DIY diamond suspension.

[luke8888]

I've got some diamond powder in various grit sizes including some very small grits I would like to make into suspension to be used for manual lapping.

Could someone let me know what are the ingredients of those commercialy sold diamond suspensions? Do they use water/alcohol? Is it just diamond, solvent and some coloring, or are there any stabilisers that delay separation etc?

Perhaps someone has a recipe that works? I'm interested in using diamond grit down to half a micron so I probably should use a solvent that doesn't contain any water(alcohol?) as water might be too viscous for such small grit. Is this correct? My lapping plates are serrated BTW.

 

[DavidScott]

I just use RO water and have yet to see any problems down to .1 microns. I use it to load strops that are used dry to sharpen knives, with the results inspected under a microscope.

 

[Conrad Hoffman]

I've seen both water based and oil based. You can add a bit of propylene glycol (RV antifreeze) to help the suspension if need be. I've seen ancient machinists handbook that told how to separate diamond powder by settling time in water. Lost arts!

 

[luke8888]

Thank you both for replies.

I just use RO water and have yet to see any problems down to .1 microns. I use it to load strops that are used dry to sharpen knives, with the results inspected under a microscope.

Is this deionised water (DO) ?

This is interesting. I heard the following anecdote. A fella was making cast iron lapping plates(serrated) using a 3 plate method. He got pretty good flatness, but he couldn't get his final micro grit to work at all. He talked to Tom Lipton after seeing his lapping video. Tom told him not to use 70% isopropanol as liquid with his finest diamond powder because the water content will make the boundary layer too thick and the grit will not work.

I imagine sharpening is somewhat different than lapping in that much lower area is at contact. Also you're presenting the object at an angle so any liquid will work, even oil, because it has easy path to squeeze out. That's just based on what I heard. I'd love someone who did flat lapping to confirm/deny this!

I've seen both water based and oil based. You can add a bit of propylene glycol (RV antifreeze) to help the suspension if need be. I've seen ancient machinists handbook that told how to separate diamond powder by settling time in water. Lost arts!

Please see the anecdote above :-) in small grits (1 micron etc) I saw water and oil soluble rather than water and oil based, but perhaps they were based on those liquids too? I don't know.

I saw propylene glycol as an ingredient in an oil soluble one. But it was a 15 micron grit so I don't know if it's not going to be too viscous for small grits. Perhaps adding some alcohol can thin it out?

Also, in context of the above I'm looking for any materials anyone might have about the difference in lapping with free abrasive (rolling around - as in two plates of steel and Sic powder) vs charging abrasive (a soft lap, for example copper, diamond abrasive and hard part to lap). I'm interested in learning more about the difference in surfaces resulting from both processes.

 

[luke8888]

I suppose not many people make their own slurries then...

Anyway. I decided to measure film thickness of few lubricants available to me. Perhaps it will be useful to someone.

It was measured with gage blocks of surface area similar to the lands of my laps so your mileage may vary, but at least it is a basis for comparison. The "squeezing" was done applying quite substantial hand pressure and rubbing both blocks at each other for few seconds(rubbing longer doesn't seem to change the results) .

All values in microns(u).

ISO Vg 10 oil 4~5u , squeezes by hand down to 1.8u.
Kerosene 1.5~2.5u, squeezes to 1u.
95% Ethyl alcohol 2~2.5u, squeezes to 0.2u.
Monoethylene glycol 1.5u, squeezes to 0.5u.
Deionised water 1u, squeezes to nothing? (water was difficult to measure for some reason).

What about all the people that run lapping machines. Do you adjust your lubricant viscosity based on the grit you're running? If so. What lubricant do you run for 1 micron etc?

 

[eKretz]

If using a suspension, the finer the grit the less viscous the fluid needs to be if you want efficient lapping. As you've mentioned, viscous fluids can nearly or even completely prevent the abrasive from doing any work. That can be a benefit in some cases, as a simple switch of the fluid can result in a somewhat different surface finish. I don't think that many would bother to make their own slurries in a professional setting.

This doesn't really apply to the knife strop example given above - he's only using the fluid to carry/distribute abrasive particles to the strop, then letting the fluid evaporate away. He mentioned using the strops dry.

If you want to find some interesting reading about the differences between charged laps and laps used with free abrasive, use the search term "2 body abrasion vs 3 body abrasion." You will find plenty to read.

 

[luke8888]

If using a suspension, the finer the grit the less viscous the fluid needs to be if you want efficient lapping. As you've mentioned, viscous fluids can nearly or even completely prevent the abrasive from doing any work. That can be a benefit in some cases, as a simple switch of the fluid can result in a somewhat different surface finish. I don't think that many would bother to make their own slurries in a professional setting.

This doesn't really apply to the knife strop example given above - he's only using the fluid to carry/distribute abrasive particles to the strop, then letting the fluid evaporate away. He mentioned using the strops dry.

If you want to find some interesting reading about the differences between charged laps and laps used with free abrasive, use the search term "2 body abrasion vs 3 body abrasion." You will find plenty to read.

Thank you for the answer and the search term. I think you're right probably no one makes their own slurries in a professional setting. For me this is primarily a hobby with some extra side income from time to time so I can do this kind of stuff out of personal interest only.

 

[alpacca458]

Just a few thoughts

Diamond is strongly hydrophobic, given the chance, fine diamond will float on the surface of water rather than be wetted and mix (the bonds in diamond are as "covalent" as bonds get - trying to get water to wet diamond is even more extreme than trying to get water to wet parafin wax).

Oils, and none polar solvents will wet diamond.

You might also be able to distribute fine grained diamond abrasive in emulsion where water is the continuous phase, The diamond sticking to the oil droplets.

The problem then arises of persuading your diamond to embed in the surface of your lapping plates, rather than staying loose and rolling around.

I remember reading about the process of sizing diamonds and preparing laps fine lapping in one of the early 20th century "machinery" magazine booklets on guage making, but that would be 16 or more years ago now

You'll probably find scans of the book on archive.org

 

[luke8888]

Just a few thoughts

Diamond is strongly hydrophobic, given the chance, fine diamond will float on the surface of water rather than be wetted and mix (the bonds in diamond are as "covalent" as bonds get - trying to get water to wet diamond is even more extreme than trying to get water to wet parafin wax).

Oils, and none polar solvents will wet diamond.

You might also be able to distribute fine grained diamond abrasive in emulsion where water is the continuous phase, The diamond sticking to the oil droplets.

The problem then arises of persuading your diamond to embed in the surface of your lapping plates, rather than staying loose and rolling around.

I remember reading about the process of sizing diamonds and preparing laps fine lapping in one of the early 20th century "machinery" magazine booklets on guage making, but that would be 16 or more years ago now

You'll probably find scans of the book on archive.org

Very interesting. After few tests I settled on other solvents than water for a number of reasons. Mainly related to viscosity of water and having to use rust inhibitors that may stay as solids after water evaporates (or risk rust).

Perhaps I should try again, but with a surfactant this time to lower surface tension. This might improve the wetting of the diamond powder. I have mixed feelings about using water. On one hand I now have a full set of diamond grit slurries I'm happy with, without using water(except as inpurity). Not having to worry about rust is a major benefit. On the other hand water is very nice from the ecological standpoint.

So I'm definitely interested in experimenting with water based slurries in future. I think I will try adding a surfactant to it ne t time to try to alleviate the problem you raised.

Regarding books I found just two old source texts on lapping. I don't remember their titles now (I'll look them up if anyone is interested), but all later literature seems to quote them verbatim. Even my "current" 30th edition of Machinery's Handbook in its lapping chapter has the same text copy/pasted from this 19th/early 20th century book. There is nothing wrong with it, but it is somewhat amusing to read about emery and lard oil in a book printed in 2016 :-)