self tapping screws

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domnic

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Self tapping screws. where in the 2011 CODE does it say I cant use a self tapping screw to connect a grounding lug ( egc ) to a metal box ect. ?
 
domnic said:
Self tapping screws. where in the 2011 CODE does it say I cant use a self tapping screw to connect a grounding lug ( egc ) to a metal box ect. ?
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250.8 but that isn't really what it says or doesn't say so read it and then if you have any questions, ask away.
 
Strathead said:
250.8 but that isn't really what it says or doesn't say so read it and then if you have any questions, ask away.
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A self sapping screw is not a machine screw or a thread-forming screw. by definition.
 
domnic said:
A self sapping screw is not a machine screw or a thread-forming screw. by definition.
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Curious by who's definition. Virtually every self tapping screw I have seen is also a thread forming screw. The old "sheet metal screws" were generally self-tapping but they weren't self drilling.
 
Strathead said:
How do you figure that? If I tap a hole I am literally forming the threads of the hole. So how could you look at a self tapping screw and say it isn't thread forming?
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We're talking about two different screw types, shelf tapping sheet metal screws (non-compliant) and self tapping thread forming screws (compliant). Sheet metal screws do not comply with the minimum threads per inch.
 
infinity said:
We're talking about two different screw types, shelf tapping sheet metal screws (non-compliant) and self tapping thread forming screws (compliant). Sheet metal screws do not comply with the minimum threads per inch.
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I respectfully get nit picky here. I know what you are getting at, but Thread forming screws that don't create 2 threads of engagement are non-compliant. There is zero that defines a sheet metal screw as having a thread pitch that does. And, I regularly see 6-32 modified truss head screws (to use the precise description) call sheet metal screws and since studs are sheet metal and these screws are used to screw them together, I deem the moniker accurate.

The codes says what it says to be clear and remove semantics from the picture.
 
infinity said:
We're talking about two different screw types, shelf tapping sheet metal screws (non-compliant) and self tapping thread forming screws (compliant). Sheet metal screws do not comply with the minimum threads per inch.
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I agree. this is what 250.8 says about it.

250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Equipment grounding conductors,
grounding electrode conductors, and bonding jumpers shall be
connected by one or more of the following means:
...
(5) Machine screw-type fasteners that engage not less than
two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less than
two threads in the enclosure
...
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very clearly a self tapping machine screw is allowed. no where does it allow a sheet metal screw.
 
Strathead said:
I respectfully get nit picky here. I know what you are getting at, but Thread forming screws that don't create 2 threads of engagement are non-compliant. There is zero that defines a sheet metal screw as having a thread pitch that does. And, I regularly see 6-32 modified truss head screws (to use the precise description) call sheet metal screws and since studs are sheet metal and these screws are used to screw them together, I deem the moniker accurate.

The codes says what it says to be clear and remove semantics from the picture.
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But what the code SAYS is "machine screws" very specifically.

(5) Machine screw-type fasteners that engage not less than
two threads or are secured with a nut
(6) Thread-forming machine screws that engage not less
than two threads in the enclosure
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So a sheet metal screw that is NOT a machine screw would not qualify. In order to ensure having at least 2 threads in 14ga sheet metal you need at least 24 threads/inch, if it's 16ga, you need 32 threads/inch. That is going to be a machine screw.
 
Jraef said:
But what the code SAYS is "machine screws" very specifically.


So a sheet metal screw that is NOT a machine screw would not qualify. In order to ensure having at least 2 threads in 14ga sheet metal you need at least 24 threads/inch, if it's 16ga, you need 32 threads/inch. That is going to be a machine screw.
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I don't think a sheet metal screw can be a machine screw. The way they are threaded is different. Machine screws are required to be either to the coarse (UNC) or fine (UNF) thread standard.

Sheet metal screws use what are called "spaced" threads. I could not remember what they were called so had to google it. i am not sure what it means but they are always coarser than UNC, IIRC.
 
Define:
The screws that look like drywall screws = no
The screws that look like 6/32 machine screws (can thread a nut on them) = yes
 
What matters is the thread count, and strictly speaking the material hardness. The screw tip doesn't matter.

In my experience people usually use the term 'self tapper' to refer to what would more strictly be defined as self-drilling screws, i.e the screw tip resembles a jobber type drill. I would add that I hardly ever have seem such drills with a code compliant thread count for bonding. And that is enough to say that usually you can't use 'self tappers'.
 
Strathead said:
I respectfully get nit picky here. I know what you are getting at, but Thread forming screws that don't create 2 threads of engagement are non-compliant. There is zero that defines a sheet metal screw as having a thread pitch that does. And, I regularly see 6-32 modified truss head screws (to use the precise description) call sheet metal screws and since studs are sheet metal and these screws are used to screw them together, I deem the moniker accurate.

The codes says what it says to be clear and remove semantics from the picture.
Click to expand...
Correct - if they don't create 2 threads of engagement they definitely fail NEC.

Jraef said:
So a sheet metal screw that is NOT a machine screw would not qualify. In order to ensure having at least 2 threads in 14ga sheet metal you need at least 24 threads/inch, if it's 16ga, you need 32 threads/inch. That is going to be a machine screw.
Click to expand...

Here is a screw that is self drilling and would have 24 thread/inch - should be acceptable to use in 14ga sheet metal.

Kind of long for what most of us would prefer most of the time but should be NEC compliant. Such screws can't be found much shorter than this one though, the design of this one probably couldn't be much less then 1 inch, look how long the drill point is and then the amount of taper where the threads start.
 
kwired said:
Correct - if they don't create 2 threads of engagement they definitely fail NEC.



Here is a screw that is self drilling and would have 24 thread/inch - should be acceptable to use in 14ga sheet metal.

Kind of long for what most of us would prefer most of the time but should be NEC compliant. Such screws can't be found much shorter than this one though, the design of this one probably couldn't be much less then 1 inch, look how long the drill point is and then the amount of taper where the threads start.
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Yep, I would agree, compliant (so long at it is going into 14ga steel). And that is what I would consider a "machine screw". The drill bit tip is just a bonus.
 
To me a sheet metal screw is one that has an expanding thread tip cross section rather than a tap configuration. It will bend (displace rather than cut) one or two thread grooves into the metal and the resulting metal surface will be distorted rather than flat.

A tap is designed to remove metal, not just displace it.

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jaggedben said:
What matters is the thread count, and strictly speaking the material hardness. The screw tip doesn't matter.

In my experience people usually use the term 'self tapper' to refer to what would more strictly be defined as self-drilling screws, i.e the screw tip resembles a jobber type drill. I would add that I hardly ever have seem such drills with a code compliant thread count for bonding. And that is enough to say that usually you can't use 'self tappers'.
Click to expand...

Look up 6-32 self drilling screw and you will see about 100 of those rare beasts.
 
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