Turning Ceramic Material on a Lathe

I have been attempting to turn down the diameter of some small ceramic magnets. I have been using my regular carbide inserts and have not been
having much luck. I am flooding the magnet with water, and my lathe is running at its highest rpms. I am only taking off about 1 to 2 thousands at a time and turning my carriage very very slowly. I have successfully managed to turn down 2 magnets so far, but have failed at about 8. Basically, the magnets just crumble if I accidentally take off to much, or twitch with my hand etc....
Is there a specific insert designed for turning ceramic, or are there any special techniques that I haven't mentioned? Is there a recommended insert radius for ceramic?
On my cnc mill, I designed a simple jig to securely hold the magnet. Using a diamond coated 3/16" 4 flute end mill spinning at about 8000rpm with coolant, I am able to machine down the thickness with no problems. I then use a ceramic 1/4" drill bit to drill a hole down the center, again with no problems. But when it comes to turning down the diameter I just can't do it with the tools I have.
Thanks for any info. I am hoping to succeed in this final machining operation.
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You probably answered your own question. Unless you have a "machinable" ceramic like Macor or lava you'll need diamond tooling. I believe diamond tipped inserts are available. Randy
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The company we purchase them from can machine them, but the lead times are very long and we just can't wait. They did say we should use diamond tooling, but I have not seen any diamond coated turning inserts. I will have to look a bit more.
Thanks
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Look them up on Google. They've been around for some years now. I was writing articles about them back in the '90s.
-- Ed Huntress
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How about some diamond coated (or similar) sand paper mounted to the back of one of my turning tools. The magnet is only 2mm thick.
Not sure what grit would be best though. Maybe a 220 or 400? Then I could flood that with water, and change it frequently. Just a thought.
Ed: What radius insert would you recommend for turning a ceramic magnet that is 2mm thick. I am going from 12.5mm dia. down to 10mm dia.
Thanks
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Got a toolpost grinder? Carbide or diamond grit burr/stone?
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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Terry wrote:

In spite of the fact that I don't know what I'm talking about, the phrase "tool post grinder" keeps bubbling up to the top of my head -- the sandpaper will just make a set of grooves unless it's moving.
McMaster has diamond-tipped lathe bits, page 2370.
--

Tim Wescott
Wescott Design Services
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I don't know of any such sandpaper (it may be around, I've just not heard of it), but how much material do you have to remove?

First, that kind of unusual-but-practical information is not something I would know from experience. My way of finding out such things is to look it up or to go directly to the experts and ask them. This is 'way outside of my knowledge.
Second, if you have to remove 2.5 mm of hard material, I think the sandpaper idea will become a career <g>, but you'd better get some other opinions.
Third, although I have no specific info, here are a couple of general ideas. Sintered ceramics usually are ground, not single-point turned. When they machine engineering ceramics in production it's usually done with green (unsintered) ceramics, or with glassy ceramics that are made to be machined (Macor, for example).
I don't know anything about the machining properties of magnet ceramics. In general, again, you turn or mill friable materials with the smallest cutting edge and the slightest feedrate you can -- a sharp tool, in other words.
But ceramics vary widely in their properties, and this is one you could only learn from experience, unless you trip across someone else's account of how it's done -- from experience.
Good luck. Probably it's doable, somehow. I just don't know where to send you for information, beyond the makers of the material itself.
-- Ed Huntress
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learn from experience, unless you trip across someone else's account of how it's done -- from experience.<<
When I worked for Dow they brought in a ceramic they wanted me to try and drill with the ultrasonic drill we used on glass. After 2 days drilling and wearing out 4 bits I was able to make a 3/16" wide dent about 1/8" deep in it.
This stuff had a Rockwell rating of 97 for hardness!
Randy Hansen SC Glass Tech. Scam Diego, Comi-fornia
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how
and
Some of the engineering ceramics are unbelievable.
Back when GE had a division working on making injection-molded, silicon-nitride turbocharger turbines (around 1980), they sent us editors a sample of the material, which was a stick around 4" long, 1/4" wide, and a little more than 3/16" thick. Silicon nitride is as hard as the hammers of hell and I expected it to be brittle. So I closed the door to my office, propped the stick up against the bottom of the door at a 45-deg. angle, and stomped on it with the heel of my shoe.
It dug a 1/2" gouge in my door. But it didn't break the first time. I really had to hit it very hard before it broke.
-- Ed Huntress
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...
really
The funny thing about this story is Si3N4 isn't really all that strong - between porcelain and steel for reaction bonded stuff. http://www.matweb.com/search/SpecificMaterial.asp?bassnum=CM0019 Some of the tougher processes, HIP for instance, get up there though.
I don't suppose your sample was monocrystalline?
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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No. Self-sintered. Maybe HIPped, I don't know. Tough as hell.
-- Ed Huntress
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Whoops, I should point out something else: The powder that will get in the air from either grinding or turning (amount unknown) could turn out to be VERY nasty, even highly toxic, stuff. You'd better do some checking. A lot of exotic materials that are harmless as solid pieces turn into skull-and-crossbones stuff when they're turned into fine particulates.
-- Ed Huntress
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It's my experience that magnets really like to stick together. I haven't ground any so can't say if the average grinder would throw the dust hard enough though.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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from http://www.wondermagnet.com/magfaq.html
Can I cut, drill or machine magnets to my own sizes and shapes?
Yes and no. AlNiCo magnets are very easy to machine in any way you wish. NdFeB magnets are by nature very hard and brittle. Although they can be cut, drilled and machined, it should ONLY be done by folks who are experienced with ceramics. If the magnets get over about 300 deg F, they will lose their magnetism permanently. They are flammable, and it is not difficult while grinding or machining to get them (or the chips and dusts from cutting) so hot hot they ignite. If they do ignite, the fumes are toxic and the material burns very fast and hot, like Magnesium! In our experience any machining of these magnets should be done with diamond tools under lots of coolant with good ventilation and the risk of fire in mind.

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I purchased the diamond tip turning bit from McMaster that Tim found. I will be here today. I'll give it another try. I am using lots of coolant. The tool and magnet never even get warm. As far as the dust. I would imagine the magnet would keep it down, as long as the coolant. But I will be more cautious of it.
Randy, not quite sure how I would use a diamond griding wheel on my lathe to turn down the diameter. I need to be accurate to a couple of thousands.
I tried machining some of those AlNiCo magnets last week. I think they were the AlNiCo 8 ones which had around 5% cobalt in them. That was a challenge. I wore out a solid cobalt drill bit, which barely put a dent in the magnet and a 3/16" solid carbide endmill which rounded the corners off only after a coulpe of passes.
As most can probably tell by now, I am pretty new to machining these things. But I am sure learning alot!
Thanks.
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One thing you may now learn is that diamond-coated tools, or those tipped with a polycrystalline diamond compact, are not usually sharp unless you specificy that you want them sharp.
Anyhow, let us know how it works.
-- Ed Huntress
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Yes, I read that somewhere. I'll just have to give it a try.
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I received the diamond tip turning tool yesterday, and it worked great. I was able to turn down the diameter of a couple of magnets with a perfect finish and no chipping.
Thanks for all the suggestions and help.
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I'm glad that worked out for you, Terry. Unless the ceramic gives the diamond a major workout, it should last you a long time. I have one I found 7 or 8 years ago, unused and lying in a trash can (no kidding), that I pull out when I have to turn fiberglass and carbon-fiber composites, mostly ferrule pieces for flyrods. It's great for that and it shows no signs of wear, even under high magnification. At this rate, it will last the rest of my life.
-- Ed Huntress
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