Choose a roundover router bit by the radius you want on the wood, then check that its shank, overall diameter and permitted speed suit your router. A “1/4-inch bit” is an incomplete description: that measurement could refer to the curve it cuts or the shaft held by the collet. Reading the specification table prevents an expensive mismatch.
My recommendation is to choose the radius from the finished edge you want, then check the complete bit specification. I would keep the first acceptable offcut as a reference for the remaining parts: its curve and pencil marks give you something concrete to compare, while a note saying only ‘quarter-inch bit’ leaves too much unresolved.
Separate the five measurements
A useful real example is the Freud 34-110 roundover bit. Its specification lists a 1/4-inch radius, 1/4-inch shank, 1-1/8-inch overall diameter and 5/8-inch bearing diameter, with a maximum speed of 18,000 rpm. The equal radius and shank figures are coincidental; each describes a different part of the tool.
| Specification | What it describes | Your check |
|---|---|---|
| Radius | The size of the curved cutting profile. | Does that curve suit the finished edge? |
| Shank diameter | The shaft held by the router collet. | Does it match the collet exactly? |
| Overall diameter | The cutter's widest diameter. | Is the cutter permitted in this router and clear of its base? |
| Bearing diameter | The guide that follows the edge. | Is it the specified bearing for the intended profile? |
| Maximum rpm | The upper speed limit for this bit. | Can you set a permitted speed within both tool and bit instructions? |
Use the product drawing to identify its dimension letters; letters can differ between catalogs. For metric and imperial shafts, our router shank size guide explains why near-matching measurements are not interchangeable.
Choose the curve on a full-size drawing
Trace the end of your actual board on paper. Draw the proposed quarter-circle into its corner, using the bit radius as the circle radius. A 1/8-inch radius is 3.175 mm; a 1/4-inch radius is 6.35 mm. That second option removes a visibly larger corner. Compare the drawing with nearby joints, hardware and any edge you intend to leave square.
Here is a geometry example: on a 19 mm thick board, matching 6 mm quarter-rounds on the top and bottom leave a 7 mm straight section between them: 19 − 6 − 6 = 7. That calculation describes the desired profile only. It does not establish that the bearing will have adequate support during both cuts, or that your router setup can make them. Those checks still need a full-size offcut.
Make one reference sample before the project
- Choose a sound offcut with the same thickness and edge preparation as the finished part. Make it large enough to secure and support properly; a tiny loose sample is unsuitable.
- Mark the reference face and the edge to be rounded. Decide which neighboring edges must stay square. These pencil marks keep a reversible test from becoming an unwanted profile on the project.
- With power disconnected, install and adjust the correct bit according to the router manual. Check its allowed speed and clearance. Read the manufacturer's router-bit safety guidance (PDF), before cutting.
- Secure the offcut and plan the cut using the feed direction and pass sizes specified for your setup. Keep the base supported and your hands away from the cutter. Stop the router completely before inspecting or adjusting.
- Inspect the finished profile from the end. Check whether the curve meets the flat faces cleanly, whether an unwanted shoulder remains, and whether the bearing tracked a clean edge. Correct the setup on the sample first.
- Label the accepted sample with the bit model, radius and reference face. Keep it beside the work while routing the matching parts.
A roundover and a bead are different profiles
Some roundover bits can also produce a decorative bead with a manufacturer-approved bearing change. Freud describes this option for the 34-110 family. A replacement bearing therefore deserves a specification check: diameter affects the profile. Do not substitute a bearing just because it fits the screw.
When shopping, save the complete model number with your chosen profile. That makes a later replacement or matching part much easier to identify than a note that says only “quarter-inch router bit.”
Original editorial illustration.
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