Threading aluminium enclosure?

[Phend]

Tap hole sizes vary for different materials.
Usually a slightly smaller drill is used for plastic and aluminum.
Drill sizes determine the percentage of the resulting thread contact.
If you drill with a very oversize drill, the tap will go in easy, sometimes really easy.
If you drill to small,  the tap will be hard to turn.
In any case you should turn the tap one turn then reverse some to break the chips.
Else you might break the tap.
Small taps are very delicate.
And if broken ,,, that really sucks.
Depending on what you eventually end up with, this chart might help.
It is primarily used in England.

[ThePracticalPeasant]

If I'm interpreting/understanding correctly, you're going to cast a pedal body and screw a piece of sheet-metal to the back (just like a Boss compact pedal, for example)

I'd leave the casting beefy in the corners where the holes need to be tapped;
Stack the parts to gang-drill under-sized holes in both the back-plate and body at once with a drill press to ensure they stay straight. Drilling together ensures correct alignment.
Use a good drill/tap chart to size the bit and follow the holes in the body so the threads can be cut;
Tap the holes, a t-wrench will work just fine with a bit of care;
Drill out the holes in the back-plate to the nominal fastener size and cut a countersink as required.

Edit: I searched Amazon for "#8-40 drill and tap" and was returned a two-piece Bosch pack for $7CAD. You should have no problem finding a similar "kit" for any sized hole for very little cost. I'd also like to emphasize "drill press". It is exceedingly difficult to drill holes this small accurately enough to cut snug fitting threads by hand. I paid $20 at a garage sale for a small bench-top drill press that would easily handle this work.

[Tobyk]

If I'm interpreting/understanding correctly, you're going to cast a pedal body and screw a piece of sheet-metal to the back (just like a Boss compact pedal, for example)

I'd leave the casting beefy in the corners where the holes need to be tapped;
Stack the parts to gang-drill under-sized holes in both the back-plate and body at once with a drill press to ensure they stay straight. Drilling together ensures correct alignment.
Use a good drill/tap chart to size the bit and follow the holes in the body so the threads can be cut;
Tap the holes, a t-wrench will work just fine with a bit of care;
Drill out the holes in the back-plate to the nominal fastener size and cut a countersink as required.

Edit: I searched Amazon for "#8-40 drill and tap" and was returned a two-piece Bosch pack for $7CAD. You should have no problem finding a similar "kit" for any sized hole for very little cost. I'd also like to emphasize "drill press". It is exceedingly difficult to drill holes this small accurately enough to cut snug fitting threads by hand. I paid $20 at a garage sale for a small bench-top drill press that would easily handle this work.

Great info from you all.
Two questions: why is the back plate always sheet metal? Couldn't it be aluminium of the same thickness as the rest of the body? That way I could draw it all in CAD, and have it perfectly lined up. And what about the blind holes, couldn't they also be part of the casting mold, and then I'd just tap them with a t-wrench?

[ThePracticalPeasant]

Two questions: why is the back plate always sheet metal? Couldn't it be aluminium of the same thickness as the rest of the body? That way I could draw it all in CAD, and have it perfectly lined up.

Price I suspect. And also strength. 20ga would suffice if steel, I suspect you'd need closer to 1/16" with aluminum. This also means the recess for it needs to be deeper...

And what about the blind holes

Tapping a blind hole is no problem so as long as it's deep enough...

couldn't they also be part of the casting mold, and then I'd just tap them with a t-wrench?

Though I have personally never cast any aluminum, I doubt you'll be able to create a sand mold with enough strength/detail to reliably cast the required hole: likely under 1/8" in diameter, at least 1/2" deep, with walls accurate enough to run a tap into. Not only will that be an exceptionally delicate shaft of sand, I suspect the grain size exceeds the thread-pitch, meaning the sand itself will create inconsistencies that are greater than the size of the thread being cut, requiring the hole to be drilled out anyway. That said, maybe with lost-PLA casting, but a 3D print per casting seems pretty expensive and I doubt would pay back compared to doing it "my way":

Again, assuming it goes together like a BOSS compact stompbox (and assuming you're going to be making these in bulk in some way):

Cast your "top" with big thick corners. Mark the hole positions on one back cover plate and set it in the correct position into/onto the top part. Clamp the parts if you don't have a rabbet to receive the plate. Drill the (four?) holes, undersized, through the cover and as deep as possible into the casting. Stack up your full supply of back-plates with the now-drilled one on top, to gang-drill them all with exactly the same layout. The top one is your master, use it to drill the pilot holes in the rest of the castings. Size-up to the nominal diameter of your machine screws and drill out the rest of the back plates and chamfer the holes. Grab the drill per the tap chart for your machine screws and drill then tap the castings.

No reason you can't have the back-plates laser-cut with the holes drilled and chamfered, or use aluminum if for some reason it works out easier/cheaper for you...

A few things (depending on your experience you may or may not find this useful):
CA glue is your friend when working with metal; the tiniest drop to hold parts together, a bit of heat and they come apart. It might help to make a plywood jig the casting fits into upside down to give it a nice flat face for it to rest on while working. When gang-drilling steel (and possibly aluminum, though I don't know as I have far less experience), if the parts are not pressed firmly together, burrs will develop and push the parts apart which leads to bit wandering, especially with small drill sizes, which in turn leads to an inaccurate representation of the drill pattern, especially deep in the stack. Now that I think about it, making a small plywood box in which two plates can be placed stacked up, might ultimately be faster and more accurate than trying to drill them all at once. If you do have the backs CNC'd in some way, I'd cut/order one that was drilled under-sized and not chamfered for use as a drilling template, as the deeper and straighter a hole, the easier it is to replicate it on the next part...

/two cents

[Phend]

Sounds like fun and a learning experience, I sand casted Al in high school.
You will need a casting box, sand (not playground sand), sprue and riser pins, mallot.
A graphite crucible, a propane oven, crucible lifting tool, gloves, face gear, apron.
Extra set of hands, milling machine to level the bottom, and more.
We cast block and rod shape items, never did thin wall parts and tap holes are not castable.
I would certainly start by casing a simple block.

[Tobyk]

Sounds like fun and a learning experience, I sand casted Al in high school.
You will need a casting box, sand (not playground sand), sprue and riser pins, mallot.
A graphite crucible, a propane oven, crucible lifting tool, gloves, face gear, apron.
Extra set of hands, milling machine to level the bottom, and more.
We cast block and rod shape items, never did thin wall parts and tap holes are not castable.
I would certainly start by casing a simple block.

Two questions: why is the back plate always sheet metal? Couldn't it be aluminium of the same thickness as the rest of the body? That way I could draw it all in CAD, and have it perfectly lined up.

Price I suspect. And also strength. 20ga would suffice if steel, I suspect you'd need closer to 1/16" with aluminum. This also means the recess for it needs to be deeper...

And what about the blind holes

Tapping a blind hole is no problem so as long as it's deep enough...

couldn't they also be part of the casting mold, and then I'd just tap them with a t-wrench?

Though I have personally never cast any aluminum, I doubt you'll be able to create a sand mold with enough strength/detail to reliably cast the required hole: likely under 1/8" in diameter, at least 1/2" deep, with walls accurate enough to run a tap into. Not only will that be an exceptionally delicate shaft of sand, I suspect the grain size exceeds the thread-pitch, meaning the sand itself will create inconsistencies that are greater than the size of the thread being cut, requiring the hole to be drilled out anyway. That said, maybe with lost-PLA casting, but a 3D print per casting seems pretty expensive and I doubt would pay back compared to doing it "my way":

Again, assuming it goes together like a BOSS compact stompbox (and assuming you're going to be making these in bulk in some way):

Cast your "top" with big thick corners. Mark the hole positions on one back cover plate and set it in the correct position into/onto the top part. Clamp the parts if you don't have a rabbet to receive the plate. Drill the (four?) holes, undersized, through the cover and as deep as possible into the casting. Stack up your full supply of back-plates with the now-drilled one on top, to gang-drill them all with exactly the same layout. The top one is your master, use it to drill the pilot holes in the rest of the castings. Size-up to the nominal diameter of your machine screws and drill out the rest of the back plates and chamfer the holes. Grab the drill per the tap chart for your machine screws and drill then tap the castings.

No reason you can't have the back-plates laser-cut with the holes drilled and chamfered, or use aluminum if for some reason it works out easier/cheaper for you...

A few things (depending on your experience you may or may not find this useful):
CA glue is your friend when working with metal; the tiniest drop to hold parts together, a bit of heat and they come apart. It might help to make a plywood jig the casting fits into upside down to give it a nice flat face for it to rest on while working. When gang-drilling steel (and possibly aluminum, though I don't know as I have far less experience), if the parts are not pressed firmly together, burrs will develop and push the parts apart which leads to bit wandering, especially with small drill sizes, which in turn leads to an inaccurate representation of the drill pattern, especially deep in the stack. Now that I think about it, making a small plywood box in which two plates can be placed stacked up, might ultimately be faster and more accurate than trying to drill them all at once. If you do have the backs CNC'd in some way, I'd cut/order one that was drilled under-sized and not chamfered for use as a drilling template, as the deeper and straighter a hole, the easier it is to replicate it on the next part...

/two cents

Thanks a bunch, good stuff!

[ThePracticalPeasant]

Thanks a bunch, good stuff!

No problem; I'm not much help with the electronomagical parts of all this, but I do have a fair bit of experience with the wood/metal working side of things... ask away and I'll do my best...

Another option, took me about six hours, is 11ga steel sheet and a FCAW/GMAW welder... and an angle-grinder and a cheap 10" disk sander to cut all the parts... and a die-grinder...

Edit: I just realized you seemed to imply that aluminum allowed CNC, does that mean you possess a router capable of cutting aluminum? if so... I'd build a jig to hold the casting in a repeatable position. First run a program to clean up the back of the casting and cut the step for the back plate then manually place the plate onto the step with a couple drops of CA glue and run the drilling and chamfer programs.  I suspect that given the thicknesses at play, you could drill to the tap size then cut the countersink deep enough into the backing plate  to widen the hole to fit the nominal size of the screw, without requiring the second drilling steps and associated risks or removing the backing plate from the casting that serves as it's fixture. Touch the plate with a torch to burn the CA glue, lift the lid off, and run the tapping program (or tap manually). Remove the casting, pair it with its lid, and repeat the whole thing as necessary.

[PRR]

sand (not playground sand)

No, special sand.

When I was young this was fairly easy: there was a casting shop in every third town. Go in with a strong bucket, say it is a school project. Today: as it happens there was a boat propeller rebuild (polish your erosion to recurve the whole prop) shop a few towns over from here, but I think they just moved south.

You might do bad work mixing mason sand with molasses?

[ThePracticalPeasant]

You might do bad work mixing mason sand with molasses?

hahaha!

It's slightly overpriced, but you can buy Petrobond on Amazon...

[Phend]

Do not forget to include you logo/name on the "pattern" , remember to make it mirrored, maybe.

[Tobyk]

Do not forget to include you logo/name on the "pattern" , remember to make it mirrored, maybe.

Had that in mind. Don't believe it need to be mirrored in the model, it will be in the sand though!