Form tap to fix anodized undersized holes in Ti?
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  1. #1
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    Default Form tap to fix anodized undersized holes in Ti?

    I have some fairly high dollar Ti6Al4V-ELI parts that came back from hard-coat anodize with some threadmilled 6-32 holes ever so slightly undersized such that the go gauge will almost but not quite pass through. Holes are through a .075" thick wall. Is it safe to use a greased form tap to resize them?

    Also, my plater claims that the anodization doesn't change the dimensions at all. Is that bull? I need to now if I need a pre-plate thread gauge to ensure this doesn't recur. Maybe I should get a go gauge that's a few tenths big, and use the standard no-go?

    Thanks!

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    The Annodize should only be a tenth or two thick at best iirc.
    So if your holes are just barely undersize, the tap would remove any coating. If you're even able to cut it with a tap.


    edit, I just saw that you're wanting to use a form tap.
    That might actually work.

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    Quote Originally Posted by Mtndew View Post
    The Annodize should only be a tenth or two thick at best iirc.
    So if your holes are just barely undersize, the tap would remove any coating. If you're even able to cut it with a tap.


    edit, I just saw that you're wanting to use a form tap.
    That might actually work.
    That's why I'm thinking form tap / roll tap. Hoping I can push the surface and keep the anodize.

    Edit: I just saw your edit; it only showed up when I hit the quote button.

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    In my (limited) experience whether a part shrinks or grows when it is anodized depends on how much material is etched off verses anodized back on. If the part is etched too long features shrink and holes grow, if too short of an etch or too much anodize the features grow and holes shrink. The only thing that can control that is consistent material, and the people doing the anodizing.

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    High dollar parts and you want to risk them with free advice from a public forum? If a gage is a little tight, a real live screw will probably be ok. I would call your customer and let them decide.

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    What Larry said. Don't try another Machining operation. (I understand what chasing threads is BTW). I'd get a SHCS and run it in and out a couple times, until the gage goes.

    R

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    Any screw actually going in there will be fine. I second asking the customer before screwing around.

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    Quote Originally Posted by dpolseno41 View Post
    Any screw actually going in there will be fine. I second asking the customer before screwing around.
    Thumbs up for the pun

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    Quote Originally Posted by Larry Dickman View Post
    High dollar parts and you want to risk them with free advice from a public forum? If a gage is a little tight, a real live screw will probably be ok. I would call your customer and let them decide.
    Most off the shelf screws that I have checked with a pitch mic are toward the low end of the tolerance, not the high end.

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    Quote Originally Posted by Dualkit View Post
    Most off the shelf screws that I have checked with a pitch mic are toward the low end of the tolerance, not the high end.
    Toward the low end of an external thread..

    $50 says any screw that needs to go in will go in.. If you can *almost* get the go gage through..

    Have you tried a screw???

    Another option.. Get a different go-gage. They aren't all the same, close, but not quite..

    Who is measuring this? What does their go-gage say??

    Warning: You should NOT do the next suggestion..

    If you are the one that has to prove the threads work with your go gage,
    MAKE it go through.. In essence a go-gage is really nothing more than
    a high dollar form tap.. If a screw goes, does it really matter if the
    threads get bigger, or the gage gets smaller???


    And then there is the question of "Does anybody really care?"..
    As long as a screw goes in.

    My guess is that somebody does, because you are worried about it..
    And I've done quite a few Ti parts, and not once, ever, have I seen
    an anodize called out.. So either the engineer was in over-kill mode,
    or it actually is important.

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    I don't want to say too much, but the anodizing is important for the application, and a correct, calibrated gauge must fit (by our inspection) or the parts are scrap. Leaning toward trying the form tap on one or two parts to see if it works; nothing to loose if it doesn't, right?

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    Quote Originally Posted by mhajicek View Post
    I don't want to say too much, but the anodizing is important for the application, and a correct, calibrated gauge must fit (by our inspection) or the parts are scrap. Leaning toward trying the form tap on one or two parts to see if it works; nothing to loose if it doesn't, right?
    #1 I third or 4th? what the guys say that a screw will fit no problems. I guess it's not an assy, and the surgeon has to fit them?
    If it was an in-house assy then no problem, but if it's a surgeon I can see why in-house inspection would fail them!

    #2 If it's that near on going in, it maybe worth trying another gauge...

    #3 Can you make a dummy plate (same mtl/thickness) and tap a few holes and get the platers to process. Then try a tap on this?

    #4 Hardcoat on aluminium is 50/50 into the material/build up. I don't know what it is for titanium

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    In the future I recommend a slightly oversized tap when the threads are going to be hard coat anodized.

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    Keep in mind that Ti is more elastic (lower modulus) than steel, so the material may flex away during form tap runthrough, then collapse back to what it was.

    Along with that, anodizes are an enhanced oxide over the normal material, and oxides tend to be more brittle. So the oxide might crack, and this may have consequences for the use of the part.

    My advise is to contact the customer and get an engineer to sign off on the "almost" thread. If that won't happen, find out if the part can be stripped and re-anodized. If that can't happen, perhaps it's better to remake the part.

    And tell your anodizer to check again on "no growth". I doubt that's the case...

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    There's a chance I am wrong, but I am 99.9% sure that the anodize coating on Ti is not actually an additive process like is on aluminum. I believe it's a micron or two thick, and it's all (or almost all) down into the metal, unlike aluminum where pores are generated which grow upwards from the surface as well as downwards into the surface.

    I think your form tap idea will work fine, and I don't think you will lose the anodize - probably not at all, and if you do - is it a problem on screw threads anyway? The way Ti anodizing works, even if you were to lose the anodize in your threads, you can just re-run them at the same (or marginally higher) voltage and the existing anodize will match the new anodize in your threads.

    Moving forward, I'd go with a slightly upsized tap made for anodizing if the problem was due to your existing tap.

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    Maybe the plater racked the parts in the holes and have minor dia. nick?
    The standard working plug gages start off at the high end of their gauge tolerance, and may reject good parts.

    Normal procedure is to use a master thread plug gauge for referee, use master gauges carefully on clean parts as they are made at the actual limits.

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    Quote Originally Posted by Bobw View Post
    Warning: You should NOT do the next suggestion..

    If you are the one that has to prove the threads work with your go gage,
    MAKE it go through.. In essence a go-gage is really nothing more than
    a high dollar form tap.. If a screw goes, does it really matter if the
    threads get bigger, or the gage gets smaller???
    Bobw, I acknowledge that you started that out with a Warning, but my immediate reaction is "Oh, FUCK No!". Please don't tell the infants to use thread gages as form taps on ceramic surface coatings, even with tongue firmly planted in cheek.

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    Quote Originally Posted by sfriedberg View Post
    Bobw, I acknowledge that you started that out with a Warning, but my immediate reaction is "Oh, FUCK No!". Please don't tell the infants to use thread gages as form taps on ceramic surface coatings, even with tongue firmly planted in cheek.
    Even if a $120 thread gage saves $5,000 of parts from the inspection nazis?

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    I think I might try greasing up a 6-32 bolt and force it back and forth with a cordless drill to try to stretch or burnish the thread til the gage goes.

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    Have you by chance double checked the minor diameter to make sure that's not the problem?


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