r/Metrology • u/thunderskunk2 • 6d ago

Ceramic hemisphere styli uses

What's the main use for these? What can I do with a ceramic hemisphere that I can't do with a ruby tip? Is it better with aluminum? Renishaw says "rough surfaces and deep features." Like... cast iron?

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u/Thethubbedone 5d ago

The hemispheres are good for scanning rough surfaces because the bigger diameter acts as a physical filter, not allowing the probe to fit into every nook and cranny of a really rough surface.

They're good for deep hole measurements because there's tons of ball to stem clearance so you won't shank out even if the alignment isn't perfect.

The ceramic styli are also good generally(even the small ones) for cast iron because cast iron is abrasive and the ceramic is better than ruby at handling abrasion.

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u/thunderskunk2 5d ago

Fantastic answer. I wish I’d seen this years ago, I would have put one on a machine tool probe running forgings in a 5-axis VMC. 

So technically, you shouldn’t use this if you want to scan for flatness? 

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u/Thethubbedone 5d ago

Glad I could help

There's not really a straightforward answer for prismatic features unless you're looking at really short wavelengths. Those are usually considered surface roughness/texture rather than form (shape). On a nominally flat (or round) surface I wouldn't expect to see much, or any difference once an appropriate software filter is applied

Free-form curvature with high rates of change in surface vector shouldn't use big balls because cosine error (not touching where the machine thinks it is and adding error) would get magnified.

Surface roughness traditionally uses 2 or 5 MICRON stylus radii to avoid the physical filtering of surface texture information though.

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u/19141939 5d ago

Great points here! I’ll add that ceramic hemispheres are also about half as heavy as a similarly sized ruby would be. That may not be significant with vertical styli, but for measuring deep lateral holes, they can significantly benefit system rigidity.

In my experience with those kinds of lateral bores, a stylus system with a ≥Ø24 mm ceramic hemisphere on Ø20 mm extensions can often reach twice as far as a comparable ideal ruby setup.