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Turning Mykroy/Mycalex (machinable ceramic) rod: rod keeps cracking when boring.

teletech

Aluminum
Joined
Jul 16, 2019
Material from MMC, billed as "Easy-to-Machine Glass-Mica Ceramic"

I've been asked to bore a 1/4" diameter thru hole in a 3/8"diameter rod. It only needs to be about 1/2" long.
I did read up in the forums and ran a lot of pecks, low feed force, and cleaned out the swarf frequently.
I did manage to drill the hole fine, but the part cracked lengthwise.
I tried different speeds, step-drilling, drilling vs. boring, etc. with pretty much the same result. I've been using a normal steel collet but wondering if I'm over-clamping or if I should just use a brass or plastic collet.
I did clamp it pretty hard at first, so wondering if I fractured the whole length of rod at first and my subsequent issues were from that.
I've been using carbide tooling, it's what I have and it's hard to justify diamond for one part since I don't even know if it's a tooling issue or clamping issue.
Also, I'd welcome recommendations about drilling and then parting off or cutting to length first then drilling.

thanks folks, this one has me frustrated.
 
I've never tried to drill machinable ceramics so right up front I will make that known. Based on the properties of most ceramics - they break under tensile or shear forces, not compressive - I would be inclined to try and use a plastic or conformal rubber collet/bushing. My goal would be to apply uniform compressive forces on the outside of the cylinder to "preload it" against the expansive forces from drilling. A hard ceramic cylinder that may not be perfect in shape will have high spots that a hard collet will press on with uneven force.
 
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I made up a run of replacement "stove top ignitors" many years back,

I didn't have good luck until I used the 5c collet to hold the piece parts.
Then it was easy. IIRC the trade name was Mycor, but it may all be the same stuff.
Swarf came off as powder, I did not use any coolant, but did use a vacuum.
The drill bit was dubbed off just a couple thou. two passes with a pocket stone.
 
I have never worked with this stuff but if the problem is clamping? related. I would try heat shrink tubing over the rod before clamping. Heat shrink tubing is avialable in different wall thicknesses & can also be had with an adhesive lining.
Good luck
Doug
 
I have never worked with this stuff but if the problem is clamping? related. I would try heat shrink tubing over the rod before clamping. Heat shrink tubing is avialable in different wall thicknesses & can also be had with an adhesive lining.
Good luck
Doug

I like the idea.

I have plenty of heatshrink tubing around.I'll give it a go. Sadly I need to get right in the middle of this rod. Perhaps I should start with .5" and turn the O.D. too so I don't have to worry about being a little off from springy clamping.

I should have mentioned I did use coolant.
 
I like the idea.

I have plenty of heatshrink tubing around.I'll give it a go. Sadly I need to get right in the middle of this rod. Perhaps I should start with .5" and turn the O.D. too so I don't have to worry about being a little off from springy clamping.

I should have mentioned I did use coolant.

You could consider paper as the wraper. Controlled length to fit your collet size. Count wraps to control diameter, then cut up a few strips. I'm sure paper could be reused.

Also, I used a pretty slow speed of revolution. Likely around 200 rpm. The waste crumbled, there is no real cutting action.

I wasn't in a hurry, the job paid well, those little igniter nibs are spendy!
 
I have done a little work with both Macor and Mycalex, and generally, you are using the correct concepts; low feed rate, coolant, clearing the drill with peck drilling. CalG mentioned one point I would stress, which is to dub off the drill edge, so you have "scraping" at the drill edge rather than "scooping". This will help to eliminate cracking at the exit end of the hole (assuming you are drilling a net shape part). If you are drilling solid rod then parting off, this may have less applicability, although an aggressive drill helix angle may still create some internal fracturing leading to failure of a thin-walled part such as yours.

As others have noted, slightly compensating for any out-of-round condition with more forgiving workhoolding may be a better approach, if the part is shown to have high spots on its diameter. The collet would have certainly been my go-to approach for the task.

If concentricity is a very close-tolerance concern, I would drill the oversize rod, then put it on some sort of mandrel fixture turned on the lathe and dress the OD for concentricity control.

Chipping and crack propagation are always the issues when making parts from brittle materials, and this is a poster child for those.
 








 
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