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, <[email protected]> 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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