"Ned Simmons" wrote in message
...
On Sat, 07 Sep 2013 12:41:00 -0500, dpb wrote:

On 9/7/2013 10:39 AM, N_Cook wrote:
I need to do a hub-pulling type operation on a small piece of equipment
that is not made to be taken apart, so no purpose-made anchor points.
There are 8 off , 1mm tapped holes with 5 threads per hole , in the
aluminium available (used for undemanding normal fixing to the chassis)
, if I drill a plate to take 8 screws . The only screws of that thread I
have are brass, probably preferable to steel anyway. For a similar
hub-pulling where there was purchase , after heating (limited as to
amount that can be used ), required about 4Kg of force to extract. Would
4Kg , assuming even distributed loading over the 8 screws be above or
below the stripping force ?
I found these sites but not applicable for my purpose it would seem

www.tribology-abc.com/calculators/e3_6e.htm
http://www.roymech.co.uk/Useful_Tabl...ead_Calcs.html

The _rough_ back-of-envelope estimate here comes out at only about 20
lbf loading each assuming a soft Al. T6061 w/ a hard fastener might be
closer to 100 lbf.

I get 30 pounds, but maybe your aluminum is softer. And I didn't have
the benefit of a napkin. g

In any case, if the screws are not silicon bronze or some other high
strength alloy, it's probably a toss-up between stripping the threads
and breaking the screws.

--
Ned Simmons

In practice I think both your estimates are overly optimistic. In this tiny,
fine thread, the clearance is usually a large fraction of the total thread
depth, so the threads just engage near the tips. Usually the failure mode is
that the tips round over instead of shearing off. Optical equipment I work
with sometimes has these tiny screws.