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  1. #1
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    Default Threads in fixtures?

    I’m working up some semi-permanent fixture plates for rotaries and similar. My plan was to make them out of 2” thick 7075, hard anodized. I am looking for some advice about the best way to hold down things to the fixture plates. I think I am overthinking it.

    My first idea was to just spot drill, drill, and tap M12 after the fixture is back from anodizing. I have heard that anodized threads can be brittle and not hold well. But then I started worrying about the thread strength.

    My next idea was to put in a Helicoil, either before or after anodizing. I am not sure this would buy me much. If I do it before anodizing I have to find an oversize STI tap, if I do it after I need some zinc chromate primer?

    Thanks for any tips here!

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    Install any Helicoils in Al AFTER anodize, doing beforehand requires careful masking to prevent attack on the stainless steel HC.

    I'd go with tapping the holes M12 STI, then have these holes masked (if they leak a bit it's not as critical, they can be chased if needed), then install the HC's in the holes as-is. Unless your coolants are corrosive or otherwise have issues, you shouldn't really have too much corrosion between the stainless and the Al.

    If there is a concern over coolant chemistry, check out how the aircraft industry handles HC inserts in Al and copy them.

    There's a type of HC that's much lower galling (Nitronic 60), I'd use these for your application. Expect to pay through the nose, err, wallet, but if these are long-term fixtures it's worth it.

    https://www.stanleyengineeredfasteni...tings-platings

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    Depending on your fixture sizes, M12 is big.
    M10 is the usual size on a 200mm dia size rotary/faceplate?
    I would ensure there's enough additional space around your chosen (M12) size that if thread strips, you can then go to something like a timesert (solid insert).

    Ref your question - I'd drill WTI core size and csk (first thread) but not tap.
    Get it anodised.
    Then tap after.
    Install stainless steel insert with WD40 or similar, and call it a day.
    I wouldn't worry about galvanics.

    FWIW, for mass production fixtures, I also allow to go larger, so when the timesert is fubarred, we can make a larger "grub screw" tapped through with the clamping thread...

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    Don't put "thread" holes in before anodize. A good spot drill goes right thru the hard layer. Drill and tap AFTER anodize, then install thread inserts. There are several types. Stay away from HC's. We all have experience with them working just fine until WHAM ... one messes up.

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    I have found helicoils do not hold up well in tooling, especially if air tools are involved. I use EZ locks. Keenserts work well, but are a bitch to change.

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    Do not use bolts but studs with a nut if possible
    You insert the stud only once So less wear

    Peter

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    I have used Keenserts instead, when going into aluminum plate with frequent removals.

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    I've never had an issue removing Keenserts. They have a specific drill size to remove them. What problems have you had removing them?

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    Quote Originally Posted by JBethell View Post
    I've never had an issue removing Keenserts. They have a specific drill size to remove them. What problems have you had removing them?
    NO.....I use them where the BOLT needs to be installed and removed frequently....

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    Quote Originally Posted by trochoidalpath View Post
    I’m working up some semi-permanent fixture plates for rotaries and similar. My plan was to make them out of 2” thick 7075, hard anodized. I am looking for some advice about the best way to hold down things to the fixture plates. I think I am overthinking it.

    My first idea was to just spot drill, drill, and tap M12 after the fixture is back from anodizing. I have heard that anodized threads can be brittle and not hold well. But then I started worrying about the thread strength.

    My next idea was to put in a Helicoil, either before or after anodizing. I am not sure this would buy me much. If I do it before anodizing I have to find an oversize STI tap, if I do it after I need some zinc chromate primer?

    Thanks for any tips here!

    I use E-Z LOKs in aluminum to hold down the mitee bites we use all the time.


    Threaded Inserts | Buy E-Z LOK Thread Inserts for Metal, Plastic & Wood Online at ezlok.com

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    I understand the size may warrant aluminum to keep the weight down, but if they wouldn't be too heavy consider using 4140 prehard instead. Much more durable, still easy to machine (just take a little longer), no galvanic corrosion issues, and no need for anodizing.

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    I also use Keenserts (or whatever McMaster calls them).

    The can be fussy about the chamfer and minor diameter so I would drill and tap after anodize. Personally I would not bother to anodize. If you need that surface hardness to hold up over time I'd just make them from 4140PH.

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    I have some steel subplates that ran production in tough alloy milling environment for 30+ years 2 shifts a day and all the threads are nice, the plates are still mostly flat within a few thou.

    I have many aluminum sub plates. Without thread inserts the threads are toast in a few years. With thread inserts the inserts start loosening and becoming a problem in about the same timeframe of daily use.

    I make a lot of fixtures from aluminum and replace/patch them when required.

    Subplates and rotary fixtures that stay on the machine I prefer steel.

    I've had several machines with subplates that cantilever a 4th off one side of the table. I find a 1.25" thick steel subplate is much more stable and rigid than a 2" thick 6061 plate doing the same job.

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    I have always used helicoils in plastic or aluminum, except (as noted above) when the torque or the temperature cycles are high.
    Then I opt for key-lock style fasteners.

    Using studs as opposed to bolts will save your fixture.
    If the stud stretches after repeated use, it can be replaced. If the threaded hole wears out, you have helicoils or key lock inserts as your fallback, and they can only be repaired once before you have to go to a larger threaded insert.

    My $0.02

    Doug

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    I'm with the 4140 crowd here.
    Since you want to anodize it ( hardcoat I hope), then my guess is it is a permanent or at least expected to be.
    In that case AL is a really piss poor choice ( a comment which I will stand by ).
    For a quick, once-in-a-while use, sure, why not AL.
    Beyond that, go steel and never look back.
    We've ruined more than a fair amount of parts by chips embedding into the AL fixture, only to dent every friggin' piece clamped in it after that!

    4140, hardened to 40-ish will be strong, hard enough as to not easily impacted by chips, soft enough to plow into it with an endmill and not make a big mess ( outside your ego of course )
    If you're concerned with the weight of it, then turn it into swiss cheese to lighten it up.

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    My business is toolmaking,
    I only use Blanchard ground STEEL. I do not care what it weighs, we build our tools to last.

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    Dunno what your going to be putting on these fixture but I vote Steel for sure and as far as pattern and thread I went with 1/2-13 with .6255 +.001 -0. c'bores 1/2" deep on a 2" spacing. This lets you use 5/8" dowels to fixture other fixtures and Kurt vises. 20210311_092237-1-.jpgimg_20210813_085320890-1-.jpg I plugged the holes with some custom Delrin plugs for easy clean up. O-ring keeps fine chips out of the threads really well. I ends up being really versatile which I need for job shop work. Never know when some engineer is gonna make you do something dumb. img_20210709_131101719-1-.jpg

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    Quote Originally Posted by mhajicek View Post
    I understand the size may warrant aluminum to keep the weight down, but if they wouldn't be too heavy consider using 4140 prehard instead. Much more durable, still easy to machine (just take a little longer), no galvanic corrosion issues, and no need for anodizing.
    Adding to this...I've made a lot of fixtures from aluminium with hard top plates.
    Used 6mm (1/4) ground flat stock, retained to the ally using M6 csk screws.
    I also did a turnkey for a customer installing a m/c shop for them, and we put in 4x verticals with manual pallet changers.
    Each of these had hard top strips - 6mm thick ground flat stock for the fixtures to sit on.

    Edit - Picture uploaded.
    Pallet base to suit = X850mm x Y520mm machine travel. Steel strips (qty 4)ran full length of pallets. Slots in end (where highlighted red) as location (drill) bush inserted into pallet. 10mm left hand end. 12mm right hand end. Fixtures located onto pallet using long drill rod - slide through fixture into pallet location bushes, add cap screws, remove drill rod.
    The wear (ground flat) strips had no location - just held down by csk screws, so could be replaced if required.
    The accuracy of fixture location was therefore constant, as it was the bush sunk in the pallet that gave location - and the fixtures were retained by c/bored cap heads going though a clearance hole in the wear plate, into the ally fixture (timesert threaded hole).
    Fast fixture changeover with datums in the program and tool library in the machines. Job changeover was less than 15 mins.
    Attached Thumbnails Attached Thumbnails capture.jpg  
    Last edited by barbter; 09-08-2021 at 07:38 AM.

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    If you need to get the weight down you can stiffen the plate at the same time...


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    That looks fantastic! How'd you pick the web pattern? I was looking into isogrids the other night and found some crazy NASA/McDonnell Douglas articles from the early 70s that talked about it, but I got lost pretty early in.


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