Aluminum Heatsink Bore for Copper Module - how much oversize should the bore be?

[smana]

hello all,

In machining an aluminum heatsink with a center bore for the copper module, how much oversize should the bore ID be in relation to the OD of the copper module?

For example, if the copper module is 35mm OD, what should the mating aluminum heatsink ID be?

If the ID of the bore is not oversize enough, we will have a binding problem when trying to insert the copper module into the bore.
If the ID of the bore is too large, the heat sinking effect will decrease due to lack of direct contact between the two parts.

Is there a 'rule of thumb' on this?

 

[farbe2]

If the ID of the bore is not oversize enough, we will have a binding problem when trying to insert the copper module into the bore.
If the ID of the bore is too large, the heat sinking effect will decrease due to lack of direct contact between the two parts.

Is there a 'rule of thumb' on this?

You are spot on.
The best ID for the bore is just in-between too large and too small.

Keep in mind that bad surface finishes on the bore/copper module will change the "fit". A little "not round" and nothing will fit.
So the best thing you can to is: try and error.
Make it bigger until it just barely fits.

 

[smana]

Sounds good.

Only thing is, I am using an ebay machine shop so I prefer to get it right the first time without having to send it back or do any minor hand-sanding on my own.

I suppose if the ID bore on the aluminum heatsink got too large, I can always apply copper tape on the copper module OD to compensate.

 

[Thorsteenster]

If it's a machine shop I'd imagine they would know a good tolerance to machine to for a nice tight snug fit.
Maybe a machinist on here will pipe up, do you have the exact OD of the module?
If you're having it machined, could also look at having w/no slot and press the module into place.
Removing wouldn't be as easy but can always just remove the diode instead of the diode/module for swaps.

 

[farbe2]

It would be best if your machinist can touch / measure the part that you want to have in there. The exact part.

You know about tolerances right?

If you say your part needs to be 20mm in diameter, depending on the acceptable tolerances, you will get something between 20,2mm and 19,8mm (for m tolerances ISO2768)

So without having the part that needs to go in there or knowing the acceptable press fit / slip fit, there is no way you are getting a good fitting part.

 

[smana]

Thank you for the advice, I will wait for the copper module to get to me, then measure, and discuss with machinist and let him decide.

 

[Thorsteenster]

Have you got a caliper to measure it with?
You want to get a precise measurement, like farbe2 suggested, good idea to send the module to the machinist to get the best fit.

 

[smana]

Yep, got the caliper. I will first measure the OD at several places and see how round it is.

But first, I have to receive it in the mail, lol.