type of tapping screws

[MSU2000]

guys,

I'm getting all confused between sheet metal screws, thread forming screws, thread cutting screws, and self drilling screws. What are the differences and usage of such those screws, when to use it, how to choose it, are they interchangeable?

Can you shed some info on this.

Thanks!
MSU2000

 

[IFRs]

Sheet metal screws are normally seen in thin metal.
Thread forming screws tap their own hole, mostly I've seen machine threads.
Thread cutting screws I think are the same as thread forming screws, perhaps intended for different materials or thicknesses.
Self drilling screws make their own hole, then form threads as they cut into the hole.

 

[limct]

MSU2000,
the difference of thread cutting and thread forming is that the former cuts out threads (usually on hard material like metal, eg. ST screw) while the latter only form the thread (through deforming process on softer material, mostly plastic, eg Eco-Sync screw).
you may want to look into this website for infos of various fasteners

http://www.bossard.com/techinfo/en/index.html

Best regards,
ct

 

[IFRs]

Check out Fastenal.com technical info

 

[CoryPad]

Sheet metal screws are used for sheet metal, typically untapped holes. They have spaced threads, are not robust, but they are used widely.

Thread cutting screws cut internal threads into a mating part (typically metal). They usually are machine threads. They produce weaker internal threads because the threads are sharp and cut across the grain structure.

Thread forming screws form internal threads into a mating part (either plastic or metal). They typically are machine threads. These are stronger than cut threads because they don't cut across the grain structure, and they induce residual compressive stresses. Among all the options here, these are usually the only ones that can be used in a structural joint.

Self drilling screws, unlike the other screws, drill their own hole in addition to making their own threads. These need to be very hard to drill a hole in another material. Usually, they are used for non-robust joints, like sheet metal and thread cutting screws.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ajack1]

Do any screws really “cut” a thread in the same way as a tap does, actually produce swarf?

I know “self tapping” screws are used for internal trim in the automotive business these look similar to a wood screw, but do they cut or move metal?

In white goods (house hold appliances) again “self tapping” screws are used that look more like a standard bolt and usually fit in an extruded hole but again to the best of my knowledge they move metal rather than cut it.

 

[MSU2000]

Ok,

Thanks for the info.

1. Thread forming screw actually forms thread, that means they don't remove material like the thread cutting screw does.
2. Sheet metal screw do not intent to cut or form any threads in the mating material instead it "clamps" the two surface between the screw pitch.
3. Self drilling screw drills hole and cut thread to follow.

Am I close?

Thanks again,
MSU2000

 

[CoryPad]

MSU2000,

Yes, you are close.

ajack1,

Yes, thread cutting screws produce swarf.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MSU2000]

Ok, Let me take it a little furher.

When do I use Thread forming screw instead of sheet metal screw when joining 2 sheet metals. In this case, my sheet metals are .048" and .094" with countersunk for flat head.

Thanks
MSU2000