Flat lapping aluminum

[garnetram]

Has anyone ever machined and lapped Mic-6 aluminum plates? 8 inch disc from 1/4" to 1/2" thick. The final flatness spec is .0002" Which has proven to be a difficult goal.

Any help would be awesome!

[the4thseal]

my experience has been that aluminum is too soft for lapping. the compound's grit embeds in the aluminum.though you might using the term lapping,differently than me. there are surface grinding discs that will do aluminum. good luck

[Mcgyver]

do you need it .0002 flat, or just .0002 dimensional thickness? The later isn't too bad, the first is a challenge. whats the application that it would require this flatness instead of just dimension? apologies if i'm telling grandma how to suck eggs, but flatness isn't often neccissary as thin parts are usually sandwiched or fastened down so dimensional accuracy might be important but flatness is not..... point being maybe the customer doesnt actually need it .0002 flat.

getting thin things flat is a challenge I agree with 4th on the dangers of lapping AL, BUT you can get non embedding lapping compound that will work....however, how are you going to hold it and lap such that you are distoring the part? it would be very tough to do this via lapping i would think.

thats the challenge, any way you to go hold it changes its shape. if it was steel you could grind on a mag chuck but even then its a royal pita as the force of the chuck will pull it out of shape and you achieve flatness by shimming.

The only way i know that will work is to scrape it in, that way there is no clamping force while the areas requireing material removal are identified. not impossible to hit your spec with that approach, but its time consuming....make sure you charge enough because its like gauge making work instead of part making

interested to see what other ideas come forth...

[the4thseal]

mmmmmmmmmmmmmmmmmmmm... scraping.(think homer and donuts). a topic near and dear.

[scadvice]

...look for a shop in your area. I've had parts done to .0005 = with just DD only. There are shops in Calif. that can do it. The cost the last time I had it done was around 8 bucks ea. for 80 pcs., as I recall. As I said that was just double disc grinding cost not the lapping. From time to time I see and bid on small lappers on ebay but haven't won one yet.
Hard drive makers double lap alum. all the time.
Steve.

[ImanCarrot]

Use true tools and aluminium oxide with water.

I can lap 12" disks flat to a twentieth of a wavelength of light. That's 1/20th of 633 nanometers. ie: 32nm peak to valley across the whole aperture. That's 0.0012".

It can be done, just takes a lot of skill.

See Engis or Hyprez for more advice, or any company that operates a Lapmaster.

Incidentaly, to go lower than a twentieth of a wavelength is difficult since light starts diffracting at that level and acts more like a particle than a wave. ie it's blinking hard to measure to that accuracy.

[garnetram]

We have done some scraping here, but for this application It would not be suitable.....I forgot to mention a few "minor" details! LOL

Top and bottom need to be paralell withing .0004" and both surfaces need #4 finish.....

Sorry I forgot that!

On another note we have noticed that as a rule after maching these disc to thickness and a reasonable machined flatness (some of theese disc have through details in then such as pockets and tapped holes) and after sitting a few days they tend to "move" on us. Thus requiring rework. Would stress relieving help? Has any ever stress relieve Mic-6? I have found a little info on the net but have not actually tried it yet.

We would love to do all of this "in-house" if possible. As a rule our customer requires a turnaround too fast to allow for outsourcing, and we have had too many parts such as this come back damaged from either the vendors shipping dept or UPS.

[steve323]

Originally Posted by ImanCarrot I can lap 12" disks flat to a twentieth of a wavelength of light. That's 1/20th of 633 nanometers. ie: 32nm peak to valley across the whole aperture. That's 0.0012".

Do you mean something close to 1.2E-9 inches? Wow.

Steve

[ImanCarrot]

I make ir 1.2 e-6 inches?

1 twentieth of a wavelength where the wavelength is, say, 633nm (0.000633mm)=

0.000633/20mm

= 3.2e-5mm

divide by 25.4 to get inches

3.2e-5/25.4

= 1.2e-6 inches.



Oh Garnetram, I dunno what mic6 ally is, I useualy use L111 ally which is an aircraft/military spec (I think the commercial equivalent is 6082, but not sure- anyone else know?)

IT IS ESSENTIAL WHEN LAPPING TO THIS LEVEL THAT YOU USE CAST ALLY, NOT ROLLED OR EXTRUDED OR ANYTHING ELSE OTHERWISE YOU WILL GET STRESSES.

Heat treating to anneal the stresses might help but I doubt it on rolled ally which bends like a bannanna when machined.

Oh, how are you holding it? if you're vacuuming it down then you can get print- through from the rear surface or chuck which will afeect the flatness.

You could get it Diamond Machined- that's get your flatness no problem. And thickness tolerance.

[Mcgyver]

Originally Posted by ImanCarrot I can lap 12" disks flat to a twentieth of a wavelength of light. That's 1/20th of 633 nanometers. ie: 32nm peak to valley across the whole aperture. That's 0.0012". It can be done, just takes a lot of skill. .

how thick?

pending some specialize lapping machine, with regular shop equipment, that challenge is how how hold it. by hand, vacuum, holddowns, whatever, its going to move a 1/4" 6" disk more than .0002; ie holding will distort so the warp/bow/whatever in the work reappears when the pressure is released - even very slight pressure

Oh, how are you holding it? if you're vacuuming it down then you can get print- through from the rear surface or chuck which will affect the flatness

how do you hold it? what ever way, unless you map the surface with a very good indicator and then shim will pull a 1/4 disk many thou to its own plane. I agree movements a potential problem, and could be even with cast although less than rolled....you sure want it stabilized before you try to get a flat section to 1/10ths. would also depend on whether you started from flat stock or a slice off a 6 inch bar. in steel I'd know what to do but not sure offhand how to normalize/stabilize/season AL

You could get it Diamond Machined- that's get your flatness no problem. And thickness tolerance.

can you explain how that works so that flatness isnt' a problem - you still have to hold it down, right? what type of machine?

Iman, can you give some more insight into how exactly you're lapping these disks? are doing this work by hand or with some special lapping machine? I'm interested in the process and would like hear more about it

[ImanCarrot]

Lapping by hand although for production quantites you'd probably buy a lapmaster machine. I think you get a better accuracy by hand (less edge roll-off), but you need to touch in your own true tools whcih is an art itself!

www.lapmaster.co.uk (I have no connection with the company)
Engis are good too for consumables
www.engis.com

Diamond Machining- essentialy what you do is hold the component on a vaccuum chuck. this vaccuum chuck has itself been diamond machined flat. The rear of the component is lapped flat and vaccumed down to the chuck. The front of the surface is then cut. No print through, no stresses= perfect opticaly flat surface and an optical mirror finish.

Aluminium is perfectly suited to diamond machining (some folk call it diamond turning), but it's costly. Actualy to give it its full name- Single Point Diamond Machining (wiki it for a fuller expanation).

[Mcgyver]

i know single point diamond machining, once you get one surface flat life gets easy(ier) the question I have is the details on how you establish that first reference flat on the one side via lapping. Can you give some more detail about how you hand lap this? are you using some optical device to show where the work is high and subsequently lapping there? It can't just be relying on the flatness of the lapping plate as the work is so thin hand pressure alone will distort, at least to the tolerance we're talking about. thx