Threading aluminium enclosure?

[Tobyk]

Could someone please walk me through how to thread holes for the back plate screws on an aluminium enclosure? Is it needed at all? I'm guessing aluminium is too soft to screw into many times, correct? I'm planning on using #6-32 bolts (3,5 mm). Should I drill a 3,5 mm hole then, or slightly larger? And then just put the thread in, maybe with some epoxy?

[tootsMcgee]

Could someone please walk me through how to thread holes for the back plate screws on an aluminium enclosure? Is it needed at all? I'm guessing aluminium is too soft to screw into many times, correct? I'm planning on using #6-32 bolts (3,5 mm). Should I drill a 3,5 mm hole then, or slightly larger? And then just put the thread in, maybe with some epoxy?

All the enclosures I've used have had pre-threaded holes and matching (usually countersunk) screws. A few have been a little tight at first from powder coating overspray. But I've never had to explicitly thread anything. I'm getting mine pre tapped and finished though from stompboxparts.

What enclosure are you (planning to be?) using?

[Tobyk]

Could someone please walk me through how to thread holes for the back plate screws on an aluminium enclosure? Is it needed at all? I'm guessing aluminium is too soft to screw into many times, correct? I'm planning on using #6-32 bolts (3,5 mm). Should I drill a 3,5 mm hole then, or slightly larger? And then just put the thread in, maybe with some epoxy?

All the enclosures I've used have had pre-threaded holes and matching (usually countersunk) screws. A few have been a little tight at first from powder coating overspray. But I've never had to explicitly thread anything. I'm getting mine pre tapped and finished though from stompboxparts.

What enclosure are you (planning to be?) using?

Thanks, I should have mentioned that I'm planning to sand cast my own enclusure, not buy one.

[davent]

There will be a prescribed drill size based on screw size and number of threads and maybe the material you're tapping. Usually an odd size just identified by a number - #36  for a 6/32.

[stallik]

The enclosures I use all seem to use  M3.5 x 0.6 thread. The correct drill size is 2.9mm
Though I use 3mm since it's easier to get hold of.

M3.5 thread is also used on UK electrical sockets so repair tools are cheap and readily available (M3.5 tap in a screwdriver handle) I have one in my pedal toolkit in case of overspray/cross threading issues.

Sorry, I can't help with imperial measurements

[MetalGuy]

There are different grades of aluminum. Some are easier to drill/thread than others.
When drilling holes in aluminum use couple of drops of alcohol on the drilling spot/area.
Same applies when threading.

[jorg777]

Sand-casting your own enclosures is pretty ambitious!  I'll bet you have some crazy shapes in mind.   

[Tobyk]

The enclosures I use all seem to use  M3.5 x 0.6 thread. The correct drill size is 2.9mm
Though I use 3mm since it's easier to get hold of.

M3.5 thread is also used on UK electrical sockets so repair tools are cheap and readily available (M3.5 tap in a screwdriver handle) I have one in my pedal toolkit in case of overspray/cross threading issues.

Sorry, I can't help with imperial measurements

Thanks, but how can a 3,5 mm thread fit in a 2.9 mm hole? Sorry, new to this..

[Tobyk]

Sand-casting your own enclosures is pretty ambitious!  I'll bet you have some crazy shapes in mind.   

You bet!😀

[FiveseveN]

Thanks, but how can a 3,5 mm thread fit in a 2.9 mm hole?

3,5 mm is the major diameter (tip of threads), the maximum minor diameter (base of threads) is 2,829 mm. You drill (close) to the minor one and then the tap cuts the threads around (to the exterior of) that hole.

[Tobyk]

Thanks, but how can a 3,5 mm thread fit in a 2.9 mm hole?

3,5 mm is the major diameter (tip of threads), the maximum minor diameter (base of threads) is 2,829 mm. You drill (close) to the minor one and then the tap cuts the threads around (to the exterior of) that hole.

Thanks, this place is great. I'm not sure we're talking about the same thing here though. I think you are speaking of the act of tapping a hole, correct? This I'm hesitant to do since aluminium is soft. So I'm thinking of using steel thread inserts. Does the same apply to those?

[Matthew Sanford]

Essentially the drilled hole allows the shank to pass through, then you cut threads with the tap - same shank size.

Are you planning to anodize/hardcoat the cast aluminum? That may help softness issues

[FiveseveN]

I'm thinking of using steel thread inserts. Does the same apply to those?

Well those come in many varieties, some also threaded on the outside. Though I wouldn't worry about the softness of Al as long as you pick a reasonable diameter & pitch.

[davent]

You can tap holes in wood and not have problems, no need to overtighten four screws holding a light piece of aluminum fighting the pull of gravity.

[Phend]

Steel threaded inserts are for car engine blocks, not a static wimpy effect box.
Don't over engineer it, just tap the holes, use a drill one size under what the chart says to.

[Eddododo]

I'm sure I'll be repeating a point already made, but nothing too complicated... Pick the screw you want to use, look up its inner diameter, Drill a hole that size, tap it.

Aluminum is indeed soft, but nothing to panic about, just be careful. The main thing you want to consider is that you are tapping blind holes.. If I were doing this, I would probably opt to do it by hand so I can be slow and deliberate and blow out chips as I go. A spiral flute tap will encourage chips up and out of the hole, but I would make patience and attention your main chip management strategy..

If you're casting your own, you could always fill the hole if you screw it up!

Threaded inserts are a decent backup plan if something goes awry.. I would personally probably just fill with hard marine epoxy and screw into that if I stripped a hole out while tapping, but if you tap it well, you shouldn't have to worry about it stripping later

[PRR]

how can a 3,5 mm thread fit in a 2.9 mm hole?

The classic start point is to drill same as the root of the thread.  You can buy combination bits that both drill and tap.


Drill to the root and let the teeth chew the grooves. You may drill a bit over or under-size for easier tapping or better strength.

BTW, combo drill/taps are terrific in heavy sheet metal (240V electrical boxes) but may be a bad choice for blind holes. You need high RPM to drill but then the tap runs too fast and breaks. Anyway they are solid in sets, mostly bigger than pedal-work wants.

In alternate processes like thread-rolling you might drill diameter halfway between tip and root and displace base metal +/- for grooves.

[Tobyk]

Thanks a lot everyone, guess I won't be needing inserts then. I looked up Hammond enclosures and they are indeed just tapped. Now I'm thinking the hole and thread could then just be made as part of the casting mold, but I guess sand casting is too rough for that to work.. I wonder if Hammond has the tapped holes as part of their die cast molds or if they drill/tap them afterwards..
Does anyone know?

[FiveseveN]

How would they get the mold out of the holes if that were the case?

[PRR]

tapped holes as part of their die cast molds

Motor-car engine blocks are traditionally cast metal. A big part of the post-casting operations is the drilling and tapping machines.
https://youtu.be/bnb7zXicXsc?t=375 

(This is the Ford Flathead V-8. If you know the Windsor OHV 289/302/351, it was made on the same machinery with different molds and drills.)