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  1. #1
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    Material Advice please

    I've designed some jigs that hold optical components while they are being coated. I've had them manufactured from Stainless Steel, but they are very heavy and the machining takes ages.

    Is there any other material I could use instead of SS please? Aluminium is out due to the high temperatures involved as is plastic and wood. I'm thinking about some sort of ceramic but am not too up on this at all.

    The parts are about 10 inch square plate, 0.25 inch thick and have a big hole about 8" diameter in the centre. The diameter is not whole- there are one or two straight sections off it (cutting down the volume of the circle. The circle thickness (0.25") is angled at 60 degrees and the straight edge at 45 degrees.

    There are about 8 smaller 4mm dia holes round the big circle and four M5 screw holes at each corner of the square.

    The whole thing operates at about 300 degrees celcius and very high vacuum so it needs to be about the same thermal expansion co-efficient as glass although I've designed some "give" for this.

    Any ideas appreciated- I have searched for days for a suitable material to no avail... can you machine ceramic like this? can you put a threaded M5 hole in ceramic?

    Thanks in advance.

    PS- it needs 4 locating pins too which at the moment I get push fitted and spot wleded in place. It looks like this:
    Attached Thumbnails Attached Thumbnails Jig.jpg  
    I love deadlines- I like the whooshing sound they make as they fly by.

  2. #2
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    Did you do a search for machinable ceramics? I used them decades ago for making bushings and supports for use at high temperatures.

    I think the name machinable ceramic is a bit of a misnomer because they have to be fired after machining to bake them to the final ceramic form. The machinable form is a sort of pre-ceramic.

    One thing you might want to consider is the possibility of out-gassing from a ceramic. Most ceramics have some porosity and I think the machinable ones are based on sintering and they may have even more porosity than normal ceramics.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  3. #3
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    Many thanks Geof, you came up trumps yet again. I found some stuff called "Shapal" which is a machinable ceramic as you suggested. It's got a low co- effiecient of expansion and looks perfect (I can get round not using M5 screw holes).

    My only concern (as you rightly pointed out) was out- gassing which is a nightmare for optical coating plants, but they say that this material is used for vacuum deposition crucibles (which we use in the coating chambers), so if it's good enough for that it'll be good enough for my jigs.

    We've coated using the stainless jigs and it seemed to be ok, apart from a couple of lenses chipping due to the difference in expansion co- efficients between glass and SS, but we're still at the prototype stage.

    I'll ask the company to send me a sample to see how brittle it is and will redesign for the production batches if it's acceptable.

    Many thanks again Geof.

    Iain.
    I love deadlines- I like the whooshing sound they make as they fly by.

  4. #4
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    what about using titanium if weight is an issue? It will hold up better than Al.

  5. #5
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    Update: I got a sample bit of shapal, it seems to machine ok and I did some impact tests on it- it's not brittle (ie will stand up to rough handling).

    Then I looked at the price.

    My jigs are around 7 inch square. A bit of shapal around 2 inch square and 0.4" thick weighs in at about 400 quid *gulp* anything larger than this is "Price on Application", which we all know means giving the company accountant a heart attack lol!
    I love deadlines- I like the whooshing sound they make as they fly by.

  6. #6
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    What about leaving the material stainless and having the shape lazer cut then just do finish machining. We do it all the time at my shop and it doesn't cost that much per pc. to have done.

  7. #7
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    Quote Originally Posted by ImanCarrot View Post
    Update: I got a sample bit of shapal, it seems to machine ok and I did some impact tests on it- it's not brittle (ie will stand up to rough handling).

    Then I looked at the price.

    My jigs are around 7 inch square. A bit of shapal around 2 inch square and 0.4" thick weighs in at about 400 quid *gulp* anything larger than this is "Price on Application", which we all know means giving the company accountant a heart attack lol!
    When I worked with machinable ceramics it was in a govt lab back in the 1970's. You must know what that means?
    An open mind is a virtue...so long as all the common sense has not leaked out.

  8. #8
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    lol yep, I was a Civil Servant scientist myself, working at a massive Govt research site... money was no object. The weird thing was we could get funding to buy anything, but hiring people was always a problem.
    I love deadlines- I like the whooshing sound they make as they fly by.

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