Slotted Countersink(?)

In my years of airplane structure modifications, one method of removing some of this "uncertainty" was to "match drill" the part that is uncertain from an existing hole as assembly. Is that an option?

Steve R.

"Cam" wrote in news:1133206598.202049.263690 @f14g2000cwb.googlegroups.com:

There are situations in which this would be perfectly fine. I've done it to allow panel adjustment in one direction while maintaing a flat surface. There are situations in which this would be a disaster. It all depends on the particular loads, screw spacing, and other design specific factors about which we know nothing.

I've done it a few times, and it works if the loads are low. A better option may be a slotted counterbore and button heads, or better yet, low head socket capscrews. Head height is about .085" on both styles for a #8, #6 is .072.

Ned Simmons

Reducing a screw head fit from its area to line contact does not seem smart. It would be better to get a nut fixing that is compliant to misalignment. A classical design is the rivetted loose nut plate, where the trapped nut can move a little - but this is a high-labor aerospace approach. How about a spring nut ( u-nut, fast nut) with an elongated hole? This could accomodate a fair misalignment. Or drill and rivet in situ (undesirable) or use an expanding countersunk bolt? (tacky)

Putting a fixing under a close cover seems like its begging to be designed out, in my mind

Regards

Brian Whatcott Altus OK

If you have the hole slotted, the countersink should match. Carrying loads along the length of the slot through the countersink and screw head is unwise.

You should have capability elsewhere to carry loads in the direction of the slot. Or the loads are small enough for friction to reaction.

Thanks for all your helpful suggestions. Based on y'all's feedback, I've decided to punt on the problem and round up manufacturer's drawings on the hole locations. It's the only way to reduce the uncertainty in the hole location (I can only measure to about 1/32 inch) and avoid potential problems with line contact or severely reduced contact area. This joint might see significant load.

This in itself is no small feat. Try calling up the customer service department of a major network equipment manufacturer and ask for a mechanical drawing of their mounting hole pattern. It's like squeezing blood from a turnip. Lots of call transfers and people saying "uhhh...."

Thanks mightily for the help!

Cam

"Cam" wrote in news:1133473452.878044.29950 @g49g2000cwa.googlegroups.com:

Yes, been there, done that. Took a month to get some drawings from a major machine manufacturer, after they said 'sure, no problem'.....