Machining heat sink fins

Hey guys
I am machining some copper heat sinks. These are off the shelf parts being modified. I have ten to machine. I am just adding some holes and need to machine the fins for clearance on one end. The fins are .03 thick and .800 tall and as you can imagine they are moving and flapping around. Any ideas ? The forming of these is inconsistent and putting spacers in did not work out.

fill it with wax
When you're done, just melt the wax out

fill them with machinable wax or hot glue and melt it out again.

I did this exact thing about a year ago. I bought some alum strip and sandwiched it between the fins and then clamped it from the outside. I had to machine some of the fins off too and it worked perfect. I think it was 1/16 thick aluminum but I cant remember the exact distance between fins on the one I did.

Clamp the fin side with a piece of hard rubber?

If you do not have the waxes that were recomended maybe 158F cerrosafe would work.

Clamp with rubber or cardboard as close to where you are cutting as possible. They might jingle a bit but it should leave a decent finish. Its tough to say exactly what you're trying to do but id give it a try. I have never used the wax but it sounds kind of like a mess. It might be worth while to try and conventional mill to pull away from the solid base.

Good suggestions. My first thought if I were doing this without having the wax at hand would be to clamp from fin end to base using some sort of compliant pad with a backer plate on the fin side. Neoprene or silicone rubber, well-cleaned so the friction is high.

I'm going with the wax. My only reservation is how can I be sure to get it all out ? I don't want a film left. As we know these are going to see some heat and my customer will not be happy if he finds wax run off in his electronics. How can I be sure to remove it all ? I was thinking of sending them out to be etched just to clean them up, they sat with coolant on them and got stained I want them to be bright again. Will etching remove the wax or will the wax act like a masking agent.

Thanks for the replies guys

Ron

I've cut a hundreds of flimsy finned heat sinks.

The simplest solution is forget everything you know about tool paths.

Look at the approach angle of the cutter to the fin, and imagine a tool path that is always pulling lengthwise on the fin.
IOW, radial depth of cut on the fin you are cutting into.
Those fins are springs, and are trying their best to jam into the cutter.
Worst case is driving down the center line of the fin, and you are in the right hand lane.
If the cutters moves the fin, it will pull it deeper into the cut. Ugly.
If you are in your left lane, any bending of the fin will reduce the cutter grab, and it will cut smooth.

Cutting one fin at a time might sound slow, but you can really get the job done quick.

I can visualize it, and have made tons of parts doing it that way. And mangled a few at first, until I figured it out.
I was planning to melt wax, exotic clamps, etc. Once I worked out the tool paths, it was load and GO!
But I don't know how to describe it...

Don't worry too much about the exact fin spacing. As long as you are close enough to control the engagement angle it works great.

No need for clamping individual fins.

Another trick on that job was being able to cut a flat spot and use vacuum to clamp down the belly.
This helped to stabilize the part, in addition to the clamps.

Clamping around the edges put stress and pressure into the middle of the part, and as you cut it wants to react, somehow.
Pulling it down from the bottom helped out a LOT on a part with a lot of hogging fins in the middle.

Sorry if I don't know how to describe it better.

wheelieking71 said:

Clamp the fin side with a piece of hard rubber?

Click to expand...

Or balsa wood from a model shop, it pushes between the fins when clamped.

std wax is easy, melt out the most with a hot air gun, then use any petroleum based degrease and the wax will be totally gone. Warm degrease is even more effective.

adama said:

std wax is easy, melt out the most with a hot air gun, then use any petroleum based degrease and the wax will be totally gone. Warm degrease is even more effective.

Click to expand...

Or just boil 'em in water. As the wax melts, it floats to the surface. Skim it off, pull your parts out, Bob's your uncle!

MSC (and others) sells a metal that melts at about 160 degrees. You can use boiling water to melt it. I've used it to construct light duty fixtures for strange shaped parts. Melt it and pour it into the fins. Aluminum foil makes a great temporary mold. I'm not sure how big your parts are but if they're small it should work well. Did for me. Her's the link

Steel Foundry Metals | MSCDirect.com

Regards

Robbie

I keep some Plaster of Paris around for this. Throw it in some hot water when done. Works great as an adhesive for very thin plates too.
i_r_

If you are really quick, you can put the piece in water then freeze it. I had to cut apart an air conditioner for a demonstration model. It worked great, but all I did was bandsaw the section.

rb07565 said:

MSC (and others) sells a metal that melts at about 160 degrees. You can use boiling water to melt it. I've used it to construct light duty fixtures for strange shaped parts. Melt it and pour it into the fins. Aluminum foil makes a great temporary mold. I'm not sure how big your parts are but if they're small it should work well. Did for me. Her's the link

Steel Foundry Metals | MSCDirect.com

Regards

Robbie

Click to expand...

This stuff works great, but one thing to keep in mind. It is not permitted to come into contact with aerospace parts due to Bismuth contamination.

I don't believe that the low temp metal will be allowed near electronics due to ROHS restrictions.

I have been doing this type of work for years and have tried wax (witch works well) Bondo (witch also works well) but about a year ago I started using the Aerosol Spray-in foam and works perfect. Machine the part then aqua blast the foam out.

Spray Foam Insulation

Here's a video of some aluminum heat sinks I machine. CNC amplifier machining: http://youtu.be/GSBT_SWduR4
It took some experimentation but you need to get the fin off in one pass. Can't take a light cut. You need to machine the fins off center. I use a vacuum fixture for these parts. I don't remember the precise specs but it doesn't matter for your application. All that being said my heat sink fins aren't nearly nearly as flimsy as yours so you no doubt have a more challenging part but you should be able do it without messing around with wax.
Jordy