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Screw Size for TO-220 Heatsinks?

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andrewortman

New Member
Hello,

I'm kind of desperate. Heh.

I'm looking for the screw size for mounting this heatsink: Standard Products Found

to a TO-220 package. I've googled like crazy and my brain is now exhausted of energy to keep looking. If anyone knows the "correct" screw size (something that fits in that .150 inch hole) and how long it should be, it would be great. I found sizes such as 6-32 and M-3, but Im not sure which one I should get. Also, I can't seem to find any of the M-3 screws on Mouser, so I'm not sure if I found the correct size.

If anyone knows of a Mouser Part number, or simply what kind of screw I should get, I will love you forever.

Thanks,
Andrew
 

Boncuk

New Member
I've used 6-32 screws with the LM7805 and it's a good fit.
Hi Bill,

I'm not familiar with inch based screw definitions, but I assume that a 6-32 screw is 6X1/32 of an inch which equals 4.7625mm. The data sheet's info on the drill hole size is 4mm dia.

Using an M3 screw there is enough space for an isolating ring (if the IC has to be mounted isolated on the heat sink)

Boncuk
 

Dean Huster

Well-Known Member
Use the 4-40 if you need to insulate things with spacers and/or shoulder washers. I also found that the 6-32 works great for TO-220 packages.

A #6 screw has a major diameter of 0.1380 and has 32 threads per inch (a thread pitch of 1/32 inch). The nearest metric screw that should would would probably be M3.5x0.6 or down-size a bit to an easier-to-find M3x0.5. The "number" screws of U.S./English threads have no direct relationship to actual diameter other than the fact that the larger the number, the larger the diameter. A #10 screw has a major diameter of 3/16-inch while a #12 is 7/32-inch.

What's really disgusting here (well, maybe it really isn't) is that all the calculators at my disposal here at the computer are old enough and have been in storage and have intermittent keyboards. I had to pull out my 10-inch Post Versalog slide rule to make the above metric-to-English conversions. I guess I need to keep polished up on that baby. You never know when some nuclear burst will render all our electronics worthless. (And my bamboo slide rule charcoal.)

Dean
 
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Speakerguy

Active Member
Actually there is a formula, but it's hard to remember. I think each screw size goes up .013" from the last, but it starts with 000 size at .034 or something. I always end up just looking it up in a table.
 

Dean Huster

Well-Known Member
At first, I thought you were nuts, Speakerguy, because like you, I always use the standard machinists' table. But you are correct. The diameter advances by 0.0130 between steps and 0.0160 between double steps. I'm sure sizes are smaller than #0 simply because the teensy screws used in watches and meter movements are smaller than that. But most modern charts stop at #0. Number sizes move in single units until you hit #6 and after than they move by 2. So the standard charts list 0.1,2,3,4,5,6,8,10,12 and then move on the using the actual dimension, usually in fractions of an inch. Once you hit the fractional major diameters, they increment by 0.0625 (1/16 inch), of course.

Dean
 

Boncuk

New Member
At first, I thought you were nuts, Speakerguy, because like you, I always use the standard machinists' table. But you are correct. The diameter advances by 0.0130 between steps and 0.0160 between double steps. I'm sure sizes are smaller than #0 simply because the teensy screws used in watches and meter movements are smaller than that. But most modern charts stop at #0. Number sizes move in single units until you hit #6 and after than they move by 2. So the standard charts list 0.1,2,3,4,5,6,8,10,12 and then move on the using the actual dimension, usually in fractions of an inch. Once you hit the fractional major diameters, they increment by 0.0625 (1/16 inch), of course.

Dean
Who the heck invented that system? Metric is much easier!
 
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