SR Spherical radius
Spherical radius · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “SR 25” states a spherical radius requirement.
What controls
A prefix indicating that the dimensioned value is the radius of a spherical surface, not of a cylindrical arc. It defines a spherical cap, a concave seat or a spherical end, and describes a doubly curved surface: the same radius in every direction passing through the center. Like a regular radius, on its own it limits size but does not fix the location of the center or require tangency with adjacent surfaces; that is controlled with a profile frame or a spherical position frame.
How to verify
Coordinate measurement with points distributed in several directions and a sphere fit, which returns the actual radius and the location of the center. For a quick check on the shop floor, spherical gauges or mating form parts are used, assessing contact with bluing. Measuring the radius with a flat gauge or a caliper in a single direction does not tell you whether the surface is spherical or simply cylindrical in that section.
Common mistakes when interpreting it
- Confusing the spherical radius with an arc radius and checking it in a single section.
- Machining with an untilted ball end mill and leaving a poor finish at the pole.
- Passing the surface on the radius value without checking the location of the center.
How to get it in the workshop / if it comes out of tolerance
It appears on ball joint seats, stylus tips, rod ends and point-contact surfaces. It is machined by three-axis interpolation or with a form tool; with a ball end mill, the finish depends heavily on the stepover and on which part of the tool is cutting, because at the center of the tip the cutting speed is practically zero. On a lathe, a spherical surface is produced by interpolation or with copying (tracer) attachments.
Poor finish at the pole of the sphere: that is the area where the ball end mill cuts at almost zero speed; tilt the tool or change the strategy so it cuts with the flank. Correct radius but irregular contact in the bluing check: the surface is not spherical or the center is offset. With a form tool, profile wear translates directly into radius error.
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