[time-nuts] Machining some aluminum help!

jimlux jimlux at earthlink.net
Thu May 18 14:53:00 EDT 2017

On 5/18/17 11:16 AM, Tim Shoppa wrote:
> I'm very dubious that you need a tapped 4-40 hole to be threaded any deeper
> than 0.25". If you work with the machinist I'm sure you can come up with
> some reasonable spec that does not require a bottoming tap and will save
> you a lot of money.

The rule of thumb is that if you get 3-5 threads fully engaged that's 
good - so at 40 threads/inch, 5 threads is 1/8" Drill the hole a bit 
more than 1/4" deep (approx), tap the first 3/16" inch (so you can use a 
regular old tap), blow the chips out afterwards, use fasteners that are 
the right length.

Is the number of fasteners for mechanical strength, for RF sealing, 
thermal transfer, or what? You might be able to get by with fewer 
fasteners if you use a conductive elastomer gasket (or even copper foil 
tape which deforms when you screw the lid down).

5 fasteners in 5" is spacing them a bit less than an inch apart.
Even in Aluminum, your threads are probably stronger than your fastener.

	Threads			Fastener	
Engaged Length	0.125	in			Engaged Length
TPI	40				TPI
Diameter	0.112	in			Diameter
mean Diam	0.0995	in	Root Diam	0.087	mean Diam
Engaged area	0.0391	sq in	Area	0.00594	Engaged area
Load	200	lb			Load
Shear stress	 5,119 	psi		 33,644 	Shear stress
Ultimate strength	42	ksi	ultimate	74	Ultimate strength
Shear Strength	 24,249 	psi	proof	55,000	Shear Strength
Safety factor	4.74		Safety factor	 1.63 	Safety factor
	Aluminum			Steel	

> I bet you went to 0.25" wall square tubing only because you want to tap the
> walls for 4-40. Alternative designs can let you use much thinner material
> and a very different flange on the ends, but the costs will likely move
> towards welding/brazing rather than machining.
> Tim N3QE
> On Thu, May 18, 2017 at 12:54 PM, <cdelect at juno.com> wrote:
>> Hi,
>> I have a square aluminum tube 5" X 5" with a .25" wall it's 8 1/2" long.
>> I need 20 holes in each end tapped for 4/40 and 1/2" deep.
>> This is for a Rubidium project.
>> The local machine shop want's $360.00!!!!
>> Anyone have a machining setup that could do the work a bit cheaper?
>> If not I'll give it a try myself.
>> Please contact me off list.
>> Thanks!
>> Corby Dawson
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