Have to tap 0.21 Thk ALU Section (1/4 dia min) Making the Best of a Bad Situation, Help! 1

[vonsteimel]

Greetings,
I'll keep this short and sweet. To the Point: Forget the surrounding circumstances; I am the firefighter, not the designer... Here is the situation:

I have a 7/32 (0.219) Cast Aluminum section (unknown alloy) that, when assembled, has only 1/16 clearance on the bottom (internal) side -- I have to drill and tap the aluminum section No Smaller Than 1/4". It must withstand a constant load (there will be no shock or cyclic stresses) that will not exceed a 25lb tensile force. (but will likely be less than 20lb)

SEE THE ATTACHED SKETCH FOR DETAILS

Should I use 1/4-20? or 1/4-28? I think the 1/4-28 will be too small for such soft aluminum and will get boogerd up too easily. Perhaps 5/16 thread? if so, NF or NC?

I even though I could get a 1/4-28 heli-coil in there, since (some if not all) heli-coils expand when a bolt is screwed in (acting similair to a dry-wall screw insert) thus increasing the gripping power. I found a 1/4-28 heli-coil that is 1/4 long (the excess can be left hanging out) that is offered with both 5/16NF and 5/16NC threads.

I'm Leaning Heavily towards a standard 1/4-20 thread.... I've gotta solve this with about 24hrs. Any input afterwards will be academic, but still appreciated.
Thank you.
VS

PS: no, quitting is not an option

 

[MikeHalloran]

Is there a stripped hole there already, or do you have to drill through?
Do you have to do the machining while it's assembled?
Could you insert a flanged threaded bushing from the backside?
Can you get the chips out; will they do any damage if not all removed?
Can you get a Keensert that short?
Are you sure it's aluminum and not a zinc alloy?
I'd use Tapmagic or kerosene on the cutting tools.





Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

you're got a very small load on this ... maybe a smaller bolt with a sleeve or a shoulder to suit the 1/4" hole in whatever you're attaching here ...

Quando Omni Flunkus Moritati

 

[vonsteimel]

To the Point: Forget the surrounding circumstances; I am the firefighter, not the designer...

Mike:

Is there a stripped hole there already, or do you have to drill through? --- I can put a stripped hole there if it helps... but why? otherwise, the constraints still stand.
Do you have to do the machining while it's assembled? --- No mention of machining... but i could if needed.. again, otherwise the constraints still stand.
Could you insert a flanged threaded bushing from the backside? --- I could... but why? if a tapped hole will work?
Can you get the chips out; will they do any damage if not all removed? --- not worried about chips, thats why i didnt mention it.
Can you get a Keensert that short? --- i believe so. Is this what you think is best?
Are you sure it's aluminum and not a zinc alloy? --- its possible it is contains zinc as an alloying element but is evident by the weight it is primarily aluminum. Why is this a concern? Are you thinking welding?

rb1957, would work, but what are the benefits over a simple tapped hole?

Thanks everyone.
VS

 

[MikeHalloran]

I'm leaning toward rb1957's idea of tapping smaller than 1/4" and using a bushing as appropriate. Probably least weight, and repairable. For a 25# load, you could use a tiny instrument screw. ... assuming the load is as described.

To get 3 diameters of thread in a .210 thickness, you're talking about a 0-80 with quite a bit of cushion. Three diameters of thread is enough to break many steel screws without stripping a plastic nut. ... before you laugh, look up the ultimate tensile load for a screw in that size in whatever material you like. Other restrictions may apply; where did the 1/4" minimum come from?

Further to repairable, if you start with a really tiny screw and the unknown aluminum turns out to be porous or so gummy that it tears in tapping, you then have the option of drilling and tapping to progressively larger sizes until you get a clean hole with nice threads. ... or you have to use the Keensert anyway.




Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Thinking firefighter, How hot is this going to get in service?

I'd glue or double-side-tape a nice big plastic bumper 0.062" thick on the untappable internal.

 

[kingnero]

there are rivets that have a threaded bore. Maybe that's a viable option?

 

[racookpe1978]

What is your maximum screw diameter? Could you go up to say 1/2 inch dia?

 

[btrueblood]

Mike's last post has a valid point - do realize that you can get ~4 threads in a 1/4-20 tapped hole in that wall thickness, which ought to be sufficient for the load as described (no cycling or shock loading). Does the fastener need to be loosened/tightened or installed/removed repeatedly? If not, just tap it. If it does, than an insert of some type, in a stouter material like steel or stainless might be warranted.

 

[drawoh]

vonsteimel,

Is 25lb the external load on the fastener, or is in the clamping force of the fastener?

--
JHG

 

[c0rd0bes]

possibly late but

A clinch nit is a rivet with an inside thread

or simply grind down the helicoil before you insert it buy a 1D and take off approx just over half the lenght from the non tanged side

 

[vonsteimel]

I always called them riv-nuts... Anyway, ended up using a screw-lock (expanding) heli-coil.

It was an odd shaped light... A few had been tapped incorrectly and needed to be fixed. I was handed the project with a set of instructions/constraints which I posted above. Thanks everyone for the input.

The next run will be tapped smaller diameter and course thread to prevent the issue. Never should have been tapped 1/4" in the first place being it was so thin. Probably will shoot for a #8 and see how that goes.

Thanks again,

VS

 

[chicopee]

Fine threads may do the trick as you'll get about six full threads. 25LBS should not be a problem for aluminum.

 

[vonsteimel]

I found fine threads are a bad choice for limited engagement in a soft material. A fine thread is much shallower than a course thread and thus get mashed/scraped.etc all the more easily. Actually, I think I remember reading somewhere that it recommended using course threads in these situations specifically because of this.

It doesn't take much to ruin a fine thread in a material as soft as this cast AL housing. I think course thread would have been more robust.
Thanks,

VS