Drilling Ceramic

I need to drill an array of holes in a ceramic (Macor) plate. Each hole will be drilled from both sides, not breaking through either side. The holes are differing diameter.
Should I drill the larger diameter first, and then break through the bottom of the hole from the other side with the smaller diameter? Or should I drill the smaller diameter first, followed by the larger diameter?
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I'm assuming that the remaining material is going to be fairly thin. In this case you should drill the large holes first. Removing less material on the second pass should help prevent the remaining material from cracking.
Dwayne
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Dwayne wrote:

No, it's a quarter inch thick.

I'm not sure you understand. Each hole is drilled twice, once from each side. The resulting hole goes all the way through. The holes drilled from one side are larger diameter than the holes drilled from the other side. Which holes should be drilled first?
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I'd practice on a piece of Al or MDF to check my alignment first, but here is how I would try it. I'd drill the small hole most of the way through first. Then on the same side, drill the large hole to depth. Next flip the piece, align and drill the small holes. In other words , I'd drill the big hole first then the small hole.
Dwayne
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I would consider drilling the small hole first. It needn't be all the way through, just far enough that it will be into the volume of the big hole. And then drill the big hole from the other side. Don't break through anything.
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"Gregory L. Hansen" wrote:

That's what I've been thinking. If I were to drill the big hole first, the small hole would break through the bottom of the big hole and might chip. Chips at the bottom of the big hole might not be a problem, depending on how big they were or if they propagated up the sidewall.
Drilling the small hole first means there won't be any surface material around the region when the second hole meets the first hole.
I asked in this newsgroup, in case there was some factor I had overlooked. So far, I haven't heard anything to dissuade me from drilling the small holes first.
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Bear in mind that I have no experience drilling Macor. It just seemed a sensible way to do it.
I think the best advice was to get a spare peice and practice.
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Gregory L. Hansen wrote:

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I didn't see the first few articles so I don't know all about your requirement. What used to work for me was this. I drilled holes in thin ceramic using a sand blaster. You mask the ceramic with a rubber sheet except for the hole you want to make. Then sand blast away with a fine grit. The grit grinds away at the ceramic but just bounces off the rubber. The hole with not be a perfect cylinder, but it will be a hole and may server your needs.
Al
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Al wrote:

Thanks for the suggestion, but the plate is 1/4 inch thick, and the holes will be about 0.150 inch in diameter for most of their length. And, I do care about the sidewall being as close to vertical as possible.
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Well, you can make the walls somewhatg vertical, but the rim at the input side will be rounded a bit. And the hole will be wider at the input side than at the output side. You might try coming in from both sides. Then you will have slightly tapering holes, wider at the top than at the bottom. You will have to start with a smaller mask than the final size of the hole.
Part of the problem is that you have a turbulent flow in the hole with particles and air both entering and leaving. This can cause a problem with such a form factor. You might try pulsing the airflow so that the ingress and egress of material will alternate.
You'll have to experiment. It does seem like you have a challange here.
Can't you have the ceramic plates made with the holes already in them?
Al
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Al wrote:

This seems like a lot more trouble than drilling. The plate is Macor, which is formulated to be machinable, but nevertheless is subject to chipping and cracking if not handle right.

It should be completely do-able with Macor, but I want to maximize the chance of success on the first attempt. If it fails, the first piece will be the practice run, and I'll have to spend another $100 for another piece of Macor.

This is for a prototype. Only one will be made. It would probably be more expensive to cast a custom ceramic plate than to drill a Macor plate. Also, the precision of the drilled Macor plate will be higher. And, the crack resistance of Macor (as compared to other ceramics) is a plus.
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Why not drill both holes from the same side? Drill the large hole to desired depth, then replace with the smaller drill bit and continue drilling all the way through. You would save a ton of time.
Mark
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redbelly wrote:

Macor chips really badly when you do that.
Cheers,
Phil Hobbs
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