Countersinking undercut flat head machine screws

[fesodes]

Hi,

I'm trying to secure two 2mm thick metal plates together. Since I need a flush surface, I'll be using 8-32 undercut flat head machine screws and 8-32 nuts to do this. Here is a diagram of my setup

http://www.flickr.com/photos/39031409@N05/3589517471/).

Unfortunately, I don't know what information I need to provide to the metal stamping company such that the holes on the metal plate would accept these undercut flat head screws. Would giving the head dimensions, major diameter, and angle(82 degrees) of the screw be enough?

Lastly, I am aware that a 8-32 machine screw has a max major diameter of about .1631. But I don't know what the hole diameter(represented by red letter 'A' in diagram) should be on metal plate #2 to hold the screw tightly. Would a hole diameter of 4.5mm be too tight or too loose?

Thanks in advance.

 

[KENAT]

I've never used undercut screws so I'll hold off on that part of your question. For typical CSK diameter (with tolerance, i.e. at the minimum dia must be at least as much as the maximum sharp dia of the screw head) of csk and included angle is appropriate.

However, with regards to calculating the required hole diameter, this relies not only on the size of the screw but assuming a pattern of fasteners is being used also on the location tolerance of the holes/countersinks in the mating parts.

You have what in Tolerancing circles is called a fixed fastener situation, because the CSK are self centering. The spreadsheet in thread1103-221602 (later version on 4 Sep 08 3:26 post) might be of use. However, unless you and/or your shop are comfortable with using GD&T terminology you may not be able to take full advantage of it.

#8 screw max major dia is .164"
4.5 mm is .177" typical drill tolerance for this size +.005 -.001 so assume min hole dia is .176

.176-.164 = .012 tolerance budget for location of the holes. This value has to be split between the holes on each part, lets say 50-50.

So the holes in one part and csk's in the other have a positional tolerance of .006". This equates to a +- dimension of +-.002 because of 'squaring the circle'.

This is pretty tight, I'd suggest a larger hole size.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[hendersdc]

The dimensions you should provide to the metal stamping company are:

-diameter at the top of the countersink
-countersink angle
-diameter of the through hole

 

[mcgyvr]

Given a .078" (2mm) thick material and a #8 undercut 82 deg fastener.
I would call out the required hole as
.200" thru by .336" x 82deg C'cink.

If the material was thicker than .0915" and a free fit of the screw was required I would call out .177" thru by .336" x 82deg C'sink.
If a close fit was required the material would have to be thicker than .0958" and I would call out .1695" thru by .336" x 82deg C'sink.

With thin material (thinner than height of regular flat head screw height) you basically ignore the typical required thru hole size for that screw as the thru hole will be dictated by the dia of the c'sink, angle of the c'cink and material thickness.