lapping...

I see these new lapping kits that are coming out and they only include a 1/4 sheet of each grit of sand paper.. This seems odd to me as every time ive attempted to lap my cpu, ive used atleast 1/2 a sheet of 3m brand paper and i dont do it for as long as yuor supposed to.


maybe im doing something wrong but this is how i do it...

1. get a bucket and put a small bit of dish washing liquid in it
2. get the sand paper and cpu block wet
3. move heatsink in 1 linear motion in 1 direction. (after about 3 or 4 "lines" my paper is grey and is very smooth from the metal shavings.
4. dip paper and heatsink into the bucket and use fingers to push off the shavings that wont leave on thier own.
5. Repeat step 3 till sand paper is ruined.


Now i dont do this for very long as i get bored quickly, but i started w/ a quick 600 grit, then 800, then 1500 3m sanding. I dont get the "mirror" shine that others seem to get unless the digicams really make it look better then it is, AND i go through more sand paper.

Whats goin on?

The mirror finish has nothing to do with flatness. Mark a line down the middle of your heatsink with a black magic marker. Then sand with your 600grit like you have been. Check the bottom of the heatsink every now and then and check to see if the balck line is gone. Once it is then you have technically gotten it as flat as possiable with that grit. Draw the black line again and sand with the next highest grit. Once the line is gone again, draw another untill you have gone through all the grits in your kit. Then your heatsink is completly lapped. I never understood the whole mirror finish thing personally. I mean, your never gonna see it anyway. Anything past 800grit, you won't really notice a temperature change anyway. People always think that a mirrior finish means flat. IT DOESN'T!

mirror finish can be synthetically made w/ polish etc, but in an essence.. mirror does mean flat.

As light reflects off the metal, the rough nature of the metal will deflect th elight in all directions, not giving you much "mirror" shine. But as things are sanded and lapped down using higher grits, the metal becomes less rough and more light is reflected directly back instead of at odd angles, THUS giving it a mirror shine.

So unless your using some sort of shiney polish - flatter = mirror finish.

yeah but once you go past 800 grit, YOU WILL NOT SEE A TEMP CHANGE. and even if you do...it will be insignificant.

Thus mirrior finish=waste of time.

One time, I saw a 15°C difference after lapping from 800 to 1000 paper. I'm not positive, but it *MIGHT* have been because I hardly had the block mounted on the CPU, the first time around. I'm pretty sure it was the sandpaper though.

Theres is NO WAY IN HELL there would be a 15*C change just between 800 & 1000grit sandpaper!!!!!! Impossiable. It was your mounting job that changed the temps!!!!!

though the mirror finish may not add much of a temp difference, the fact that i still burn up more sand paper w/ high grits and STILL dont have the mirror finish - gets me. i'm shooting for that finish, but it seems weird that my 3m sheets are burning up alot faster then other peoples and thier getting better results.

i just want to know what i may be doing wrong.

hehehe.....that was good

You could also use a Dremel with some polishing compound as a final step. Works for me

Biz, you are using the waterproof sandpaper right?

Yeah...I'm thinking the stuff is just coming off the sandpaper due to the water. It happened to me just the day before when I used a 400-grit sheet of sandpaper. I completely forgot to check if it was wet/dry sandpaper. Make sure you do your best to apply pressure evenly on every part of the HS.

vonkaar, I think your temp drop may have something to do with mounting+thermal compound application. Are you sure you didn't change anything when you lapped the HS again?

When I lap a Heatsink I use 600 grit only, and set it up in the kitchen sink on a sheet of safety glass positioned at a 45° angle so that as I sand, the water rinses away the removed particles as they come off...I use several sheets of paper, changing them out as I work in order to ensure a good "bite" thruout the lapping process...
I learned to "lap" in the aviation industry, where we were taught to use figure eights under a constant even pressure to get a nice flat surface. The real trick, no matter what "pattern" you use, is to make darn sure you use an even pressure thruout the process.
After learning to "lap" a full engine set of pushrod covers, which is anywhere from 24 to 36 individual parts (or more on the really BIG radials), I find that dealing with a single heatsink is a piece of cake in comparison...Nothing like having all 18 cylinders on a radial engine leaking oil like a pig to convince you of the need to make sure it is "right" the first time around...

holy crap people, I was being funny... If I had used a larger number than 15°, maybe people would have realized that. Here, I'll change my statement =p.

I saw a 400°c change from using 600 to 1000 grit sandpaper when I lapped my heatsink.

Also... squirtle632, dremels *an* polishing compounds are generally considered BAD. A dremel rubs at a round surface, which is exactly what you are trying to avoid by lapping. Flat is the reason you see people mounting their sandpaper on glass. FLAT is 200x more important than shine. Using a dremel at any point (50 grit to 2000 grit wheel, if such a thing exists) is completely defeating the purpose of a 'flat' lapping. Even a polishing wheel can potentially remove layers that you worked hard at getting flat.

Polishing compound is also a controversial no-no. This isn't the thread for that debate, but the consensus is that the rubbing gunk stays in the heatsink. If you use it to polish the heatsink, you'll need to COMPLETELY remove it's residue with contact-cleaner or acid... or it could potentially interfere with your TIM.