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Old 1st Oct 2020, 12:37 pm   #1
astral highway
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Default Drilling large finned heatsink

I wish to mount an IGBT module (half bridge) on a large heatsink.

If I align two of the mounting holes above the thin part of the heatsink, the other two are above one of the fins. It means I would have a difficult time drilling and tapping that side.

I want this to be:

1) effective
2) perfect

Any hints or ideas please l? Assume I have only basic tools and facilities , no pillar drill for me, only a battery Makita
I would be grateful for tips on what kinds of bolts to use - self-tapping ideally ?
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Old 1st Oct 2020, 12:55 pm   #2
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Default Re: Drilling large finned heatsink

If you want two holes to line up the only way to be certain they will is to drill them together. Not strictly true, but I do it whenever I can.

Use a drill the same size as the holes in the module to spot drill one of the mounting holes to a shallow depth, then use the correct tapping size drill and follow up with tap or taps in the case of a blind hole.

Mount the module with a single screw, line it up correctly, then drill and tap the remaining holes in a similar manner.

It's important that the holes be drilled at 90 degrees to the heat sink's surface. When I had to drill holes by hand I found it useful to have an assistant to monitor the drill's angle in a fore and aft direction while I looked after the side to side direction. It's easier to use a powered hand held drill as you can concentrate on the drilling angle more easily whilst not turning a handle.

When tapping I find it easiest to hold the work piece in one hand and the tap wrench in the other. Then you can rotate the work piece by 90 degrees to make sure the tap is entering at 90 degrees in all directions.

I can do the job for you if you're prepared to pay the postage both ways.
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Old 1st Oct 2020, 1:59 pm   #3
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Default Re: Drilling large finned heatsink

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Originally Posted by Station X View Post
I can do the job for you if you're prepared to pay the postage both ways
Hi Graham,

Thanks for that detailed description. It's really helpful. I think, given the value of the module and the heatsink, I'll take you up on your kind offer for you to do the job.

I'll PM you now.

Thanks!
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Old 1st Oct 2020, 6:17 pm   #4
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Default Re: Drilling large finned heatsink

Stick it on with "output", https://cpc.farnell.com/loctite/outp...315/dp/SA00606 good idea until I saw the price.
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Old 1st Oct 2020, 6:32 pm   #5
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Default Re: Drilling large finned heatsink

I had a look at the datasheet. 'Self-shimming 5-6 mil' must mean it has 0.005" thick particles in it. Fairly conductive thermally, and a breakdown voltage of 26kV/mm

OK, so what's a good thermal conductor, a good electrical insulator and so**ing expensive?

What comes to mind is either 'A girl's best friend' or that BeO stuff Bang and Olufsen didn't make.

Odd they don't mention what it is. I think they'd have mentioned the former, I'm not sure that the latter needs warnings if it's not in a form that could be inhaled.

Sufficiently expensive that it's academic. I was just curious enough to look.

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Old 1st Oct 2020, 6:38 pm   #6
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Default Re: Drilling large finned heatsink

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Originally Posted by merlinmaxwell View Post
Stick it on with "output", ... good idea until I saw the price.
Wow. That is a cheeky little price!
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Old 1st Oct 2020, 6:40 pm   #7
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Default Re: Drilling large finned heatsink

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Sufficiently expensive that it's academic...
Yes... I'll pass on this one. I wonder who buys this?
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Old 1st Oct 2020, 7:09 pm   #8
David G4EBT
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Default Re: Drilling large finned heatsink

Data sheet says:

Quote:

‘Must be stored at between 2 and 8 degrees C. Below 2 or above 8 C adversely affects the properties’.

Unquote.

No wonder they’ve only got one in stock! (Shelf life?).

Screws don’t have a limited shelf life and when components are screwed onto heat sinks they don’t come unstuck.
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Old 1st Oct 2020, 8:14 pm   #9
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Default Re: Drilling large finned heatsink

Has anyone addressed the OP's original requirement:
If I align two of the mounting holes above the thin part of the heatsink, the other two are above one of the fins. It means I would have a difficult time drilling and tapping that side.

I want this to be:

1) effective
2) perfect
This would need a short length of ali channel to mount the device above the heatsink: the channel can then be screwed through onto blind holes anywhere on the heatsink
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Old 1st Oct 2020, 8:20 pm   #10
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Default Re: Drilling large finned heatsink

I had assumed that the OP's problem was deep drilling into the fins, the holes in the thin part of the heat sink being drilled right through.

I'll find out when I get the job and can use channel or W.H.Y if necessary.
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Old 1st Oct 2020, 8:36 pm   #11
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Default Re: Drilling large finned heatsink

I've never liked fixing devices [whether semiconductors or conduction-cooled power-valves] to heatsinks using tapped holes; my preference has always been for the fixings to be free-floating within the heatsink through-holes (so as to allow the hot-device to 'settle' in good contact with the heatsink) and use nuts-and-bolts.

In the past I've had inconvenient fins milled away so as to provide access to a nut-runner so a hot-device can be properly bolted-down.

Also - side-note - properly torque the fixings: don't just 'do it up till it feels right', use the device-manufacturer's recommended torque. Getting it right can reduce the thermal-resistance of the device-to-heatsink interface by a good 10%,meaning cooler runningand longer life. [Equally, nothing good's been reported about cracking a Beryllium-Oxide thermal-conduction 'slab' because your technician thought he could do-it-without-a-torque-wrench]
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Old 2nd Oct 2020, 7:36 am   #12
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Default Re: Drilling large finned heatsink

Quote:
Originally Posted by David G4EBT View Post
Data sheet says:

Quote:

‘Must be stored at between 2 and 8 degrees C. Below 2 or above 8 C adversely affects the properties’.

Unquote.

No wonder they’ve only got one in stock! (Shelf life?).

Screws don’t have a limited shelf life and when components are screwed onto heat sinks they don’t come unstuck.
Being CPC it won’t be stored properly either.

With these general mechanical issues I practice avoidance and try and look at the problem from different perspectives.
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Old 2nd Oct 2020, 1:07 pm   #13
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Default Re: Drilling large finned heatsink

The filler in Loctite 315 is listed as Aluminium Hydroxide (60-70%), so not quite as exotic as the price might suggest...

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Old 2nd Oct 2020, 1:46 pm   #14
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Default Re: Drilling large finned heatsink

Quote:
Originally Posted by G6Tanuki View Post
In the past I've had inconvenient fins milled away so as to provide access to a nut-runner so a hot-device can be properly bolted-down.
Yes, this would be on the list of ideal ways to deal with the problem. Only not so good for someone with no access to a workshop! I'm pretty inventive and do well with my makeshift facilities but I know the limits of what's practicable. So I'm very happy to pass the job over to Graham (post - #10)

Your point about torque - yes, a good one, thanks!
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Old 2nd Oct 2020, 9:12 pm   #15
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Default Re: Drilling large finned heatsink

Al,

Does your heatsink have a flat channel between the rows of fins and is your IGBT module too wide to fit between the fins?

If that is the case, it would seem you need to mill off part of the adjacent fins to widen the channel at that point. This presents the problem that the contact surface of the heatsink needs to be very flat, which is difficult to do without basic machine tools. Next best would be to remove the entire length of a fin, or fins for symmetry, and to allow you to carefully file the widened channel flat, using a suitable flat surface, such as the machined face of an engineers square and marking out fliud to gauge this.

If using the device as its own template is difficult because of projections, it might be usedful to make a template with just the device mounting holes by carefully marking out and drilling a suitable peice of sheet material. The dimensions willbe given on the device's data sheet.

For fixing, you might like to try tri-lobular self-threading screws such as TapTite https://taptite.com/assets/files/tap...i-reminc_5.pdf. These are fairly readily available but the key to their success is to use exactly the correct drill size to drill the hole the screw will form the thread in. (This is slightly bigger than the conventional tapping drill size). The thread formed will then take an ordinary screw if necessary.

To be honest, the cost of a large heatsink will be a fair proportion of the cost of a basic pillar drill which, if it can be justified, makes very many jobs much easier and more precise.

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Old 2nd Oct 2020, 9:30 pm   #16
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Default Re: Drilling large finned heatsink

I'm beginning to wish I hadn't agreed to take on this job which was originally specified by the OP as drilling and tapping four holes, a job I can easily do using my milling machine which I retro-fitted with a Digital Read Out (DRO). I'd co-ordinate drill the holes to match the manufacturer's spec for the module.

Once I get my hands on the heat sink I'll reassess the job and discuss it with the OP. If drilled rather than tapped holes are needed and part of the fins need machining away I can do it. It's rather more work than I'd anticipated though.
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Old 3rd Oct 2020, 12:45 pm   #17
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Default Re: Drilling large finned heatsink

I have now received the heatsink from all Al.

The flat part is 10mm thick, so there's plenty of "meat" for tapped holes without infringing on the fins.

The holes in the module, which I don't have, are drilled 6.5mm diameter presumably for M6 fasteners.

What I'm proposing is to fit M5 studs which can be fitted with washers and M5 nuts torqued as required. There'll be plenty of clearance in the holes to allow for expansion of the heatsink and module.

Any constructive comments?
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Old 3rd Oct 2020, 1:05 pm   #18
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Default Re: Drilling large finned heatsink

Sounds eminently sensible, and the use of studs means if there is much repeated assembly/disassembly during development, it isn't the tapped holes in extrusion-type aluminium that take the wear.

David
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