(Micro) convexing from pasted MDF

[Mr.Wizard]

An amateur clockmaker found that a pasted MDF lap produced an insufficiently flat result when polishing steel hardware, and changed to tin. I thought it relevant for those seeking ultimate geometric precision. Start at 7 minutes 30 seconds in.

 

[ToddS]

Just like a with a strop/blade, micro-convexity is caused by the loose fibers on the surface, not from the compressibility of the substrate.

 

[Mr.Wizard]

Nice to have a reply from you! Is this (video) correctly what you would call micro-convexity, or is that something that would not be seen by the unaided eye?

 

[Obsessed with Edges]

Also noticing the part was spinning under driven power against the MDF wheel (at 8:00 into the video). If done by hand, the give of the MDF would've been less of an issue. At driven speeds, most any metal is going to make the MDF deform out of flat, as it's very soft against any driven tool.

By hand, MDF works well as a stropping substrate for flat-bevelled knife edges or tools like chisels, so long as the edge itself isn't allowed to dig into it under pressure and by which the MDF would be gouged or otherwise deformed. My favorite finisher for chisel edges is an MDF block with green compound applied (dry compound suspended in mineral oil and 'painted' onto the MDF).

 

[ToddS]

Nice to have a reply from you! Is this (video) correctly what you would call micro-convexity, or is that something that would not be seen by the unaided eye?

Yes, you easily can see very small variations in flatness in a mirror surface by looking at reflections of distant straight lines (like a ceiling grid) or point sources of light.
Micro-convexity of a blade bevel is typically less than one micron "out of flat."

 

[HeavyHanded]

For clockwork precision you need metal, stone, or glass. Machine shops don't do finish passes on MDF.

A nice test is to use a micrometer and give a light press on the dial. You'd be surprised how much the thickness can vary depending on what torque the ratchet is set.