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Old 13th October 2009, 01:26 AM   #11
imix500 is offline imix500  United States
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Hey Conrad, not being a machinist- how do you order their taps? Looks like there is an "H" number that corresponds to how much tolerance you want in the threads, but what is a happy medium for your average hobbyist or non-production application?
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Old 13th October 2009, 02:22 AM   #12
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Originally Posted by KevinHeem View Post
You don't need anything fancy. Go to Lowes, Home Depot, or local hardware and get a 6-32 tap, 7/64 drill bit, and Tap handle. Grab some short (1/2") 6-32 screws, washers, and cutting oil.
Have Fun,
50 bucks later...
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Old 13th October 2009, 03:51 AM   #13
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H3 or H4 (or even H5) is fine- the larger the number the larger the tap and these are middle-of-the-road general purpose classes. Here's an example of what I might choose from MSC:
MSC Item Detail

BTW, if you buy cutting taps don't confuse spiral pointed with spiral fluted. You want spiral fluted for stringy gummy materials like heat sinks.

I may be barking up the wrong tree, but at least I'm barking!
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Old 13th October 2009, 04:08 AM   #14
T in AZ is offline T in AZ  United States
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Self-Clinching Captive Fastener or threaded insert is the way to go, no taping and you won’t have any problems with steel threads on an aluminum heat sink and if you get cross threaded just punch out the insert and replace it.
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Old 13th October 2009, 11:01 AM   #15
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More thoughts- as above, any way you can avoid the whole problem and not tap the gummy heatsink is good. I like molded sockets for TO-3 devices so all you need are some holes with the right spacing.

One of the first things they do with soft aluminum parts on a race car is drill out the threads, re-tap and install Heilcoils. I've also seen this done with heatsinks. Gets expensive. Gives you an almost indestructable thread- great if you blow a lot of output devices.

More on the Balax forming taps- be sure to get their drill size chart and stick to the larger end of the range. The top of the thread will have a "butt crack" where the metal is displaced. If this is too wide, use a smaller drill so there's less tendency to cross thread the fasteners. Too narrow and you risk breaking the tap, so usually one sneaks up on it in a production situation. I've never had to deviate from the published chart.

There's a certain joy in accurately locating holes so the parts fit together and the screws just spin in. It's how you separate good and bad metal workers. The method is simple and once you learn never to skip a step, works every time. 1) Using a scribe holder with a machinists scale, or height gage, lay out the hole location. Use Dykem Blue or a Magic Marker prior to scribing if you need better visibility or don't want to put deep scratches in the metal. If you don't have a surface plate, use a sheet of plate glass. 2) Center punch the marks. Check with a magnifier and readjust as necessary by striking again with the punch at an angle. Yes, you can "move" a center punch divot. 3) Start the hole using a short solid center drill that picks up on the mark. Use oil. Never ever skip this step. 4) Now, drill the hole! Use oil. The hole should be within a few thousandths of where it's supposed to be.

I may be barking up the wrong tree, but at least I'm barking!
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Old 13th October 2009, 11:59 AM   #16
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Originally Posted by mrwireless View Post
Can anyone suggest how best to attach circuit boards/transistors to an aluminium heatsink? Obviously some sort of screw is needed but i don't want to use self-tappers because i know it's going to get cross threaded at some point. Should i be using machine screws? Is it best to tap the hole first? I'd like a reliable fix that can be done/undone without degrading. Any advice much appreciated, thanks.
I have always used M3 screws like DIN 84. Drill a hole, countersink it, use a M3 screw tap and ready...
The equipment is reasonable cheap, but you need some basic metal working skills.
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Old 13th October 2009, 12:48 PM   #17
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Default Threaded Holes

Can't see that this has been mentioned, but I use thread forming screws. These are different in that they in effect, forge a thread, in a plain drilled hole, as opposed to a self tapping screw which cuts one. The resultant thread is stronger and suitable for use by standard screws as well. As with all standard tapping it is important to have an accurately drilled hole of the correct size .
Taptite is one well known trade name.

REMINC CONTI TAPTITE® Thread Forming Products

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Old 15th October 2009, 03:57 PM   #18
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If you don't have a surface plate,
What kind of DIYer wouldn't have a surface plate? I just stick the heat sink in my CNC mill to spot drill, drill through and tap.
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