How to flatten a wide heatsink

This isn't rocketry related, it's for my home brewery, but I need some basic guidance, and you guys have tons of experience that will help.
I bought a 12" long x 3.5" wide extruded aluminum heatsink, on which I intend to mount a couple of SSRs. As explained by the vendor, and confirmed by my measurement, there's a slight concave deflection (~0.5mm) along the narrow dimension. I haven't got a metal shop, so while milling it flat is probably the ideal solution, but would require me to send it out, and perhaps pay more for the planing than the piece cost. If that's what I need to do, I will- I just don't want to do something that's beyond necessary.
Available to me is a 4"x48" belt sander that many of us have for rocketry. If this were ferrous material, I'd shy away immediately, but 6063 is non-magnetic, so my motor should survive. I'm wondering if I'd be able to zip off that slight curvature (there's 1/4" stock material as the base) with my existing tool, or if I should bite the bullet and have it done on a mill?
I plan to use heat-sink compound, but 0.5mm is more of a deviation than it's meant to overcome...but if I have some ripple/sanding artifact but am mostly flat, I think it'll be fine for my use. What are your thoughts?

In your situation I would probably just sand it. Not sure how much it might clog up your belt. I know it can really clog up grinding wheels. Also, some of the belt grit could get embedded in the Al.

Once close to flat, working it with a fine file may give a better surface for heatsinking.

I realized after posting that I was being overly cautious- if I "eat" a belt, boo hoo...ran this across the belt sander, changed belts, ran it again, and then ran it against the wheel, just to see if I'm planar. The scratches tell a story! I'm now within a few microns of flat (enough for government work, as they say)! Thanks for letting me put my irrational fears online, and think through them while I wait for the ideas that I thought I needed.

By the way, OverTheTop, is this the same OTT that posted the Velociraptor build from Binder designs on an Australian rocketry forum? I'm getting ready to build the Velociraptor myself, and have used your thread several times to prepare!

If you have a thick piece of glass or a piece of granite countertop just rubber cement or double sided tape a piece of 80 grit emery cloth to it and sand it flat on that. Sand in one direction, not back and forth. When the sanding marks cover the surface switch to about half the grit (so from 80 go to 120 or 180). When you switch grits change the sanding direction by 90° each time and sand to half the depth of the previous sanding marks. Stop when you get the finish you want.

soopirV said:

By the way, OverTheTop, is this the same OTT that posted the Velociraptor build from Binder designs on an Australian rocketry forum? I'm getting ready to build the Velociraptor myself, and have used your thread several times to prepare!

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Certainly is me .

The Velociraptor (and now Velociraptor R since I resurrected it) is a great and regular flier for me. I will be flying it in about three weeks as a test bed for the Vertical Trajectory System on its maiden flight. Glad the build thread is of assistance . You going to do the claw fins?

Steve Shannon said:

If you have a thick piece of glass or a piece of granite countertop just rubber cement or double sided tape a piece of 80 grit emery cloth to it and sand it flat on that. Sand in one direction, not back and forth. When the sanding marks cover the surface switch to about half the grit (so from 80 go to 120 or 180). When you switch grits change the sanding direction by 90° each time and sand to half the depth of the previous sanding marks. Stop when you get the finish you want.

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This right here, if it's good enough for lapping delidded CPUs to get ~2deg thermal margin, it's good enough for just about anything. You can take it to mirror ( which is not necessarily what's wanted for heat transfer ) no problem.

A fairly smooth piece of concrete also works in a pinch ^_^

OverTheTop said:

In your situation I would probably just sand it. Not sure how much it might clog up your belt. I know it can really clog up grinding wheels. Also, some of the belt grit could get embedded in the Al.

Once close to flat, working it with a fine file may give a better surface for heatsinking.

Click to expand...

Sanding aluminum on a belt is one thing, but applying aluminum to a grinding wheel is extremely dangerous. Grinding aluminum forces aluminum into the pores of the wheel, which can shatter the wheel, sending shards into your face and other important body parts. I suggest all avoid the temptation.

cherokeej said:

Sanding aluminum on a belt is one thing, but applying aluminum to a grinding wheel is extremely dangerous. Grinding aluminum forces aluminum into the pores of the wheel, which can shatter the wheel, sending shards into your face and other important body parts. I suggest all avoid the temptation.

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It is something that not a lot of people are aware of. The usual result is a clogged wheel, but the extreme result is fragmentation. Too many time I go to a grinder in the common work area at work and it is clogged up

The bad part comes when you come in behind Mr. Aluminum at the grinder, with a piece of steel. You're taking a chunk of weld off your part, and that's when the wheel lets go. Give my regards to the folks at the ER.

You should talk to your shop safety steward, and get Mr. Aluminum to knock it off before he hurts someone.

Spent over three decades in a shop environment, and watched too many guys leave the shop in a damn ambulance. And brother, that sux.

Why try to flatten it? For a few dollars you can get some thermal pad that would fill the gap. Depending on the size of the SSRs, even a thermal paste might work, but the pads are less messy. Pads are available from all the usual suspects DigiKey, eBay, Amazon, etc.

Thermal conductivity of the pad gets worse as it gets thicker. If there is enough thermal margin in the design it is acceptable and easy.

@soopirV , you said it was for a [home] brewery project - controlling kettle heating elements?

cherokeej said:

Sanding aluminum on a belt is one thing, but applying aluminum to a grinding wheel is extremely dangerous. Grinding aluminum forces aluminum into the pores of the wheel, which can shatter the wheel, sending shards into your face and other important body parts. I suggest all avoid the temptation.

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This is good and important advice- when I said wheel, i meant the sanding disk, not a grinding wheel, but it's great to remind!

Charles_McG said:

@soopirV , you said it was for a [home] brewery project - controlling kettle heating elements?

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Bingo! Am not an electric brewery (use propane burner for the boil), but I'm reworking my control panel to be more robust using theelectricbrewery.com as an inspiration. Fellow brewer I trust?

soopirV said:

Bingo! Am not an electric brewery (use propane burner for the boil), but I'm reworking my control panel to be more robust using theelectricbrewery.com as an inspiration. Fellow brewer I trust?

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Cidermaker and distiller. Well, actually, my wife runs the still.

cherokeej said:

Sanding aluminum on a belt is one thing, but applying aluminum to a grinding wheel is extremely dangerous. Grinding aluminum forces aluminum into the pores of the wheel, which can shatter the wheel, sending shards into your face and other important body parts. I suggest all avoid the temptation.

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Did not know that. Thanks for the warning.

Very cool- and only now do I see your signature and your avatar , been a rough day!
Have wanted to try distillation, but of course the law is against me. Have you got a website?

soopirV said:

Very cool- and only now do I see your signature and your avatar , been a rough day!
Have wanted to try distillation, but of course the law is against me. Have you got a website?

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Yes, the law is against you. I file lots of paperwork every month to stay legal.

I have a website : www.appletrue.com but it's just informational. I gave up on shipping after The Granholm Decision and the wave of permit requirements that followed. But we do have very limited distribution from Hawaii to DC. Mostly Wisconsin, of course.