Lapping plate material questions

[jccaclimber]

I've used planetary lappers with cast iron plates and SiC abrasive in the past. I'd like to understand the process a bit better, and am using hand lapping to get some practice with the fundamentals.

Question 1: Low speed tool sharpeners like Glendo/Accu-Finish have a smooth ceramic plate as the lap when using diamond slurry (different from the plated diamond wheels usually used). I've made the mistake of using a harder lap than my workpiece with diamond paste and had issues with embedding diamonds in my steel workpiece. I don't remember this ever being an issue when using the ceramic lap on the Accu-finish, which I assume is much harder than my workpiece as well. Is there something special about the ceramic, or is it just that I've never noticed the diamonds embedding?

Question 2: With steel workpieces, is there any benefit in using copper vs. cast iron for the lap so long as my steel workpiece is harder than the cast iron? With cast iron would gray vs. ductile matter? If I need to go copper am I going to see much difference between tempers and alloys, or is this a case where close is well close enough?

 

[boslab]

I’ve played with a lapping machine for bits of hydraulic motors, it was fairly big like upside down Blanchard idea, plate was cast iron and had these conditioning rings to run for flattening, exact procedure I don’t recall, and we had small lappers for metallurgical samples, copper disks were common, it would seem that the lap is softer than the work as you say,
Mark
Lapping Machines | Diamond Compound & Ultrasonic Cleaning Machines - Kemet

 

[eKretz]

Probably the fact that the ceramic lap surface is being used to lap items that are hard to embed. High speed steel and tungsten carbide are generally what's being lapped on a surface like that. Neither of those are likely to embed anything. I wouldn't use it to lap copper or aluminum, or even softer steel though. You might run into those embedding problems then.

Fair question 2, iron works just fine, I have several iron laps. Copper works too, so does granite! As I recall there was a type of lap made from granite with diamond embedded marketed to toolmakers some years ago, I forget the brand now.

 

[boslab]

I remember something about the different modes of lapping, I think there was the embedded or abrasive but there was this other mode called “rolling” lapping
I suppose a hard ceramic lends itself to the rolling kind, I’m not sure if the slurry is a different mesh or size particle, some research probably in order
Mark

 

[jccaclimber]

Probably the fact that the ceramic lap surface is being used to lap items that are hard to embed. High speed steel and tungsten carbide are generally what's being lapped on a surface like that. Neither of those are likely to embed anything. I wouldn't use it to lap copper or aluminum, or even softer steel though. You might run into those embedding problems then.

Fair question 2, iron works just fine, I have several iron laps. Copper works too, so does granite! As I recall there was a type of lap made from granite with diamond embedded marketed to toolmakers some years ago, I forget the brand now.

No picture handy, but I've been using the wider bell shaped tip from a Starrett 25R indicator set as a test piece. Partly because it was the only piece of hardened steel I had on hand that I was willing to re-grind. I'd hate to have to grind down a 1-2-3 block because I messed up an edge. Under high magnification it has 1 to 5 micron specs which don't move. I assumed those to be embedded diamond. They don't wash off, and on occasion that I do remove one they leave a solid drag mark on their way out. I assumed this was embedded diamond from a foolish attempt at using a larger piece of hard steel as a lap, but it's really just a guess. That's interesting about granite, it wouldn't be hard to grind some countertop pieces flat to experiment with, although it wouldn't have grooves.
Do you have any issues with fine scratches from impurities in the cast iron? Boslab's link above seemed to recommend it for general purpose lapping, but maybe not for higher surface finish requirements.

Would you advise starting with a softer material (say an non-hardened steel) as a workpiece, or does it matter? I remember that lapping down tungsten carbide gauges always took forever on the planetary lap, but I was using SiC grit that we had on hand for steel, not diamond or something harder like that.

 

[eKretz]

I remember something about the different modes of lapping, I think there was the embedded or abrasive but there was this other mode called “rolling” lapping
I suppose a hard ceramic lends itself to the rolling kind, I’m not sure if the slurry is a different mesh or size particle, some research probably in order
Mark

Yes, you got it. The embedded abrasive lap performs what is called "two body" lapping. Loose abrasive rolling between two surfaces is "three body" lapping. Surface finish for the same abrasive size is generally finer with two-body abrasion.

 

[eKretz]

No picture handy, but I've been using the wider bell shaped tip from a Starrett 25R indicator set as a test piece. Partly because it was the only piece of hardened steel I had on hand that I was willing to re-grind. I'd hate to have to grind down a 1-2-3 block because I messed up an edge. Under high magnification it has 1 to 5 micron specs which don't move. I assumed those to be embedded diamond. They don't wash off, and on occasion that I do remove one they leave a solid drag mark on their way out. I assumed this was embedded diamond from a foolish attempt at using a larger piece of hard steel as a lap, but it's really just a guess. That's interesting about granite, it wouldn't be hard to grind some countertop pieces flat to experiment with, although it wouldn't have grooves.
Do you have any issues with fine scratches from impurities in the cast iron? Boslab's link above seemed to recommend it for general purpose lapping, but maybe not for higher surface finish requirements.

Would you advise starting with a softer material (say an non-hardened steel) as a workpiece, or does it matter? I remember that lapping down tungsten carbide gauges always took forever on the planetary lap, but I was using SiC grit that we had on hand for steel, not diamond or something harder like that.

Coarser lapping works just fine with cast iron, finer lapping works better with softer laps. For certain optical polishing for instance, they even use pitch as a lap! I generally don't use cast iron to go too fine, so no I never noticed any stray scratches. Softer stuff does lap faster, of course, so what you may want to start with depends on the work you want to do. If your goal is to lap in some 1-2-3 blocks you may want to start with some hardened steel so your experience is more applicable to your end goal.

The Starrett piece may not be very hard. Can a file touch it? The level of hardness I was inferring was more like a piece of HSS - i.e. a cutting tool. Tungsten carbide is on yet another level of hardness, so lapping that should absolutely be done with diamond abrasive if possible. And lack of grooves in granite is not difficult to rectify, you can easily grind them in with a diamond disc.

 

[boslab]

I have a small lapping machine, checked the disks, most are copper but there is an aluminium one and a glass one, I’ve never used those as I just use it to sharpen chisels as it came out of a bin, fixed it, the glass one uses self adhesive lapping film, silicon carbide, 1.5 micron is on it, the control arm is missing but I don’t use it anyway it’s just a glorified sander to me
In the sheet it also lists a lead and a brass platten (6” disk)
I’m guessing the brass was in fact a bronze one but it’s handwritten notes
Seems like granite might be handy, yet another project, I do have a piece of granite worktop, wonder if that would work, there’s MDF on it at the moment with lapping compound to polish chisels and planes, does a good job of that btw
Mark

 

[jccaclimber]

The end goal is probably things in the mid to low 50 HRC range, but with an optical level finish, so perhaps one cast iron one for a coarse grit, then I'll have to consider something for the finer grit sizes.

The Kemet page mentioned ceramic as their hardest lap for "where a cleaner lapping process is required". I wonder if that's less stray particles that could cause a scratch, or something else?

Several other threads on here advise aluminum as a poor lap material, but it's come up in this thread and on the Kemet page. Perhaps it's only useful for finer polishes and low pressures?

 

[eKretz]

The end goal is probably things in the mid to low 50 HRC range, but with an optical level finish, so perhaps one cast iron one for a coarse grit, then I'll have to consider something for the finer grit sizes.

The Kemet page mentioned ceramic as their hardest lap for "where a cleaner lapping process is required". I wonder if that's less stray particles that could cause a scratch, or something else?

Several other threads on here advise aluminum as a poor lap material, but it's come up in this thread and on the Kemet page. Perhaps it's only useful for finer polishes and low pressures?

Yeah it's the same for all soft materials, not so good for coarse grit. They wear very quickly, of course. Even with embedded abrasive, some of it is going to come loose and then begin to abrade the lap as well. The "cleaner" process probably just means that less of the material from the lap gets smeared and rubbed against the material being lapped. Some materials may become contaminated by this. The harder the lap, the less of it that will be abraded away during the process.

 

[dian]

interesting question about the ceramic lap. ceramics can be 1:3 in hardness. i could not find any trace of acufinish ceramic plates but found some other info. reading it the issue of tumbling action vs. embeded particles seems unresolved. i guess the main idea it to have a very flat surface. but then again if you use a norbide stick on it? they stress not overcharging. that would mean the diamond is supposed to be caught in the "pores"? but then "loose on the surface" is mentioned as well.

maybe there are different types of ceramic plates? then ceramic might polish steel just by itself. finaly the 4 hours of "conditioning" got me.

Ceramic Laps How-to and Tips
Using the Ceramic Lap – United States Faceters Guild
Advice For Properly Charging Ceramic Laps
http://www.aussiesapphire.com/Storage/MatrixLap.pdf

o.k., it exists: Accu-Finish(R) Ceramic Lap: 6-inch Ultra-Fine (Ultra Polishing) - GRS

i have seen aluminum foil used online. there are phenolic and acrylic laps.

 

[jwhartley]

Check out some of Tom Lipton's (Oxtools) videos on youtube. He has several that discuss lapping. He has a series that goes through the construction of some cast iron lapping plates. Also videos on soft (copper) lapping plates.

-Jess

 

[Joe Gwinn]

Interesting data point: Many years ago, my wife and I took the tour of the Waterford glassworks in Ireland, where we saw the grizzled craftsmen who could carry on a conversation while cutting lead crystal (glass) freehand.

The tools were rotating soft copper wheels charged with diamond dust, lubricated with olive ("sweet") oil. I don't know the rotational rate, but they did not throw oil at all. The glass was cut fairly quickly, so the freehand work looked a bit like a slow graceful dance.

 

[jccaclimber]

As an additional data point, what I thought were impregnated diamonds turned out to be tiny little pits. I got to looking at the part after it sat overnight and saw the distinct rust stalagmites growing up from the surface. I ran it across the lap once, and sure enough little pits were left behind. I've stopped experimenting with water as a lubricant and have been using light oils (still need to get some lighter fluid or kerosene).

Aluminum results in scratches every time for me, although I've only tried it on a piece that I used a hack saw to put grooves in, so there may be other junk trapped in them which I wasn't able to clean up with a toothbrush.

I don't have any copper on hand, but I do have some brass which worked reasonably well.

Wood also leaves a nice surface, although I suspect it isn't holding form as well, mainly because I don't have it as flat.

Still struggling with really tiny scratches that are only visible with the correct lighting or under the microscope.

I've seen the Tom Lipton's videos before, but it was good to watch them again.

 

[jccaclimber]

It's been a while, but I figure I should update this even if I don't have any pictures.

I finally got some copper bar off of McMaster, cut a few grooves in it, and tried that. Initially I had a bunch of scratching, and in then end I found that my copper laps simply weren't flat enough. Once I'd used them a while and they flattened out things greatly improved. The other thing I did was swap over to 1 um and 0.1 um slurry from McMaster. A couple drops and even without rolling it in to the surface it seems to work well. I did not try lighter fluid or anything thinner than spindle 10 for my prior diamond paste, so I can't say how well that would/would not have worked.

The really big difference seemed to be getting my copper laps flat enough.

I'm still getting scratches on the order of 1 to 5 um wide so I have a bit more to learn, but they are fewer than before and there's a clear line where my now flat surface is slowly eliminating the edge rounding my previous efforts made.

 

[jccaclimber]

One more thing of interest to add to this. I don't yet have an optical flat (anyone have one they want to part with?) but I do have some very (λ/5?) flat clear glass windows from an old project and a (non monochromatic) diffuse light source.
I figured it costs me nothing to put one on top and see what happens. Did it produce fringes? Yes, and it as really cool to see that in person. It does tell me that I'm going in the right direction. Do they move all over the place if you look at them funny because my glass is way to thin? Yes, they do.

So, it'll work for me to verify if a DIY monochromatic light source works, but if you happen to be wondering if you get by without a thicker proper flat, the answer is resoundingly no.