Expanding TO-3 Holes/ Dremel question

I've got to mount some TO-3 devices on my heatsink. The heatsink is pre drilled for TO-3 devices, and also has threaded mounting holes to mount the TO-3 with #6 screws.

The problem is that there are no shoulder washers that fit a TO-3 hole and which are wide enough to pass a #6 screw. All of them are for #4 screws. The heat sink's original use had the TO-3 devices uninsulated from the heat sink with #6 screws.

Thus, to insulate the TO-3 from the heatsink, I came up with the idea of making the TO-3 holes bigger- so I can use standard shoulder washers designed for #6 screws.

I've got a tiny round hand file and recently borrowed a dremel with lots of attachements/tips, seems like the dremel would do the job a lot easier than hand filing 16 devices. I've go no experience using this except that I know not to apply pressure- let the tool do the work. It has var speed up to 30,000 or 40,000 RPM or so.

I've tried this attachment or tip- not sure of the correct name:-


All that happens is that the thing wears out as soon as you contact it to metal, even lightly. I'm wondering:

(a) will it hurt the transistor to dremel or file its holes to make them larger (will the die detach on the inside of the package?)

(b) what dremel tip/attachment would be good for this purpose? Some common ones are here: link to dremel tools/tips

Hi Igreen,

Forget that. Oversize the holes and do a deep countersink then add overlength plastic tubing of the right diameter to take your screws. With a bit of care the tubing will take up the space the countersink generates and provides the insulation. A thickwall tubing would be best. Test for no shorts.

Not the best but workable at a pinch.

Sorry, I misinterpreted.

You want to use the existing tapped mounting holes on the heatsink, with your #6 screws/ It may not be possible. Something has to give. Maybe you can oversize (drill out) the holes on the To3 case to suit a slightly larger shoulder washer.

Or you can lose the threaded holes and drill out the heatsink so you can sleeve and insulate down there.


I was thinking about what you said and maybe I misinterpreted.

Forget that. Oversize the holes and do a deep countersink then add overlength plastic tubing of the right diameter to take your screws

If you are saying oversize the #6 threaded holes with a countertap drill bit and use a countersunk screw with heat shrink protecting the periphery of the screw- I think that might work.

But yes, my idea was to make the screw holes on the Transistor larger and use a standard shoulder washer. any other ideas-- no drill press is avail to me.
Widening holes in aluminum is a lot easier with a dremel than trying to clear holes in steel TO3 casings, i think.

Deburring with a Dremel is nice, or drilling holes in pcb's, for steel the power of Dremel's falls short.
I blueprinted my alloy TH700 automatic transmission casing with a Dremel, any other stuff i tried gave the same result as you describe: worn bits in 15 seconds.
Likely the TO3's will become too hot if you try to widen their holes with a grinding tool, dont think it matters to the Die whether it kills itself or a Dremel bit cooks it.

Easiest way without a drill stand,imo, is to widen the holes in the heatsink with a driver and a hammer, with a small block of wood or metal on the other side to keep the pressure in the area of the hole, and grind the edges with the Dremel afterwards.
Filing them wider by hand requires vast filing experience, likely that the holes will not line up afterwards.
You could ask Magura for ideas, he is a professional toolmaker.
Re: ??

lgreen said:
any other ideas-- no drill press is avail to me.

You might use non metal screws, but I don't think they would be strong enough to apply enough pressure. If you don't have drill press, you may rent one from Home Depot or similar outlets. I don't see a way of enlarging holes in aluminum without proper equipment (hand drill being a minimum requirement)
What about a clamp

Today, in response to the post by Davesaudio, I tried a nylon
screw- it couldn't be that easy could it?

The nylon screw can ony provide so much force until it slips and you have to retighten it. And so on...and so on.
I am not convinced the 6-32 gives sufficient force though its
better than 4-40 which gives almost no force.

But what do you think about using the nylon screws in
conjunction with a metal bar clamping over the top of the TO-3
little tophat dealie

Kind of like my Gainclone clamp, below?
(gainclone here: parttimeprojectswebsite)


The nylon screws could hold the TO-3 in place and a metal bar on the tophat could press the TO-3 to the sink? No?

Do you have a tapered reamer - they're ideal for enlarging the hole on the TO3s. You have size 6 shoulder washers?

I would definately NOT use nylon screws - they are simply not strong enough and, if you overtighten them the heads spin off and you have to drill out the threaded part and virtually retap them to remove the remaining nylon. It was my nightmare once just with some TO126's.

Re: What about a clamp

lgreen said:

Ah ha. Another 750K jpg to complain about. I'm going to have a friend of mine with broadband write a little web page that downloads that pic over and over again for a whole week while he sleeps. When you get your next server bill for Terabytes of downloads you'll think twice about posting such large graphic files.