Definition of a machine screw

[eawilliams]

2008 NEC. In 250.8(B) under methods not permitted, the use of sheet metal screws has been deleted. A new 250.8(A)(6) was added to permit the use of thread-forming MACHINE screws to be used. I think this is an unsafe deletion because I don't believe that a sheet metal screw is a machine screw. The thread-forming screw are those #8 and #10 green screw for fastening EGC to boxes now in use.

 

[raider1]

So what is the definition of a sheet metal screw?

I guess that I don't understand why you feel the change to the 2008 NEC is an unsafe deletion of the prohibition for the use of "sheet metal screws".

Chris

 

[petersonra]

2008 NEC. In 250.8(B) under methods not permitted, the use of sheet metal screws has been deleted. A new 250.8(A)(6) was added to permit the use of thread-forming MACHINE screws to be used. I think this is an unsafe deletion because I don't believe that a sheet metal screw is a machine screw. The thread-forming screw are those #8 and #10 green screw for fastening EGC to boxes now in use.

A thread forming machine screw is not a sheet metal screw. It is a machine screw with hardened threads that can act as a sort of tap, allowing you to drill a hole and not have to tap the hole before threading in the screw.

 

[480sparky]

A thread forming machine screw is not a sheet metal screw. It is a machine screw with hardened threads that can act as a sort of tap, allowing you to drill a hole and not have to tap the hole before threading in the screw.

Like these:

 

[nyerinfl]

The way I look at it, a machine screw has a flat head, a sheet metal screw is a tapping screw with a pointed head. Machines can be used for threaded purposes and with nuts and other threaded fittings, while a sheet metal is for straight drilling and not threadable (wood screws, self tappers, etc.). The rule I think is correct, as a threaded connection is a more mechanically secure connection than a drilled in screw.

 

[RAYMFL]

Sheet metal screw are generally tapered and coarse. That's what I have been taught...right or wrong. Referring to the NEC, to require a minimum thread pitch for screws or bolts may be a better approach. Don't have the 2008 code, but is there a requirement for 2 threads in contact with a certain gauge metal. Possibly could figure the TPI from that.

 

[infinity]

The rule I think is correct, as a threaded connection is a more mechanically secure connection than a drilled in screw.

I would disagree. I've installed many 10-32 green grounding screws and stripped some of them from over tightening in a metal box. A self drilling screw (i.e.-Caddy SM8) drilled through the box is almost impossible to strip and very hard to remove since it's extremely tight.

 

[eawilliams]

The problem is not with self threading, it is the two threads required to be in contact. #/32 as you know is 32 threads to the inch, which makes a requirement for two threads to be in contact and I believe that a sheet metal does not meet this requirement. Thanks

 

[infinity]

The problem is not with self threading, it is the two threads required to be in contact. #/32 as you know is 32 threads to the inch, which makes a requirement for two threads to be in contact and I believe that a sheet metal does not meet this requirement. Thanks

I agree, but IMO the requirement is that great to begin with since two threads in a thin metal box can easily be stripped during tightening. My guess is that this requirement keeps a lot of guys from making grounding connections with sheetrock screws.

 

[eawilliams]

In the examples BF and BT do not look like machine screws in what I have been shown to what is a machine screw.

 

[hillbilly]

Ever notice... on most new water heaters the grounding conductor is attached to the small JB cover with a green machine screw, and then the cover is attached to the heater with a sheetmetal screw?

Kinda defeats the purpose (IMO).

steve

 

[Twoskinsoneman]

2008 NEC. In 250.8(B) under methods not permitted, the use of sheet metal screws has been deleted. A new 250.8(A)(6) was added to permit the use of thread-forming MACHINE screws to be used. I think this is an unsafe deletion because I don't believe that a sheet metal screw is a machine screw. The thread-forming screw are those #8 and #10 green screw for fastening EGC to boxes now in use.

These are what I use.

"Self Drilling Excellent for drilling and tapping threads into a wide variety of metals and plastics." Discription from McMaster-carr

​

 

[GeorgeB]

The problem is not with self threading, it is the two threads required to be in contact. #/32 as you know is 32 threads to the inch, which makes a requirement for two threads to be in contact and I believe that a sheet metal does not meet this requirement. Thanks

In a much earlier job when I was doing fastener engineering, piercing a hole in sheet metal into a female die allowed a sufficient distance to thread and meet the multiple threads engagement. We specified minimum of 2, minimum of 40% of diameter of screw engaged if screw and "nut" were same material. For a 10-32, 2 threads is 2/32" (0.063), 40% is 0.075 so 40% controlled. Steel 0.040 thick was pretty easy to "draw" by piercing to get sufficient engagement. Even 0.032 could pass spec if done in good tooling.

 

[kbsparky]

These are what I use.

"Self Drilling Excellent for drilling and tapping threads into a wide variety of metals and plastics." Discription from McMaster-carr

WE call these TEK screws.

And I have used these to fasten things like ground bars, and grounding pigtails, etc. They hold quite well, IMO.

Do any of you believe that the Code prohibits the use of these?

 

[iwire]

Do any of you believe that the Code prohibits the use of these?

Yes, that type of Tek screw is a form of sheet metal screw (IMO) and prohibited under the current reading of 250.8.

You can purchase Tek screws with machine threads 10-32, 1/4-20 etc.

 

[peter]

The obvious thing I see is that the sheet metal screws have a flat portion along the shank whereas a machine screw doesn't. A cross section of a machine screw would show a series of 60? thread angles will little or no space at the roots.
I am disappointed in the Code panel's decision to eliminate the "use of sheet metal screws for grounding is prohibited" sentence. It is/was the one clear sentence in the entire code book. But then I suppose they had to follow the section of their textual standards manual which states "all sections and statements in this Code must be stated as obliquely and as obscurely as possible. Clear English statements are not to be permissable."
~Peter

 

[eawilliams]

After reading some of the posts it appears without the prohibition of the use of sheet metal screws some people are going to take it that is OK to use them. They are not checking the requirement for two threads to be in contact.