ot: How flat is a CD?

[dave]

Or a hard disc platter? Just wondered. (Google so far finding flatness measuring kit - not specific for DVD or disc platter).

[Andy Champ]

Very flat. I'll be surprised if you can measure it.

The pit depth on a CD is IIRC ~0.2uM and over shortish area the disc has to be _a lot_ flatter than that. It ought to be flat over the whole thing, but the drive can track a bit.

I think hard discs are better than CDs.

Andy

[pcb1962]

CDs are not terribly flat. I've just measured a difference of 0.08mm (about 3 thou) between various points on the surface of a CD, using the 'flatness measuring kit' here

'd expect the platters of a hard disk to be considerably flatter.

[Brian L Johnson]

Yes, if you break apart a hard disc unit and play with the platters inside themselves, you'll find that they are VERY smooth. Put 2 platters together and press, and you can't separate them any other way than sliding them apart.

[Andy Champ]

I'd expect the platters of a hard disk to be considerably flatter.

Well that's a DVD, not a CD...

...but I'd expect that to be flatter, if anything. Do you move the disc or the device? What's the substrate? Which direction are the lumps and bumps?

I'm honestly surprised. I assume you aren't talking about the ring near the middle?

Andy

[pcb1962]

You're absolutely right, it's also a recordable one, so the plastic may be softer and more prone to distortion than a factory pressed item. I'll try the same test with a proper CD tomorrow.

I moved both, the result was the same. The substrate is an inspection grade granite surface table, unmeasurable deflection of the DTI across the whole surface. The high and low spots on the DVD were at random points across the surface, no discernible pattern.

Only the data area.

[Andy Dingley]

CDs don't need to be flat - they're often significantly conical. Circumferential variation would be bad for focus, but drives cope fine with a gradual refocussing from track to track.

HD platters are good, but a bit small.

Granite chopping boards are better than you'd expect (take the varnish off though). Good enough for a model engineer's surface plate, stable and cheap too.