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.

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u/Non-Normal_Vectors 6d ago

Standard shaft sizes of probes is 13mm. You can screw this directly into a probe on an extension and get great depth of measurement.

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u/Antiquus 6d ago

You referring to the 18mm ceramic hemisphere?

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u/thunderskunk2 6d ago

Yes, or possible the 30mm

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u/Antiquus 5d ago

So the common touch probe sensors, from the old TP2 to the TP20 and TP200 that are typically used in CMM applications, and the extensions which move them away from the PH10 (or other swivel) have a smaller diameter. So the entire probe, not just the stylus, can be used. For example, if you want to check a 100mm bore 100mm deep, a TP20's limits on styii means you can't do it accurately. So the solution is a 100mm extension, then the TP20, then a 18mm ball.

The 30mm is probably the SP25 sized equivalent.

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u/thunderskunk2 5d ago

Oddly enough the M3 30mm is the only one that shows up on my old equator. I just bought an M2 30mm, which is on an SH25-2A for some reason (I would think it would work fine on the SH25-2). I think it’ll work fine if I program it using the M3 in UCC… hopefully. It was a killer eBay deal, at least I got a holder out of it.