[time-nuts] Machining some aluminum help!

[Van Horn, David]

For popping off the lid, a few threaded holes in the lid would be nice.  Insert screws and let them push against the body and push the lid off.


-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of ed breya
Sent: Thursday, May 25, 2017 3:08 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Machining some aluminum help!

This is the first time I've looked at time-nuts in about a month, and I noticed the run about this project. I'm very late to the party, but have a few suggestions that may help - if it's not too late. I quickly scanned many of the posts, and agree with many of the ideas. Please forgive if my suggestions are redundant to what's already been said.

First, I assume that the aluminum box is a simple extrusion, so it will likely be a soft alloy that will tend to gall horribly with machining - especially bad for anything that needs high precision. With these kinds of material, go big, starting with bigger fasteners. With 1/4" walls, you can easily up it to 6-32 or 8-32, as long as the holes are fairly shallow, and you can jig it up for good centering and plumbness. With
4-40 and blind holes, you're just asking for trouble - especially taking a chance 40 times. The tap drill will be quite skinny, and prone to deform and wander as it goes in, and can easily be snapped off when it stalls due to the galling - and that's just the drilling stage - the tapping will be worse.

Bigger threads give you a chance to get it done with fewer fasteners and holes, and much less grief. The thread depth should allow for at least one pitch-diameter of penetration for strength, but preferably two or more, so you don't have to worry about finding exact right screw lengths that won't bottom out. Depending on the thickness of the end plates, you could get down to two or three fasteners per side to hold the small pressure needed.

If you're using a drill press, punch mark the hole centers, then use a center drill to make the pilot holes for the tap drill. If you're freehand drilling, put the piece on the floor and drill downward, keeping it as plumb as possible. Definitely use an oil or other lubricant for all the drilling and tapping operations.

For sealing, I'd recommend against fancy o-ring features and such - these are also harder to machine cleanly in soft aluminum, and add unnecessary complexity. If the end caps need regular remove and replace operations, then go with a pliable gasket, have more fasteners to get more uniform compression, and make the threads deeper so they'll last longer. If the sealing is one-time, or seldom needs to be broken, I'd recommend using a gasket sealing goop that will work fine with few fasteners and even rough surface finish. My favorite is Permatex #2 "Form-A-Gasket Sealant," which I've used for all sorts of stuff over fifty years (back then it was Radiator Specialties brand). Don't use a silicone goop unless you want to spend a lot of time scraping off the old stuff if it needs to be opened. If you do use a goop, it's a good idea to machine in features for prying the lids off, such as gasket-plane screwdriver slots, or extra tapped through-holes in line with the mounting holes on of the lid.

Ed
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