CAD/CAM blocks and discs: the sintering factor is where accuracy is lost

CAD/CAM machining is often described as inherently accurate, in contrast to casting techniques. In reality it introduces errors of its own — different from the traditional ones, but not negligible.

The first limit is geometric and concerns bur diameter. A 1-millimetre bur cannot reproduce an internal angle with a radius below half a millimetre: that detail is rounded off, however precise the file.

This is why preparations with sharp angles are reproduced with rounded radii, and why a chamfer mills better than a ninety-degree shoulder. Preparation geometry should take account of it.

The second limit concerns zirconia and its sintering shrinkage. Discs are milled in the pre-sintered state, far softer, and then fired: during firing the material contracts by roughly 20 to 25 per cent.

The software compensates by multiplying dimensions by a factor declared by the manufacturer and printed on every disc. If that value is inaccurate, or if the furnace fails to reach the programmed temperature, the restoration comes out uniformly off-size.

A crown that will not seat, or that rocks around its whole perimeter without an identifiable point of interference, almost always indicates a sintering-factor or furnace-calibration problem, not a milling one.

The distinction between wet and dry machining is not organisational but material-dependent. Glass ceramics and composites are milled wet because heat would damage them; pre-sintered zirconia is machined dry because water would be absorbed by the porous material.

Milling zirconia wet means having to dry it thoroughly before firing, and incomplete drying produces cracks during sintering.

Among materials, lithium disilicate is milled in the pre-crystallised state, recognisable by its violet colour, and develops its mechanical properties only after heat treatment. Milling and cementing it without crystallisation yields a brittle restoration.

Millable composites require no firing and are immediately usable, with the advantage of intraoral repairability. They suit long-term provisionals and trial rehabilitations.

Bur wear is the most neglected laboratory variable. A worn bur produces imprecise margins and rough surfaces, and the system does not always flag it. Usage counters exist precisely because wear is not visible to the eye.

Milling strategy affects both time and quality. A fine pass over the entire surface improves finish but triples the duration; the sensible approach uses rapid passes on external surfaces and fine passes only at the margin.

In summary: bur diameter limits reproducible detail, the sintering factor determines zirconia fit, and bur wear is the commonest cause of imprecise margins. CAD/CAM accuracy is real but not automatic.