heavy. Lift it with two hands and store it flat so it does not warp.
Working notes
- Mark the radius on the rim so you do not mix up the top and back dishes.
- Replace the abrasive before it glues up. Dull paper rounds the brace ends instead of flattening them.
- Check the dish for flatness against a straightedge before you trust it.
- Vacuum the dish between plates — grit left in it scratches the next top.
Related
Bracing chisel
Shapes braces to a measured taper without tearing the grain.
Spokeshave
Shapes curved surfaces a plane cannot reach.
Go-bar deck
Clamps a whole braced plate at once, evenly.
Binding router bit
Cuts the channel that takes the binding and purfling.