Self-threading screws are not suitable for multiple use applications; the hole in the sheet metal (I presume) will enlarge.
If just once or twice for testing, go to a slightly larger screw diameter each time.
For multiple use, tack weld a nut to the backside of hole and use machine screws.

The end frame is a casting with a considerable amount of wall thickness. Itz NOT a sheet metal. The self-threading screw is tightened into a blind hole on the casting.

For sealing and water leak problems, we plan to replace the bolt & nut assembly, by self-threading screws. So definitely we want to make sure that, when the screw is backed out and retightened, it still doesn't loose the holding power.

any suggestions or encouragements?

Assuming adequate thread engagement (diametral and axial), thread forming screws do not loose significant preload capacity after loosening and retightening.  For your application, I recommend TAPTITE fasteners.  Learn more at:

http://www.taptite.com

http://www.camcar.textron.com/NewHTML/taptitedata.html

CoryPad,
Thanks, that seems like a good product.

A lot of good points have been provided to you from the various sources.  I have always said that one test is worth 10,000 opinions.  Many years ago I worked with CAMCAR to introduce thread formers to a major OEM transmission manufacturer.  After many studies we found that there was no degradation in clamp vs time, temperature, vibration etc. However, if something needed repair in house we would just go to the next IMPERIAL SIZE UP (if metric screws were used used initially) and that screw would be color coded as a repair screw for field information.

CoryPad provided you with some excellent web sites to explore but rmember that there are other sites that give more detail.  The only correction I might add is to use www.taptite.net, not the www.taptite.com website provided by REMINC, the originator of the Taptite product.

You must have plenty of wall thickness to allow for the larger screw.  See my article in American Fastener Journal about two years ago (www.fastenerjournal.com) referencing proper boss design for thread formers in aluminum.

If you can't find it, email me and I will be glad to send you a fre copy of the initial draft.

fastech
www.fastenertechnology.com

Bill P.
CSFT, Inc.

fastech,

If there are other sites that give more detail, perhaps you could supply them?

The two TAPTITE URL's lead to identical information from REMINC.  Perhaps you are thinking of when Continental/Midland had the taptite.com URL.

I would like to direct you to www.shakeproof.com and in particular the Tap/R product.  The design basically has high relief on two sides of the screw point that transition into a round-bodied thread.

When dealing with castings, the Taptite literature will tell you that a thread engagement length equivalent to at least 2 screw diameters is needed to avoid stripping problems.

The Tap/R's round bodied thread typically requires only one diameter of full form thread engagement to take advantage of the full strength of the tapping screw. Plus there is no problem with reusability with the round-bodied thread (it can be manufactured to ANSI tapping screw standards or to a class of fit).

Let me know if you would like some more info.

Hi CoryPad;

If you go to the www.taptite.net site (Which is REMINC) and click on the US area, click on applications, then on FAQ, you will find your very question answered at about the 12-13 question.

It sounds like what you really need is joint analysis study.  One can be developed using ANOVA tables or Taguchi study, etc.  Then conducting the study. That kind of goes back to my second sentence.

One thing you will find is that every manufacturer will have their own gimmick version of a tri-lobular but be sure you use the one that works for your application.

I have worked on several projects similar to the one you are speaking of for Black & Decker and a window manufacturer (putting thread formers into aluminum) plus my 38 years with GM.

Bill P.
CSFT, Inc.

fastech,

I think you confused me with the originator (mpoc) of this post.

I think you are confused about Reminc websites - they use both taptite.net and taptite.com, and both have identical information.

Bill (fastech):

I can't find the article that you wrote,referencing proper boss design for thread formers in aluminum. Could you email me the initial draft to bgruban@yahoo.com?

Thanks,
bernard