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Date: Thu, 31 Jul 2008 20:11:04 -0700
From: Dale Rogers <dale.r@cox.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Safety wire
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Ernest Christley wrote:
> Dale Rogers wrote:
>> Bill,
>>
>> "Case hardened" or "Through-hardened"?  Either way, ditto on the 
>> cobalt drill bits, or for hole sizes larger than #20, maybe even 
>> "solid carbide".
>>
>> FWIW, I do case hardening.  It is typically done to give a 
>> wear-resistant surface on a part that otherwise needs to be somewhat 
>> ductile underneath - which won't be the metal's strongest state.
> But do you really want to use hardened bolts anchored in aluminum?  
> Lynn said it the other day.  It would be better to choose a bolt where 
> you could get some stretch out of it before it pulls out of the anchor 
> material.
>

Ernest,

   Generally, there isn't much choice.  Threaded fastener specifications 
normally include some degree of heat-treating - hardening and then 
drawing the temper to achieve a "grade" of hardness.  I have seen bolts 
made from aluminum, but I don't know as I'd want to use them on any 
critical component of an airplane.  Yes internal threads in aluminum 
aren't as strong as those in steel, but we see steel cap screws in 
aluminum threads in all kinds of automotive applications.  The strength 
of an internal thread in aluminum can be improved by swaging the thread 
with a thread-former instead of cutting it with a conventional tap.

Dale R.
COZY MkIV #0497
Ch. 12; Ch's 13, 16, 22 and 23 in-progress