Machining parameters for bisque alumina

Hi all,

I'm working on a project where I'm spending a lot of time machining bisque-fired alumina. For reference, bisque-fired alumina is aluminum oxide (Al2O3) powder plus some additives which has been pressed into a form and partially sintered. The partial sintering produces small, relatively weak bond areas between adjacent alumina particles, which break easily. This enables this material to be machined using conventional milling/turning tools instead of requiring grinding. It has the consistency of very hard chalk. I get my material from McMaster (PN 8484K61, for example), and they get it from a company called Superior Technical Ceramics. It can be purchased from other companies, including Aremco and Cotronics.

I'm posting today because I'd like to find out if anybody here has definitive recommendations for feeds and speeds for machining this material. I've been working with it for quite some time, and have feeds/speeds that work...kinda. However, I go through tooling at an alarming rate, and I'd like to improve this.

Here's what I have so far:

• Tooling: I know to use diamond-coated carbide tooling with bisque ceramic, since it's super abrasive. Recently, I was advised to use CVD coated tooling, since it's apparently a thicker coating that leaves a larger edge radius, meaning it will hold up for longer.
• Feeds & Speeds: The resources I've been able to find on machining bisque alumina are 1) Aremco's machinable ceramics recommendations (largely useless - I'm pretty sure it's just Corning's recommendations for Macor repackaged), and 2) an extensive guide from a company called sp3(very instructive - but targeted at green ceramic instead of bisque).
• Other advice: I have asked STC for recommendations. They are unwilling to give me any pointers, beyond saying that "...[their] speeds and feeds are somewhat lower (~30%) compared to [their] standard settings (for green ceramic)." This has been anecdotally confirmed to me elsewhere, with the explanation that it's better to be "digging" material out with the endmill, rather than "grinding" it away.
• Finally, some other things I've learned while working on this:
  • Fixturing is critical to avoiding breaking parts. For loosely-toleranced operations, I'll put strips of electrical tape on my vise jaws, to give a little bit of cushion between the jaws & the parts.
  • Starting with flat stock is also very important. Bisque alumina has low tensile strength, so if you put a blank that's even slightly warped in a vise, it will likely snap it.
  • Sharp tooling is just as - if not more - important than fixturing. I go through a lot of tooling
  • All of the normal machinable ceramic rules about material chipping out (avoiding drilling all the way through a part; adopting unusual milling strategies to avoid ever cutting through an edge from the inside) are relevant. I've recently had some luck with using double-angle drill bits designed for composites to drill through holes without substantial chipout (more work required to confirm)

Anyway - if anybody here has specific experience working with this material, and can help me out with optimizing machining parameters, I'd very much appreciate them. Thanks!

<bump> Any thoughts? Also, if there's some other subforum that this would be better posted in, please let me know. Thanks!

I can't help but I have a request to machine some Ultra-High-Temperature Alumina Ceramic. Search on McMaster for it. The parts will be turned. Apparently, this material is alumina that is porous enough to allow machining with carbide. (Apparently, pure alumina can only be diamond ground)

Have you any experience with Ultra-High-Temperature Alumina Ceramic?

Can you examine the edges of your worn tools under mag and report back as to what the failure mode is? Tools with flank wear will require a different solution than tools with edge chipping.

You will want to confirm failure mode of the cutting edge, but given your description of the material I'd suggest a PCD tool for this application. It will provide a harder tool surface, which would help with tool wear. DLC (what you mention is currently in use) is good, but PCD should provide a longer-lasting edge.

I'd also suggest shooting a message to Carbide Bob. He might have some good input here.

Hi all,

@laminar-flow: The material you're talking about is interesting - I've played with it before, but have not machined it. It's really more of an insulation than it is a bulk ceramic (like the bisque I mentioned earlier); has a distinct layered structure; and is much softer at a macro scale than bisque, probably more like green alumina. My guess is that you will have no trouble machining it with conventional carbide tooling, or even HSS, for small lot sizes. However, it will wreck your tools quickly because the dust is so abrasive, and if you're going to be doing a lot of these parts, go with a diamond-coated bit (I agree with Register that PCD is substantially preferable to DLC) + airflow to remove the dust.

Other notes:
- In addition to ruining your tools, the abrasive dust will ruin your machine over the long term. Proper air management (not only blowing, but vacuum) will help with this.
- We do all of our cutting without any sort of coolant. There are coolants available for machinable ceramics, but we haven't investigated them. I suspect you will want to do the same for your material, since it is much more porous than the bisque I work with. One possible advantage to coolant, though, is dust mitigation - as long as you don't let the dust-soaked slurry get down into your machine's bearings.
- The way I've been told to think about working with machinable ceramics is that it's like shoveling gravel, more than it's like slicing wood. You are using the edge of the tool to fracture the material along whatever its weak plane is (weak sintered bonds in bisque ceramic, glassy binder in Macor). You generally want to run slower speeds and higher feeds than you would expect, so that the tool is always "digging" material out, rather than "grinding" against the cut face. This is obviously super qualitative - if someone has a better cutting-science-supported explanation, please chime in.

@register: Thanks for your suggestions. I actually switched to PCD tools wherever possible a while ago, and it's made a real difference. Failure mode previously was almost always flank and corner wear --> higher cutting forces --> parts splitting during machining. After the results I mentioned previously with double-angle drills, I had Harvey Tool make me up a batch with PCD coatings, which have worked fabulously. Interestingly, Harvey released these as a stock item about one month later...

Thanks all for the help! Any other thoughts about machining bisque alumina/other machinable ceramics appreciated.

Edit: Realized I misstated what coatings I was using earlier. I switched to CVD diamond coatings (D2 coatings at RoundTool, CVD at Harvey), and they've worked great. I haven't tried actual PCD coatings, but would be interested to check them out.

This isn't exactly what you're doing but might be an interesting approach.

How One Shop Machines Advanced Ceramics

Possibly they can be roughed in with cheaper tooling and finished with the diamond grinding tools.

Aside from that one of the more economical ways to increase tool life is to increase flute count. Theoretically going from a 4 flute to a 5 flute should net you a 25% increase in tool life.

I don't know how much this relates to your work, but I had to cut grooves in Lavite, an uncured volcanic material you can machine and then fire. Using ,020" slitting saws taking a conventional cut, their life was very short. I found that if I used a climb cut the teeth would wear but in a manner that was self sharpening, putting clearance on the teeth and making a sharp point. Apparently the teeth rubbed on the material at an angle determined by the speed and feed that in effect honed the ends of the teeth.

Bill