Lapping of tin to mirror finish

reasonableman · Apr 6, 2016

Hello, I am attempting to lap tin to a mirror finish. I seem to be having problems.

Some details:
- Using a Kemet 20 precision lapping machine.
- Using diamond slurry.
- Tin is 50 mm diamter disc.

Initially I started with a disc taking it from 14 micron to 3 micron. This got a pretty good surface finish. I then tried 0.5 micron but could not see any improvement over 3 micron. Conditioning rings were ultrasonically cleaned and the plate was cleaned using copius paper towels and cleaning solution (supplied by manufacturer).

So far, although not thrilled with the finsh it was acceptable so I've tried to repeat the process. Now I cannot get anything other than a scratched finish. I've cleaned everything thoroughly, I've tried varying rotation speed, I've tried reducing the pressing weight. Everytime it is scratched. Does anyone have any suggestions?

Iridium77 · Apr 6, 2016

reasonableman said:
Hello, I am attempting to lap tin to a mirror finish. I seem to be having problems.

Some details:
- Using a Kemet 20 precision lapping machine.
- Using diamond slurry.
- Tin is 50 mm diamter disc.

Initially I started with a disc taking it from 14 micron to 3 micron. This got a pretty good surface finish. I then tried 0.5 micron but could not see any improvement over 3 micron. Conditioning rings were ultrasonically cleaned and the plate was cleaned using copius paper towels and cleaning solution (supplied by manufacturer).

So far, although not thrilled with the finsh it was acceptable so I've tried to repeat the process. Now I cannot get anything other than a scratched finish. I've cleaned everything thoroughly, I've tried varying rotation speed, I've tried reducing the pressing weight. Everytime it is scratched. Does anyone have any suggestions?

I am not an expert, but I've read a little on lapping. Your using a slurry which means the grit is rolling around so you can't get a mirror, you'll only get a matte finish. Your grit needs to embedded or charged into the lap. Now on how to do this I'm not sure.

Dan from Oakland · Apr 6, 2016

I've never tried to lap tin, but I've bored thousands of holes in babbitt which is about 90% tin. Tin is relatively soft and that is probably causing what you are seeing. Why diamond paste for such a soft material? Seems AO would work just as well, but seems to me that a free abrasive is not the way to go. You can get a near mirror finish in tin with a very sharp lapped carbide or high speed tool and conventional cutting. Hope this helps-good luck.

On edit, there is a product called Timesaver that is used to lap babbit bearings- you might try that. I don't use the stuff, but some folks use it to lap bearing fits. AFAIK it comes in various grits. But it does not give a mirror finish- so. . . . .

Conrad Hoffman · Apr 6, 2016

I think things like that are polished on cloth or even with pitch. Might try looking up speculum mirrors and see how they polished those, though the material is harder.

reasonableman · Apr 6, 2016

Thanks for the comments so far, to respond:

- We might have a copper lapping plate, I think it's currently steel. I can see if that helps.
- I'm using diamond paste because that's what we've got. We do have AlO but it's a large grit.
- Machining works well, we normally get high precison turned material but that is expensive with a long lead time. It is also very directional due to machining marks which is not ideal.
- I have tried polishing using Brasso etc. but I find that this makes the surface darker (I think the abrasive particles get embedded) and nothing can clean it up (ultrasonic, solvents).

CalG · Apr 6, 2016

To polish "specular Reflection", You will need to change to pads or some sort of soft platen like pitch. Maybe lead? I can't say, I've not done tin.

If flatness is also part of the deal, that will need to be established prior to the polish operation.

Ask here Welcome to Universal Photonics

Screwmachine · Apr 6, 2016

When lapping the lap is generally softer than the material being lapped. That's a problem with tin, it's a seriously soft material, I actually use it as a lap often.

How flat does it need to be? Are you trying to make a mirror? If you need a truly mirror surface a paper towel will scratch the heck out of tin. Front surface mirrors are usually aluminum and the cleaning procedure is laborious and risky and alu is harder than tin.

Can you give any more details on what you're trying to achieve?

Derek Smalls · Apr 6, 2016

My guess would be that a mirror finish would be almost impossible from any kind of lapping or any other straight abrasive means. I know Mothers aluminum polish works great on aluminum when used with supper fine steel wool, but it sounds like that's not a practical approach for you, even if it would work well on tin. Maybe some kind of burnishing process instead of abrading? Roller burnishing???

Spyderedge · Apr 6, 2016

Is burnishing of some form an option?

Milland · Apr 6, 2016

Aren't there some very soft polishing materials like cerium oxide that might work for tin? Might have to process the part for form factor, then polish for finish as a separate operation if you need flatness.

Mark Rand · Apr 6, 2016

Cerium oxide is about 7 on the Mohs hardness scale. That's about the same as Quartz. It'd still need to be carried on a soft plate of pitch or similar to work.

brucecu · Apr 7, 2016

It only takes a single rogue particle to ruin a finish. Check your lapping plate for larger embedded particles by shining a light at a low incidence angle and look with your nearly the same height as the plate. embedded particles will sparkle or glint. They can be dug out or punched deeper in the plate. Try the expanded urethane lapping pad from Universal Photonics. It is a softer material that retains the lapping fluid and abrasive yet is stiff enough that you won't lose too much flatness. If your willing to try a diamond turned surface, contact JAMI Enterprise or Mindrum Precision.

I would use something softer like iron oxide and finish with colloidal silica. Be sure to keep the lapping well lubricated. Tin is a soft low melting point element that seems to stick to anything.

reasonableman · Apr 8, 2016

Again, I'll need to reply to a few people at once.

With regard to application I am doing plate impact experiments. Essentially two flat plates are impacted together at high speed (1+ km/s). I look at the states created by this impact. Now generally we only care about flatness but at the moment we want to use a diagnostic that needs a mirror-like finish (exact requirements flexible).

I will try the light shining trick to find embedded particles.

I don't really know about burnishing and if it'd be suitable...

rogertoolmaker · May 6, 2016

Lapping and laps

reasonableman said:
Thanks for the comments so far, to respond:

- We might have a copper lapping plate, I think it's currently steel. I can see if that helps.
- I'm using diamond paste because that's what we've got. We do have AlO but it's a large grit.
- Machining works well, we normally get high precison turned material but that is expensive with a long lead time. It is also very directional due to machining marks which is not ideal.
- I have tried polishing using Brasso etc. but I find that this makes the surface darker (I think the abrasive particles get embedded) and nothing can clean it up (ultrasonic, solvents).

Keep in mind, the first lapping basics, the lap has to be softer than the work. If the work is softer than the lap, the work becomes charged and becomes the lap. This I feel is the reason your getiing scratches. Using diamond for the media on tin is like killing a flea with a sledge hammer. I honestly don't know what material that could serve as a lap on tin.Especially if you have flatness requirment. You might contact a supplier of metal polishing and buffing
media.

Rogertoolmaker

Winterfalke · May 6, 2016

Now I'm actually kind of curious how long tin will hold a mirror finish at over 1km/s in an atmosphere.

Milland · May 6, 2016

Winterfalke said:
Now I'm actually kind of curious how long tin will hold a mirror finish at over 1km/s in an atmosphere.

If these are kinetic impact tests (and not facing plates using explosives to mate them) then it'd guess it's done in a vacuum setup.

adama · May 8, 2016

Have you looked at the 3m sub micron diamond and alu oxide lapping films? Should be easy to take a micron flat surface to a full mirror in but a couple of minutes by hand with these simply laying on a good flat surface.

Key part of your problems probaly not grit contamination but the material simply snowballing up, You may well need less force than the weight of the part + cupios flushing as you go with something like DI water. At these levels, the swarft must be removed before the lap goes back over the part, other wise with a soft sticky metal like tin, it will just stick and snow ball - seize back on.

You may have way more luck simply burnishing it smooth aginst a hard polished disc of something else.

Mechanola · May 8, 2016

You can’t produce a mirror surface on tin by lapping.

The next and final step is to procure a polishing mop together with soap. Yes, primitive soap. Potato sap is also good for the purpose.

Milland · May 8, 2016

Maybe press it against a silicon wafer coated with mineral oil? Spin coating will give a uniform surface, and should prevent bonding (if that's possible) between the silicon and tin. You can try other low-stick surfaces too, such as man-made sapphire. Support the wafer on a thick ground steel plate to prevent bending and fracture risks.

jkopel · May 16, 2016

From left field, but could you cast the tin into a mirror polished mould?
The stuff melts at < 500F...

Lapping of tin to mirror finish

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