CNC machining ceramics: what works, what to avoid and what it costs

Yes, you can CNC machine ceramics, but how depends on the material and on whether it has been fired. Machinable ceramics like Macor and boron nitride cut with carbide tools on a normal mill or lathe. Technical ceramics like alumina and zirconia are machined before firing, in the soft green or bisque state, and made about 20% oversize to allow for shrinkage. Once they are fully fired, they are too hard for carbide and get finished with diamond grinding, which is where most of the cost is.

Below is how each case works, with figures from the manufacturers and the trade press. Every number links to its source.

Machinable ceramics: cut them with carbide

A small group of ceramics are made to be machined as they are. Macor, a glass-ceramic of about 55% mica and 45% borosilicate glass, "can be machined with ordinary metal working tools" and needs no firing afterwards. Shapal Hi-M Soft, an aluminium nitride composite, is sold as machinable to precise tolerances on standard equipment without diamond tooling. Hot-pressed boron nitride is in the same group.

For these, American Machinist says solid carbide is your best bet. Goodfellow's Macor guide recommends carbide and advises against ceramic-tipped tools.

Here are Boedeker's published Macor parameters:

Operation	Speed	Feed	Depth of cut
Turning	30–50 sfm	.002–.005 ipr	.150–.250 in
Milling	20–35 sfm	.002 ipt	.150–.200 in
Drilling 1/4 in	300 rpm	.005 ipr
Drilling 1/2 in	250 rpm	.007 ipr
Drilling 1 in	100 rpm	.012 ipr

Sources don't fully agree. Accuratus gives turning speeds of 30 to 150 sfm with carbide. Start at the low end and work up while you watch the edges.

Technical ceramics: machine them green

Alumina, zirconia, silicon nitride and silicon carbide are a different story once fired. Kyocera puts alumina at nearly three times the hardness of stainless steel, and silicon carbide at more than four. So the shaping is done earlier, while the part is still pressed powder (green) or lightly fired (bisque). Morgan Advanced Materials says it is preferable to machine a part before it has been fired, because hard grinding afterwards is slow.

The catch is shrinkage. Parts shrink about 20% during sintering, and International Syalons gives 20 to 25% for advanced ceramics in general. So the green part is programmed oversize by a scale factor. Dental labs show how exact this gets. Glidewell prints a measured scale factor on every zirconia blank, and the CAM software uses it to mill the part larger.

Green machining is also cheaper. Syalons calls it much cheaper than diamond grinding, and CoorsTek's design guide says that maximizing green machining lowers unit cost.

Fired ceramics: diamond grinding

Features that need tight tolerances after firing get ground with diamond. Superior Technical Ceramics notes that most of the cost in ceramic manufacturing comes from diamond grinding after firing. The shop profiled in Modern Machine Shop also uses PCD and PCBN tools, slows the feed to about 40 ipm on zirconia and needs a much finer grit (above 220).

Keep the cuts shallow. A Ceramic Industry guide to diamond wheels gives about 0.003 in per pass as the maximum for an 80-grit wheel and 0.00035 in for 320 grit. Ultrasonic-assisted machining, which adds roughly 20 kHz of vibration to a diamond tool, cuts drilling forces and reduces edge chipping.

Tolerances you can expect

State	Typical tolerance	Source
As fired	±1% or ±0.005 in, whichever is larger	Morgan, STC
Diamond ground	about ±0.0005 in	STC
Critical ground features	down to ±0.0002 in	Modern Machine Shop
Macor, machined	about ±0.0005 in	Morgan

Shop tips that save parts

Ceramics break out where the tool exits. Leave material there. Boedeker recommends at least .050 in on the back side for breakout. Goodfellow suggests a backing plate or chamfers at the hole entry and exit, and climb milling so the cut doesn't pull material off the edge. Put chamfers or radii on edges in the design itself. Superior Technical Ceramics notes that sharp edges tend to chip.

Clamp gently and evenly. Macor is not resilient, so Goodfellow suggests soft jaws, and magnetic chucks don't work on ceramics at all. Run wet with water-soluble coolant, except on boron nitride, which some shops cut dry because oils can contaminate it. Look after the machine too. Ceramic swarf is abrasive and can damage the machine, so keep it clean and use a settling tank for recirculated coolant.

Quoting ceramic work

The drawing decides most of the price. Every surface with a tolerance tighter than the as-fired ±1% means diamond grinding after firing, and that is the expensive part. When you quote, count those features first, ask whether the customer really needs them, and plan the rest for green machining. If you want to see how we turn a model and a drawing into a plan and a price, send us a part.

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