Minimum metal thickness for machine screw threads?

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I'm trying to fab up a 2 piece mounting plate for a lights and siren controller head unit to go into an overhead console. I need the inner piece to attach to the outer piece but won't have access to the back so can't use screws and nuts. The outer one is 2mm thick. Is that thick enough to drill and tap M4 threads for M4 screws? There will be four mounting holes and the head unit I'm mounting weighs under half a pound.
 
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OP I mess around with old gm pick ups. I can tell you that GM used a lot of what I call nut plates on different things usually under the dash. They usually are just an 11 gauge or 10 gauge oval washer that is tapped and spot welded in place. 10-24 is real common and even 1/4-20 under the dash. Can you use a nut sert? There are several different clips that could be used in your situation as well.
 
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Assuming M4x0.7 you’ll have about 2.86 full threads of engagement through a 2mm thick piece of material. If it’s going into metal, I think it’ll be okay just don’t go crazy tightening them. If it’s plastic, I’d worry about it yanking the threads out over time with vibration.
 

D60

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I agree w @Skippy722. The main bummer as noted is that it can be easy to strip then you're done without stepping up a size.

Nutserts are always a good option but require a relatively large hole. I use the AP drill attachment, but not sure it even goes as small as M4.

If you wanna go more crude you can get tapered self-drilling "stitch" sheetmetal screws, assuming you have the depth available on the backside of the mounting surface. These things hold surprisingly well as demonstrated by every time I plug/unplug into my surface mounted 6-50R's in the shop for plasma and welders
 
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I think if I was to do this, I would tap the hole as carefully as you can because you don’t want any wobble with the 1st thread during bolt engagement. The thru hole has got to be perfect and maybe no chamfering to maximize thickness. Dry fit a screw and get a sense how much torque you can apply. If it starts to get too easy and the threads are letting go, no worries. Redrill the hole oversize for clearance fit for the m4 fastener and jb weld a flange nut on the outer plate to create your threads.
 
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Might be a good opportunity to use some sort of stud that you could loctite in then (carefully) torque nuts from the front of the unit.
 

atikovi

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I was able to tap it successfully and it mounted fine.

Before with an inner backup plate barely visible,

roof interior.JPG


That opening was too tall for the new controller so I had to mount the new controller's mounting plate to a new large blank mounting plate that I had to cut the center out of.

IMG_3148.JPG


The inner M4 x 5mm socket head cap screws go into the tapped threaded holes behind them. Turned out cleaner than I expected.
 
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