What is the best aluminum for heat sink?

wrote:


Surface grinding for large mating faces that then get lapped together against a third, thinner lapping skate.
Ideally, surface grinding on a medium sized mating face, subsequently lapped.
Definitely should only need lapping on small mating situations. If the item is that far out of flat, something is wrong with the way it was made.
There is no way that "scraping" would yield true flat surfaces. His situation HAD to use huge amounts of gap filling mating paste of some kind. Scraping is NOT a precision machine operation. Anything bigger than a couple square inches should get surface ground. Anything below that size should be manufactured with a very flat face to begin with.
Lapping is certainly better for zero gap magnetic elements like transformer cores, etc.
Heat sink mating would benefit from lapping, but only if one ignores the labor cost. If you are doing a personal or proto project, sure, but if you want design for manufacture, you need to design it into your proto stages as well, and lapping is a VERY labor intensive, semi-skilled procedure.
So, ideally, in a manufacturing environment, reasonably integral, highly reliable matings are already being done every day without lapping.
If it is to be a non-permanent installation, which has service expectation or provisions to consider, the mating paste has to be one that does not form an adhesive bond.
If it is to be a permanent, one shot per part, per sink assembly, then the use of the Silver filled epoxy that the IC chip industry uses to bond chip tops to dies with is the right choice. It forms a strong, adhesive bond on the two faces, if they are clean, and has the best characteristics for permanent attachments which require good thermal transfer across that boundary.
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wrote:

Scraping mostly went out of use around 50 years ago but is still ocassionally used, and can produce a flatness of .0001 inch over a surface spanning hundreds of inches, with a surface finish of about .0005 inch, well suited to holding oil. Skill required.
(Modern hardened way machines are ground, not scraped, and most machines are never reground after manufacture, they are just demoted to lower precision work as they wear out, or just scrapped and replaced with a new machine.)
Lapping has been the ultimate method for producing accurate surfaces for hundreds of years. I once read Isaac Newton's description of the process in an optical fabrication book, a section included to show how little the process has changed. (Newton did not invent lapping, but he was credited with being the first to write a description.)
Lapping can produce flatness of better than .000001 inch (one microinch) over many tens of inches, with RMS surface finish better than one microinch. I don't know what the largest optical flat ever produced was, but telescope mirrors have been produced by lapping with geometry and surface finish with microinch accuracies over hundreds of inches.
So lapping done by an expert is about two orders of magnitude more accurate than scraping done by an expert. Alas the process is little understood outside of the optical fabrication community, with people attempting to use totally inapropriate surfaces like a piece of glass for a lapping plate, without even making it flat before and during use!
(I have lapped parts up to 40 inch OD flat to better than 10 microinches with 1 microinch surface finish in a decades ago job as a lab tech; smaller parts flat to 1 microinch. Most of the work is in the shaping of the lapping plate, which needs a very slight crown to produce microinch flatness on the workpiece.)
Typically a surface can be economically milled or otherwise machined flat to about .001 per 10 inches, ground to .0001, and lapped to .00001, at high volumes in a production shop without undue effort. Tighter tolerances cost more.
BTW there were many variants of hand work exercises for producing accurate surfaces in the old days, mostly ending with WWII. One shop I know of required apprentices to saw off a piece of 1-1/4 bar with a hacksaw (sawing 3 sides) then file it to a 1 inch cube, with thickness and squareness +/- .001 inch, using a micrometer, a machinists square and feeler guages. If the foreman found any spot outside tolerance, usually a spot where the .001 feeler guage would slip under the square, then the apprentice got to start over again. No lapping or scraping permitted. A number of these hand work exercises as well as powered machine exercises were required for the promotion to Journeyman.
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GregS wrote:

Yes. A century ago, machinist apprentices were given a metal block and a file, and told to file it flat and square. That gave them a sense of how metal behaves when machined. It's a tough task, but a possible one. It teaches you hand tool technique.
                    John Nagle
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wrote:

Now they just show you how to use an inexpensive surface grinder without embedding the shards of a 3600 RPM stone in your forehead.
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On Tue, 05 May 2009 13:47:28 -0400, Spehro Pefhany

That is so far beyond even wrong that there are not words to tell you. Go make a 4-piece set of V-blocks with hand tools and maybe then you can understand.
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Right idea, but to actually work It's three metal blocks, a tube of bluing, and a scraper.
You need two to be able to mark the high spots on each other, and you need three to avoid developing complementary curves.
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On Wed, 6 May 2009 07:56:03 -0700 (PDT), cs snipped-for-privacy@hotmail.com wrote:

An old optical technique is to rub three surfaces against one another. Eventually all three become flat. There are automatic rotating grinder things that do this.
John
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On Wed, 6 May 2009 07:56:03 -0700 (PDT), cs snipped-for-privacy@hotmail.com wrote:

Lap all four faces independently of each other, then lap the block against one face, clean it, and mate it, and then attach the other face, being sure to accommodate any non-paralell circumstance the block may have incorporated into it. The second item mentioned would be the heat source.
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John Nagle wrote:

I've mastered scraping flat--how does one ensure "square" ?
TIA, James Arthur
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James Arthur wrote:

When I was given that task, (not quite a century ago!), we were given a vernier caliper, whose jaws were "square". The metal was a chunk of cast iron, and the task was to file a one inch cube, accurate to +/- 0.001", "square" on all sides.
--
Virg Wall, P.E.

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VWWall wrote:

Thanks, that solves one mystery. I'm still curious how the original right-angle standards are made.
I guess you could reflect a laser vertically off a pool of mercury until it returns perfectly back to the source, or maybe somehow rub three different cubes together in rotation while sliding them on a surface plate.
Best, James Arthur
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wrote:

The thing with a right angle defining device (a square) is that they self prove their own accuracy.
There is no way that a square that was off by even a TINY fraction of a degree could not be detected by any person with a good eye.
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On Wed, 06 May 2009 17:48:36 -0700, SuspendedInGaffa

Sounds a lot like a baseless assertion to me.
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wrote:

It is one of the methods given by a major manufacturer and referenced in this very thread by another poster, you biased, retarded twit.
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VWWall wrote:

That would've been a pig of a job. How long did it typically take?
--
W
. | ,. w , "Some people are alive only because
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On Wed, 06 May 2009 18:00:36 GMT, James Arthur

I guess if you can rotate a block on a flat surface by 180 degrees and it still matches a reference surface then it's square.
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Spehro Pefhany wrote:

I'm thinking a plumb bob hanging down into a pool of water would make a pretty square reference angle, or maybe bounce a laser off a first-surface mirror so that it returns to the laser...
Hmmm.
Not that I need such precision. I've got the CNC bug[1], and I'm thinking of ways to make things nice. Ways to make nice ways, if you will.
[1] A CNC mill and a lathe, and the desire to make more of the same.
Cheers, James Arthur
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The lathe and mill are both Turing-complete machine tools, so that's no problem. :-)
Crossposting to RCM, which I recall had someone madly interested in something like "Godel's Theorem for Machinists"!
Tim
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You could always play with material addition rather than subtraction. There is an open-source rapid-protoype machine at http://reprap.org/
Of course, you'd still have to buy some simple structural pieces and threaded rod (and raw plastic)... --Glenn Lyford
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Tim Williams wrote:

For the benefit of rcm, Virg mentioned the old student machinist's assignment of making a 1" cube flat, square, and parallel, by hand. He said they used a vernier caliper's jaws to gauge squareness.
I've mastered scraping flat, but wondered how you make things square. Really square. Like, how do they make and calibrate reference squares? How would you get, for example, a tenth per foot, from scratch?
My reference ideas: o a plumb bob hanging into a tub of water would make a 90 deg. angle to the water's surface. o a laser bounced off a first-surface mirror back to the source
It's a curiosity thing--I realize few machines need such precision.
TIA, James Arthur
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