Attach Lexan to Steel Security Door - Need Help

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Old 03-23-12, 07:16 AM
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Attach Lexan to Steel Security Door - Need Help

Hi all. Not a frequent poster but I need some help. I need to attach a sheet of Lexan plexiglass to a steel security door that is made of 1/8" angle iron and 1/2" square steel rods. The purpose of the Lexan is to create a shield to prevent someone reaching through the bars to open the deadbolt lock from the inside. Inside lock will be a thumb-turn for safety in case of fire.

I want to screw through the Lexan into the steel from the inside so that the screw heads are not accessible from outside. Tried already with self-drilling screws (#8 x 3/4"). They would drill into the steel but will not cut threads or grab - probably the steel is too thick.

Does anyone have any suggestions for the right screw type and how to use it?

Thanks!
 
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Old 03-23-12, 07:42 AM
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I imagine you need a self threading or thread cutting screw.....not a self drilling.

Thread Cutting Screws, Pan & Hex Wash Slotted Cutting Screws
 
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Old 03-23-12, 08:00 AM
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Thanks. So, looks like I'll need to drill pilot holes in the steel and then insert thread cutting screws through the Lexan and into the steel.

Any idea what size drillbit to use for the size screw - let's say #6 or #8 screws?
 
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Old 03-23-12, 08:10 AM
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You might take it a step further and run a tap through each hole. Having a tapped hole will allow you to judge the pressure better and avoid cracking the Lexan. Use a touch of thread lock and they will stay put.

Also, if memory serves me, Lexan is different from plexiglass. Plexi is somewhat brittle and not as secure as Lexan which tends to bend and stretch without breaking.

Bud
 
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Old 03-23-12, 08:33 AM
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I really don't know about the drill size....but it will be just very slightly smaller than the outer thread size. If too small, they will snap.

Tapping with a bottoming tap (flat bottom) would be the best, but that would take a while.
 
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Old 03-23-12, 09:02 AM
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Yes, you're right. I just used "plexi" as a generic term but it's actually Lexan I will use.
 
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Old 03-23-12, 11:20 AM
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Any tap and die set will tell you the corresponding drill bit to use with any given tap.

For pre-drilling a #6-32 screw you will need a 7/64 drill bit. Im not sure what size for an #8
 
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Old 03-23-12, 11:57 AM
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Thanks. I see now that Irwin sells matching taps and drill bits in a package. I'll grab one of those.
 
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Old 03-23-12, 06:38 PM
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The pilot holes you use in the lexan need to be drilled a LOT larger than the screw, and the screws (I'd recommend a truss head screw) need to be left a little bit loose so that the lexan can expand and contract. A "little" clear silicone wouldn't hurt between the lexan and the metal as a gasket.
 
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Old 03-24-12, 03:06 AM
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If it is a security door, you may want to consider one way screws and loctite. You won't bear down on the screw too much as mentioned, and the loctite will keep the screw in place. As an example One-Way Slotted Security Fasteners: Tamper Resistant Screws
 
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