hand lapping techniques

M. Roberts · Oct 27, 2021

Guys,

I have a job that requires the faces to be flat to 0.0002" over a 10" x 12" x 0.200" thick ceramic plate. To add to the difficulty, there are 60 holes of varying sizes, including a 6.8" hole thru the center. I wasn't able to hit the spec right off of the grinder (yet! I have a few ideas to hopefully hit the spec off of the grinder.), so I decided that I had to attempt to lap it to spec, but I do not have a lap big enough for the job. I looked in McMaster, and found 9"x11" diamond paper; I ordered 4 sheets to put on my granite surface plate...I wet the surface down, and was hoping that they would adhere. I was somewhat successful, but I had issues with the edges not staying down. Luckily, I remembered that I had 24" square Mylar, 0.01" thick. I used the same procedure, wet the surface plate, place the mylar on top, and squish out the air pockets. For this trial, I used a 65 micron diamond slurry, and light to no pressure....it cut, now to see if it is flat...more to follow. Please feel free to chime in...

Mark

implmex · Oct 27, 2021

Hi Mark:
First of all, I commend you for taking this on with such an ambitious flatness tolerance over such a large surface area.
This is not easy to achieve, so if you pull it off you will have my respect forever.

If you can avoid rubbing it on abrasive sheets I recommend you avoid it...you will forever have problems dubbing off the corners at the periphery of the plate and around every hole too.
Using pressure sensitive adhesive backed sheets will be a tiny bit better than what you're using now, but not very much, if flatness is your goal.

So I would explore making a cast iron lap that uses diamond slurry.
You can grind it flat (as flat as the grinder will grind), you can match lap it with two other plates to get it flat withing fractions of a micron, and you can make it big enough to cover your entire ceramic plate with room to move, so your ceramic plate comes out flat too.

There is a Youtube poster (I think his name is "Oxtoolco" or something like that) who gets into the method in some detail...I encourage you to do a search for some of his videos.
If you can't face making the lap, there are lapping houses who can do this for you in a pinch...I don't know of any offhand, but they are around.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Scottl · Oct 27, 2021

Not easy with improvised methods. Many years ago I had to lap parts for high pressure gear pumps after they were ground flat on the surface grinder. The design had no gaskets so we had to lap all parts (keeping together as a matched set) on a cast iron lap and then lap the gears just a bit more for clearance to the side plates.

I've had mixed results with abrasive paper. Better than as ground but not as good as a real lapping plate with the groove patterns cast in.

Dan from Oakland · Oct 28, 2021

If you get close, but not close enough, these guys are pretty good at what they do:
Precision Lapping | Precision Grinding | Surface Finishes

jccaclimber · Oct 28, 2021

Is that thickness correct, 0.2"? I'd be worried about flex on something that thickness, but maybe it's stiffer than I think. I would think applying uniform pressure on something that large/thin would be difficult, even if you did have a flat lap large enough.

If you have a lap that's too small, do you have a good method of measuring it? What about treating it like a granite table and lapping local regions that are high rather than trying to put the entire part on a larger lap?

Alternatively, I don't know the price, but places like Lapmaster do offer lapping as a service and may be able to hit that one one of their larger machines if there's enough value to justify shipping it out.

How many do you have to make? Have you found that it warps as you remove material, or is it fairly stable? I've never taken enough off of a ceramic to find out.

Demon69 · Oct 28, 2021

Re cast iron lap and diamond.
You can scrape an iron surface plate flat to a granite reference, then roll diamond into the surface of the iron plate with a bearing, thats what they use to lap granite surface plates.
Lots of info and nuance in this. (20:21 bringing work to lap)

As mentioned using abrasive paper will roll the edges of the work. We're talking micro dimensions but think of having a layer of sponge between the ref and abrasive.
Using a cast lap with abrasive firmly fixed to it (rolled in) theres no 'give'. Your edges will be razor sharp and the surfaces near them flat.
Potential problems :scratchchin:

:-
heat from hands swelling the work leading to flatness out of tolerance.
uneven pressure resulting in uneven thickness of the work.

Just the 2c from some schmuck who's lapped a few granite plates, good luck with it and looking forward to see how you make out, doesn't sound easy.

sfriedberg · Oct 28, 2021

jccaclimber said:
If you have a lap that's too small, do you have a good method of measuring it? What about treating it like a granite table and lapping local regions that are high rather than trying to put the entire part on a larger lap?

Right. This is a standard practice when there's not a lap large enough to cover the whole area. Some oldtimers used to use a brass disk to take down high spots, and a brass ring to take surrounding area down to a low spot.

M. Roberts · Oct 29, 2021

Guys,

Good morning, and thanks for all of the input. Sfriedberg; that is an interesting concept you brought up; would the "oldtimers" just rub the brass directly on their work, or would they have a compound as well?

I had dropped my plate off to our QC department, and I was pretty happy with the results; one side (the critical side flat to 0.0003" (spec is 0.0002", and parallel to 0.0011" (spec is 0.0012"). They did not supply me with a map showing me where the part is high though. So, I'm almost there.

I do have to make 3 of these parts total. I am working on EDMing "blockers" to fit the contour of the part in an effort to hold it better, in addition to planning on dusting the mag chuck to ensure that there are no issues. I would like to take a picture to show what the part looks like, but I don't think that would go over very well.

I agree 100% with the thinking that the Mylar being compliant would have the tendency to roll the edges of the part (maybe QC didn't go out that far?) but, if the Mylar is thinner than the grit size of the abrasive, wouldn't it just get imbedded into it just like charging the steel/cast iron lap when one re-surfaces a granite table? IDK.

The un-even pressure from me holding onto it was another big concern; I tried to apply as little pressure as possible, and was pretty diligent about keeping my figure 8 pattern, and rotating the orientation every 10 "8's" or so.

Maybe just chalk it up to dumb luck, but I'm just glad it is working...we shall see.

Thank you,
Mark

MCritchley · Oct 29, 2021

Perhaps map the part and simply spot lap the high spots and stay away from the low spots, just like scraping. Wrap some lapping paper on a 1-2-3 block and use that as your spot lapper. Check often like scraping, it’s many trips to the plate when scraping down to tenths, lapping is no different.
Moore shows a similar technique being used to fine tune their jig borer/grinder ways in Foundations of mechanical accuracy. They use a charged grooved block to give a high spot a little rub down.

implmex · Oct 29, 2021

Hi again M.Roberts:
I said "This is not easy to achieve, so if you pull it off you will have my respect forever."

You said "I had dropped my plate off to our QC department, and I was pretty happy with the results; one side (the critical side flat to 0.0003" (spec is 0.0002", and parallel to 0.0011" (spec is 0.0012"). They did not supply me with a map showing me where the part is high though. So, I'm almost there."

Well, you now have my respect forever...well done indeed!

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

eKretz · Oct 29, 2021

M. Roberts said:
Guys,

Good morning, and thanks for all of the input. Sfriedberg; that is an interesting concept you brought up; would the "oldtimers" just rub the brass directly on their work, or would they have a compound as well?

I had dropped my plate off to our QC department, and I was pretty happy with the results; one side (the critical side flat to 0.0003" (spec is 0.0002", and parallel to 0.0011" (spec is 0.0012"). They did not supply me with a map showing me where the part is high though. So, I'm almost there.

I do have to make 3 of these parts total. I am working on EDMing "blockers" to fit the contour of the part in an effort to hold it better, in addition to planning on dusting the mag chuck to ensure that there are no issues. I would like to take a picture to show what the part looks like, but I don't think that would go over very well.

I agree 100% with the thinking that the Mylar being compliant would have the tendency to roll the edges of the part (maybe QC didn't go out that far?) but, if the Mylar is thinner than the grit size of the abrasive, wouldn't it just get imbedded into it just like charging the steel/cast iron lap when one re-surfaces a granite table? IDK.

The un-even pressure from me holding onto it was another big concern; I tried to apply as little pressure as possible, and was pretty diligent about keeping my figure 8 pattern, and rotating the orientation every 10 "8's" or so.

Maybe just chalk it up to dumb luck, but I'm just glad it is working...we shall see.

Thank you,
Mark

I would get after them for that map. Sure would help you.

Conrad Hoffman · Oct 29, 2021

I find that lapping film tends to round the edges near holes. The cast iron lap is the way to go. I'd use something smaller than the surface to be lapped and a lot of measurement.

DavidScott · Oct 29, 2021

Err, how about the "three plate method"??????? The ops needs define it perfectly. He has 3 plates that need minor lapping to make them all flat.

As others have said sandpaper WILL roll the edges significantly. What you want is loose abrasive, a tough monocrystalline diamond in your case, or maybe boron carbide, with enough water to make a slurry but not enough that it washes away. Get a spray bottle and mist the plate to add water, a small amount will take it from not enough to too much, how much will be obvious. Then lap your three plates together with figure 8 passes and semi-light pressure, it's a big surface so don't be too shy. Try 1 minute per plate per cycle, which will end up being 3 minutes per plate when you are done being the 3 plate method, clean them up and have QC inspect them. This method of lapping is way faster than using a bonded abrasive so don't overdo it until you get a feel for it. My keyboard guess is you want to use around 20-40 micron diamond for this, probably not coarser if I am off. I expect it will break down rather quickly so when you are done it will leave a finer finish on your parts than the grit you started with.

I have done a fair bit of lapping this way and will say there is no comparison between loose grit between two plates and using some bonded abrasive, the loose grit will leave a much flatter surface in far less time. The heart of the technique is to get the loose abrasive to roll between the plates and not skid, and getting the amount of liquid just right makes a big difference in how the abrasive works. Too much or little and it will want to skid vs roll. Also, keep the pressure light so you don't break the abrasive down faster. If you find you are only high in certain spots I would use the same method but with either flat aluminum, steel, or cast iron pucks to localize the abrasion.

implmex · Oct 29, 2021

Hi David:
You make an excellent point, it is the obvious way now that M.Roberts has revealed he's making three of the little beasts.
Saves making and qualifying a lap too...big bonus!

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

dcsipo · Oct 29, 2021

DavidScott said:
Err, how about the "three plate method"??????? The ops needs define it perfectly. He has 3 plates that need minor lapping to make them all flat.

As others have said sandpaper WILL roll the edges significantly. What you want is loose abrasive, a tough monocrystalline diamond in your case, or maybe boron carbide, with enough water to make a slurry but not enough that it washes away. Get a spray bottle and mist the plate to add water, a small amount will take it from not enough to too much, how much will be obvious. Then lap your three plates together with figure 8 passes and semi-light pressure, it's a big surface so don't be too shy. Try 1 minute per plate per cycle, which will end up being 3 minutes per plate when you are done being the 3 plate method, clean them up and have QC inspect them. This method of lapping is way faster than using a bonded abrasive so don't overdo it until you get a feel for it. My keyboard guess is you want to use around 20-40 micron diamond for this, probably not coarser if I am off. I expect it will break down rather quickly so when you are done it will leave a finer finish on your parts than the grit you started with.

I have done a fair bit of lapping this way and will say there is no comparison between loose grit between two plates and using some bonded abrasive, the loose grit will leave a much flatter surface in far less time. The heart of the technique is to get the loose abrasive to roll between the plates and not skid, and getting the amount of liquid just right makes a big difference in how the abrasive works. Too much or little and it will want to skid vs roll. Also, keep the pressure light so you don't break the abrasive down faster. If you find you are only high in certain spots I would use the same method but with either flat aluminum, steel, or cast iron pucks to localize the abrasion.

Yeah, all the above, and.....

Since the pieces are 0.200 a 0.500 (or thicker) ground mic6 tooling plate would be a good backer while lapping. I have no feel for how rigid the plates are, but any grit or dirt under the bottom plate may distort them enough to be out a few tenths. A thin coat of spray adhesive would keep the dirt out and keep the plates affixed to Mic6 backers securely for the lap. Would be interesting to see how well it worked.

Demon69 · Oct 29, 2021

Backing up with something is a great idea, spot glued or taped to a cheapy 12x12 granite plate even, would be like surfacing a small plate, they run much tighter than a tenths.
I still think going at things with a larger lap is the way to go, using small laps risks introducing and chasing local error :scratchchin:

, more of a pia if you dont have a gauge to check progress. I wonder what Robin would suggest, hes not backward when it comes to doing the seemingly impossible.

implmex · Oct 29, 2021

Hi Demon 73:
The problem with sticking it to any surface becomes one of being sure you didn't distort it when you stuck it down and didn't make it worse when you unstuck it again.
If he three plate laps it like David Scott recommends in post #13, he doesn't even need to make a lap.
So I'm a fan of David's suggestion, and I personally would just leave them naked and handle them very gently while I'm lapping them against one another.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Demon69 · Oct 29, 2021

implmex said:
Hi Demon 73:
The problem with sticking it to any surface becomes one of being sure you didn't distort it when you stuck it down and didn't make it worse when you unstuck it again.
If he three plate laps it like David Scott recommends in post #13, he doesn't even need to make a lap.
So I'm a fan of David's suggestion, and I personally would just leave them naked and handle them very gently while I'm lapping them against one another.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Yeah I can understand concerns over distortion. I was thinking 20-30 spots of a 'super glue' on a clean base and floating the work on, leave set then lap. Use a release to free the work. I've no experience here but have heard it done iirc.
I guess I lean this way as ive some time down scraping and have made a few laps for playing with granite, easy enough, a couple hours effort. Lapping is done dry so it's easy to see progress. I've not worked 3 surfaces to flat or have any real experience with slurry, so maybe I'm missing the easy road.
Is interesting to think on though,

Cheers
D

dcsipo · Oct 29, 2021

Demon73 said:
Yeah I can understand concerns over distortion. I was thinking 20-30 spots of a 'super glue' on a clean base and floating the work on, leave set then lap. Use a release to free the work. I've no experience here but have heard it done iirc.
I guess I lean this way as ive some time down scraping and have made a few laps for playing with granite, easy enough, a couple hours effort. Lapping is done dry so it's easy to see progress. I've not worked 3 surfaces to flat or have any real experience with slurry, so maybe I'm missing the easy road.
Is interesting to think on though,

Cheers
D

You are right superglue is a better option. Heat will release superglue easily. I was thinking spray adhesive because it is easily released as well

sfriedberg · Oct 29, 2021

M. Roberts said:
would the "oldtimers" just rub the brass directly on their work, or would they have a compound as well?

Compound. A brass lap without compound isn't going to cut anything.

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