How to measure cam follower face radius. Sort of.

The cam followers have piqued my interest- the suppliers say the face radius is probably ok, but won’t tell me what “ok” means, because it’s a trade secret.

Given that it’s a trade secret for followers that have been made since 1995, I found out* that the radius should be between 52 and 81 inches, (ideal is allegedly 72-74″) which equates to 0.0012″ – 0.0007″ height difference between the centre of the follower and points 0.70″ apart. Not easy to measure accurately, but not impossible to measure with the right equipment.

Let’s make a thing!


The block was drilled and tapped on the bench drill, using the drill vise, so the holes are parallel and in line. The follower specs call for measuring face drop at 0.700″ diameter, so that’s how far apart the threaded holes are.

Next we screw the pointed probes into the block with a bit of thread lock to keep them aligned, making sure they’re exactly the same length.



And there you have it. A DIY spherometer.


Using it is a little tricky- to get repeatable results, you need to make sure the gauge is perpendicular to the surface being measured- I just held it against a square. Zero the gauge on a piece of mirror:


…and start measuring.


I measured each follower twice at right angles, then checked that the gauge was still zeroing on the mirror surface. I measured the entire set twice too, so each follower was measured four times. The measurements and zeroing were absolutely repeatable (except for the follower which had its face ground non-perpendicular to the sides. Seriously, WTF?) – and repeatability is SCIENCE.

Just for reassurance, I measured a bunch of cylindrical and spherical things of known radius- and it turns out my little gauge is eerily accurate. I didn’t have anything as large as 80″ to measure, but it was spot on at 30″.

Now is the time on Sprockets when we math:


Easy peasy. I won’t subject you to the math, but the results were that 2 of our followers were about right at c.70″, three were a bit low at around 85″, two were at c.114″, and one was around 150″.

The reason I wanted to measure the followers is because it’ll cost $20 to post them back to the UK, and the supplier has a vested interest in saying “they’re fine, nothing wrong with them”. But since they’re definitely not right, I’m a bit more confident about spending the money now.

*It’s not what you ask, but who you ask. Though I’m not going to name any names, just to be on the safe side.

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