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Old 9th December 2010, 12:56 AM   #1
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Default Drilling and tapping Conrad heatsinks

I'm about to drill and tap to mount my F-5 IRF MOSFET's to my Conrad heatsinks. I had planned on drilling (and tapping) for 6-32 mounting hardware, but NOT all the way through the base of the heatsink. (Didn't want the mounting screws to be visible among the cooling fins of the heatsink). I'm having a hard time finding a "bottoming tap" that will allow me to tap the threads all the way to the bottom of the drilled hole. I'm to the point where I'm considering drilling "through holes" through the base and into the finned area, and using a conventional tap to run threads all the way to the fin area. (Not as esthetically pleasing, to be sure).

Your thoughts? How did you mount your MOSFET's? Partial threads in the base, or through-holes? I'd appreciate your thoughts, before I bugger up my $$$ heatsinks.

Ken
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Old 9th December 2010, 01:22 AM   #2
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Check out McMaster-Carr. They have hand taps for blind holes.

I like to use a drill chuck sans drill rather than a T-handle. The drill chuck makes it easier to turn the tap in a circular motion, so you are less likely to break a tap.

JJ
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Old 9th December 2010, 01:28 AM   #3
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McMaster-Carr buy a couple since you will probably break one or two.

Use a good lubricant (I've had good luck with WD-40) and if at all possible use a drill press urned by hand to keep the tap running straight.

Edit: JJ and I think alike.
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Old 9th December 2010, 01:41 AM   #4
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Default home made bottoming tap

Try grinding the end off a starting tap, and when grinding dip the tap in water every time you grind a bit off, don't grind until it changes colour of the tap or the hardness of the tap will be less. Make sure to leave about 3 threads on the end of the taper, tap the hole then grind off another thread off the end of the tap till there is one thread left. Use a tapping compound or a light oil. Make sure to clean out the hole every time the tap starts to get tight in the hole and start again till you get to the bottom. Full thread strength will be fine when at least 1 1/2 times the of the diameter of the screw is used(ie-1/2" bolt 3/4" deep). Tap all the holes then grind a bit off then tap all the holes again until done.
Hope this works out for you.
Peter C.
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Old 9th December 2010, 01:47 AM   #5
eyoung is offline eyoung  United States
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You Only need 6 threads for full torque ISA standard so look at your number of threads You need beyond the thickness of the transistor... Do you have a torque device that measures in inch/ounces or the equivalent newton metric???...32 tpi / 6 ~6 threads about 1/5 of an inch (.20 ")not very deep.
do the math... whats the max torque on a 6-32 machine screw???

OK I digress There a number Engineering types here that can give you more specifics the key thing here is , relax get it close use the largest screw head and the largest washer available to secure you device to the heat sink...
nuff said

sorry for the tyraid, Elwood
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Old 9th December 2010, 02:02 AM   #6
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All,

Thanks for the great advice. Probaby will go the McMaster-Carr route, and chucking it in my drill press to keep alignment true. Thanks, too--for the tip about "required depth" to give me a satifactory fastening. Re: "buy a couple because you'll probalby break one or two......" Oh, boy--something else to look forward to....!
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Old 9th December 2010, 02:08 AM   #7
eyoung is offline eyoung  United States
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If you need a tap buy 2 you always break one. Good luck
E
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Old 9th December 2010, 02:22 AM   #8
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I use self tapping screws for everything. Taps are a waste of money when you can buy screws like THIS

Follow the links for thread cutting screws - type 23.
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Old 9th December 2010, 02:39 AM   #9
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A self tapping screw would be a really good idea. You only need a few to several inch pounds of torque and most sinks are made of the lowest grade of extrudable alumium alloy and it usually has terrible machinability. Swallows taps by the handful. I suppose you'd have to have the Proper size hole for the screws to work well.

Last edited by Andrew Eckhardt; 9th December 2010 at 02:41 AM.
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Old 9th December 2010, 03:26 AM   #10
eyoung is offline eyoung  United States
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Yes you do...self tappers are a godsend I just can't bring myself to that point being a mechanic by trade. I will limit myself to machine threads>>>as I said before torque specs are infinitismal so... what ever feels good as long as the holes are perpendicular and provide aducate heat transfer, use a hammer and a nail if you can modulate the force needed to gain propper heat transfer...

Regards, Elwood
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