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K-amps 13th September 2020 03:56 AM

What are these screws called?
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What are these screws called and where can I get them? They are used in securing output transistors on heatsinks. They seem much more user friendly than the machine screws i have been using. Those need to be tapped, which sometimes ruins the sink plus takes a long time, These on the other hand are just drill and screw types, no need to tap them ... I know they are self tapping, but what kind or where can I order them?

rayma 13th September 2020 04:05 AM

Sheet metal screw. You drill a hole somewhat less in diameter than the screw, and it cuts threads as you turn it.

Pilot-hole sizes:
size-4, 3/32 inches
size 6, 7/64 inches
size 8, 1/8 inches

K-amps 13th September 2020 05:17 AM


Originally Posted by rayma (
Sheet metal screw. You drill a hole somewhat less in diameter than the screw, and it cuts threads as you turn it.

Not the same kind. The ones I photographed have larger threads, and are slotted. They are not home depot types.

Vovk Z 13th September 2020 06:03 AM

I guess they are possibly self drilling sheet metal screws?

simon7000 13th September 2020 06:39 AM

Self tapping screws.

Self-tapping screw - Wikipedia

leadbelly 13th September 2020 07:10 AM

What’s the Difference Between Self-Drilling and Self-Tapping?

JMFahey 13th September 2020 09:47 AM

All "sheet metal" screws are "self tapping" to a point because they are not constant diameter like a "mechanical screw" or "bolt" or plain "screw" intended for a pre threaded hole or nut, instead nthet "startb thinner" so they can oush metal/plastic/wood apatrt.

Easier to do on sheet metal because of not much metal being involved, so itīs relatively easy to push it aside and forming a coarse thread.

Now if sheet metal is thicker or material is not easy to push aside (it can crack) then, as in your example, tip is less "conical" but a slot is ground, so a sharp edge is formed, it can *cut* the base material instad of simply shoving it aside, now your self tapping screw becomes "self threading self tapping screw", it actually works like a built in screw tap.

These are called self tapping screws but they only work on sheet metal or relatively soft plastic (or wood):

In my book *these* are the real/full self tapping screws, notice they are way less "conical" (although always there is some taper) but they have a special tip where you have a material cutting edge:

Wikipedia distinguishes between them as:

Thread-forming vs. thread-cutting
Self-tapping screws can be divided into two classes;[3] those that displace material (especially plastic and thin metal sheets) without removing it are termed thread-forming self-tapping screws; self-tappers with sharp cutting surfaces that remove the material as they are inserted are termed thread-cutting.
First ones are cheaper and ubiquitous, material cutting ones are more specialized and somewhat harder to find.

Name alone can be confusing since lots of people call all the same, which they are not.

If you want to buy some, browse catalogs carefully and specify the right code.

K-amps 13th September 2020 11:40 AM

Thanks JMF, So I should be searching for Thread cutting screws...

K-amps 13th September 2020 05:57 PM

Anything I find that is self threading, is #6-#8 or larger. I probably need something in the #3 or #4 width that will go through TO-220 cases.... Must be a specialty screw.

russc 13th September 2020 06:57 PM

Search for 'Cross Cut Tail Tapping Screws'.
Phew, that took some finding!

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