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I work at a machine shop with high-volume parts and a lot of threaded holes. Common sizes are 1/4-28 STI and 10-32 STI. Every hole gets a helical insert. We have always had major issues with the 1/4-28 STI holes. Some inserts will go in with no problem, some will not even go halfway in. These are holes that have been through the same process and tapped with the same tap. What can be some causes of this? Also, we never have problems with any other hole size. Just 1/4-28 STI. We use pneumatic drivers and also dip the inserts in a liquid before inserting. also, every part gets anodized.

Started doing some research and testing. Like debris in the holes from buffing and tumbling, or anodizing thickness causing issues. These are all still in development.

Final questions: Has anybody had similar problems and did you find a solution? Does anodizing leave a thick enough layer to affect this? What about Alodine?

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    $\begingroup$ Is there any consistency with what holes suffer from it? Perhaps always in the latter half of tool's life cycle? Or perhaps the inserts that don't go in are always part of the same batch? Or the anodization from the same batch? Or perhaps the guilty holes are only every in specific locations? Are the inserts removable? For example, if an insert does not go in, can you remove it and stick it in a hole where an insert has been previously confirmed to go in? And can you stick an insert that has been previously confirmed to go in a hole into the hole where the insert has previously not gone in? $\endgroup$
    – DKNguyen
    Nov 14, 2023 at 21:27
  • $\begingroup$ the inserts cannot be removed without damaging them. I cant seem to see any consistancy. $\endgroup$
    – Woody
    Nov 14, 2023 at 21:40
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    $\begingroup$ Would that not be simple enough to find out by cleaning out a hole where an insert previously had not gone in? Or is the part lost when an insert is botched? I guess that would explain all the trouble you're having if you can't ever verify anything. $\endgroup$
    – DKNguyen
    Nov 14, 2023 at 22:50
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    $\begingroup$ @Woody "lazy operator" or do you mean they are not really given sufficient time to clean each hole properly? $\endgroup$
    – Solar Mike
    Nov 15, 2023 at 7:02
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    $\begingroup$ Easy enough to test by thoroughly cleaning the holes on a few parts and seeing if the issue goes away. $\endgroup$
    – Drew
    Nov 15, 2023 at 17:59

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I had a problem like this years ago. High-precision bearing balls; some fit and others did not. Turns out the vendor was shipping us parts which had been rejected as out-of-spec by his other customers. Our volumes were tiny compared to his bigger clients and this was his way of saying he didn't want our business.

We had to institute 100% incoming inspection for roundness and diameter on the balls to keep his junk out of our process- in essence, we were doing his quality control work for him. Do you have incoming inspection for the 1/4-28 STI parts?

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  • $\begingroup$ We make the parts and have recently purchased a thread gauge to prove they are good holes. Also, the helical inserts are from large reputable companies that are sometimes even called out in a spec on drawing. Can't really get around it. Dont think this is the problem. @niels nielsen $\endgroup$
    – Woody
    Nov 16, 2023 at 2:53
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I'm an EE - so not very knowledgeable on this stuff - but have had a look at some of these helicoil devices. As far as I can see the action of screwing them in reduces the diameter if they bind on anything so they should free up as a result.

I'm wondering if they are being advanced inconsistently and binding vertically rather than diametrically? Are the pneumatic drivers too heavy? Could you cut a deep slot in the driver bit so that it bottoms out in the hole and the coil's driven "diameter" can slide down the slot at a rate determined by the thread rather than the weight of the driver?

What's your success rate if you drive them in by hand?

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  • $\begingroup$ The drivers are on arms that are designed for repeatability (there is a counterbalance to keep it light. The practice used by us right now is placing the driver on the whole and resting it on the hole while driving the insert. The drivers are designed to have shims and we have them set to drive a certain, measured depth each time. We don't drive any by hand as it would be impractical. We put inserts in hundreds if not thousands of holes every day. Of course we can try it by hand if you suggest that will help in the troubleshooting process $\endgroup$
    – Woody
    Nov 14, 2023 at 22:34

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