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ultrachrome 15th April 2003 02:48 AM

help...drilled hole too big
 
I inadvertantly drilled my heatsink with too large a bit. I found some aluminum sleeves at home depot that fit perfectly to shrink the size adequately enough to tap.

My question is how should I make sure they stay in there. They fit in snug by themselves. Can I use superglue? Or should I use epoxy. Epoxy might be messy because the sleeves fit pretty snug and most of the epoxy will end up being sqeezed out.

Peter Daniel 15th April 2003 03:22 AM

You might use superglue, but I'm not sure it will hold in a long run. Remember that when things get hot, the stress might break the glue bond.

If it's for an output device, one option could be going to the next size, tapping the hole properly and if necessary enlarging the hole in a device as well. This is the most appropriate fix. After all this is what is done in aviation as well, when somebody drills a hole too big.;)

Or insert the screw from the other side and use a nut if possible. There are also threaded inserts, which can be screwed in a bigger hole and are already tapped. What is the thread size?

JoeBob 15th April 2003 03:32 AM

I don't know how your setup is, but isn't the simplest solution (and therefore the best, no?) to just make a hole next to your original hole? Or can you not move your device. I broke a tapping bit in a hole, and it was impossible to get out, so I just made a new hole slightly to the side of the original hole.

karma 15th April 2003 03:39 AM

superglue
 
hmm thats y plane's fall apart in mid air :yikes:

just joking;)

peter's right enlarge the hole on the device as well

jleaman 15th April 2003 04:20 AM

this brings a question to my thought. Is it possible to dril the original hole in a ( irfp240 ) transister a slitly bit bigger will it hurt it at all. ?

I had a dead one and drilled it to the size i needed but didnt see any thing that could damage it just black plastic. could it work. ?

Peter Daniel 15th April 2003 04:26 AM

Depends how much bigger the hole, but I don't see it could damage anything inside.

mefinnis 15th April 2003 04:29 AM

I gather from your comment on "tapping" the sleeve that this is for a threaded metal screw to mount the FETs.

Best solution if you must use the same hole is to buy some "Helicoils" of the appropriate size.

These are a commercial product designed to fix your exact problem (actually designed for when you strip the existing thread).

http://www.hillcliff-tools.com/helicoil.html

You find a helicoil with the internal thread dimensions for your mounting screw. The coil will have a defined outer thread size which you then tap into the aluminium block, then insert the coil into the thread, then mount the screw.

I'll bet any money you like Boeing have used these before ;)

Regards

Mark

Peter Daniel 15th April 2003 04:33 AM

Now I remeber that product and if not me, somebody else I knew, was using it. If enlarging hole isn't possible this is indeed the best solution, although sometimes it's not practical waiting few days for the part to arrive in mail.;)

On the aircrafts, if it's not production but maintanance, the usual problem is with removing screws, as when they get older and the same are being used on and on, the slots become worn out and the screwdriver bit begins to slip. The only way is to either drill out the head or use easy-out. Next problem is when easy-out breaks and then high speed grinder with a special rotary file is used. But even then things may go wrong.;) Fixing worn out or broken threads is not a big problem, since nothing is tapped directly in aluminum and special anchor plates are always used and it's usually a matter of replacing such plate (which is riveted).

Apogee 15th April 2003 05:08 AM

You might consider using JB Metal Weld.

I would simply tap the hole the next larger size then fill it with JB. Then just redrill it the correct size and tap it.

The purpose behind tapping it the next larger size is so the epoxy has a decent surface to "bite" into. The threads will prevent the plug from pulling out of the hole over time. I don't think it would be a problem either way because you're really not loading it too much, but this way you know for sure that it will hold up to the thermal cycling... It's good to 300oC so I think you'll be safe... It's simply specialized epoxy with metal powder mixed it. Once it's set, it drills/taps/files just like metal.

http://www.jbweld.co.uk/files/prodb.html

You can get JB Metal Weld at almost any hardware store or Home Depot, Lowes, etc... The stuff really works!

I also found this interesting use for it on the overclockers forum for PC CPUs... It could be a cool way to attach output devices in an amp (as long as you're using a proven design)...

http://www.overclockers.com/tips169/

Good Luck,

ultrachrome 15th April 2003 05:15 AM

I'd upload a picture but I just had to reformat my drive and I'm not quite to the point of reloading camera software.

I'm building the AKSA 55W amp. It's a normal looking heatsink that has a shelf to mount devices. The heatsink will make up most of the back of my enclosure which is a cabinet from the sixties that you slide the component into.

Anyway, I drilled holes from the back that exited between the fins. The hole's diameter is equal to the fin spacing. This eliminates the possibility of using a nut unless I use a screw at least 3" long to clear the end of the fins. I thought of that. I also considered repositioning the holes but I didn't want to risk screwing up again and ending up with a swiss cheese heatsink.

I went ahead and pressed the sleeves in with some superglue. Two of them tapped great but one is slipping probably from WD40 residue from when I drilled it. So what I will probably do is use some plastic inserts like they use for running screws into concrete and use a self-tapping screw into that. Won't be pretty but since the heatsink is on the back, I won't tell anyone if you won't.

Metal work is so emotionally trying.:bawling:


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