Rogue particle control with iron/steel lapping plates

As I've mentioned before I use diamond paste on iron and/or mild steel honing plates for some sharpening, and particularly for flattening blade backs.

One issue that arises often is particle control, whether due to initial surface quality, contamination over time, or after lapping the plates. This issue recently came to a head for me when I added a new cast-iron lap (from McMaster) to my rotation, and it seriously scratched the first tools that I tried it on (thankfully not good ones).

One obvious way to deal with this is to rub the plate with an "eraser" of some sort. The ideal eraser would be softer than the plate but hard enough to hold the abrasive particles. I chose to use two in sequence:

1. White ScotchBrite pads (the kind with the least/finest added abrasive). These are commonly used to remove abrasive grit and "hairs" from ski bases, and it turns out that they work fairly effectively on lapping plates as well. They helped, but weren't sufficient to achieve scratch-free results.

2. A 1/2 hard 110 Brass bar. At Rockwell B40 this is significantly softer than either cast iron or mild 1018 steel. This appeared to remove basically all residual protruding rogue particles, leaving mild scoring as evidence of their passage (the brass begins to lap the iron/steel with the newly "captured" particles. The brass bar can then be filed to remove those before using it on another plate).

I get scratch-free lapping performance with submicron diamond compounds on plates that have been given this treatment.

Full soft 110 brass (Rockwell F40) would probably be better still, but costs significantly more. I wanted to test the concept with cheap 1/2-hard material before going overboard with it.

I'm sure somebody like George will point out that this is a common strategy among knowledgable machinists. It's obvious enough that it has to be :-).

I have no helpful advice to add, unfortunately, when it comes to diamond paste. I'm curious to see one of the local experts chime in because I've had this issue myself. I spent way too much time lapping blades, plane soles, etc. when I first got into hand tools. I couldn't scrounge up the money for a real lapping plate or anything so I instead tried every type of sandpaper out there glued to plate glass, then upgraded to silicon carbide grit on a piece of steel, and ultimately concluded that the method you describe is the least frustrating. Rogue pieces of grit has been a serious issue for me, though, and I suspect that the quality of diamond paste (stuff I got was cheap) has a lot to do with it.

Now I mostly rely on SiC grit because I haven't had the rogue grit issue--it breaks down as you use it and doesn't seem to embed in a piece of steel if I don't press down too hard. So-I hope I'm not hijacking the thread here-what's the advantage of the diamond paste?

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Originally Posted by Matthew Hutchinson477

Now I mostly rely on SiC grit because I haven't had the rogue grit issue--it breaks down as you use it and doesn't seem to embed in a piece of steel if I don't press down too hard. So-I hope I'm not hijacking the thread here-what's the advantage of the diamond paste?

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Speed, speed, more speed and the ability to cut basically anything, including high-Vanadium steels and solid carbide tools. SiC can deal with basically all woodworking steels, but it's a lot slower particularly as it breaks down.

Diamond paste is also available in finer grits than loose SiC, all the way down to 0.1 um (~120000 grit). I usually stop at 0.5 um though, as I don't think there's much practical benefit for woodworking in going lower.

Hi Patrick, I'm a bit confused... Are you referring to cross-contamination? (I use diamond paste on mild steel btw) Or are you referring to the fresh (unused) state the plate arrived to you from MC?

I'd recommend a thorough washing ,tilting the lapping plate over a container of gasoline. You need to get those larger particles out of the grooves in the lapping plate. If you don't want to use gasoline,try odorless paint thinner. We washed a lot of stuff in gasoline when I was young. Best to do it outdoors,and obviously no smoking!!!

The ultimate might be a lapping plate for each grit of abrasive you use. That might get expensive,though.

Is there maybe some sort of sacrificial surface that you could put on top of a highly accurate, expensive lapping plate? Something thin enough to conform to the lapping plate surface? I normally use a piece of plate glass with the expectation that it's only gonna last so long but even that can get expensive after enough time.

George, out of curiosity, how does gasoline or paint thinner loosen things? Does it act as a lubricant? Or does it etch the lapping plate on a very small, virtually unnoticeable scale?

When I worked at Grobet File they made a very fine brass bristled brush about 9 inches long by 1" wide. The bristles were only about .005 in diameter.
Using that with a small parts cleaner that ran kerosene or simple green over the plate would remove the diamond paste particles.
But having a plate for each grit size is what most shops would do.

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Originally Posted by michael langman

When I worked at Grobet File they made a very fine brass bristled brush about 9 inches long by 1" wide. The bristles were only about .005 in diameter.
Using that with a small parts cleaner that ran kerosene or simple green over the plate would remove the diamond paste particles.
But having a plate for each grit size is what most shops would do.

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I have a plate for each grit size that I use.

The problem I was addressing here was cross-contamination and foreign contaminants, not removal of the intended grit.

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Originally Posted by george wilson

We washed a lot of stuff in gasoline when I was young. Best to do it outdoors,and obviously no smoking!!!

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Not blowing yourself up or burning yourself alive is over-rated.

I don't know anything about lapping plates but I've seen them advertised and the whole concept seems weird to me. I'm sure there's more than one something I don't understand about them.

Iron is softer than steel. Then you add abrasive and start rubbing steel on it.

It seems to me that the iron would wear away faster than the steel and you'd very quickly end up with a lapping plate that's not flat. I mean even the first steel thing you lapped would end up taking the iron out of flat faster than the iron makes the steel flat.

Obviously these things work so why doesn't it work the way I think it would?

The diamond particles imbed in the surface of the cast iron plate (which was previously lapped flat ). This acts like a binder for the particles (think it's like a glue in sandpaper). The particles then abraid the tool steel.

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Originally Posted by Paul K. Johnson

Not blowing yourself up or burning yourself alive is over-rated.

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Mineral Spirits or Xylene are slightly safer and get basically the same job done, albeit at higher cost. They're in flammability class IC vs IB for gasoline.

Odorless mineral spirits and Kerosene are in class II for that matter, so you have to work at it a bit more to blow yourself up that way.

EDIT: And then there's the ultimate hard-to-burn low-viscosity hydrocarbon, JP-7. You can drop a lit match into the stuff and nothing happens.

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Originally Posted by Patrick Chase

Mineral Spirits or Xylene are slightly safer and get basically the same job done, albeit at higher cost. They're in flammability class IC vs IB for gasoline.

Odorless mineral spirits and Kerosene are in class II for that matter, so you have to work at it a bit more to blow yourself up that way.

EDIT: And then there's the ultimate hard-to-burn low-viscosity hydrocarbon, JP-7. You can drop a lit match into the stuff and nothing happens.

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I use WD-40, bought a gallon of it years ago for the price of a spray can. It works fine and smells better.

O.K.,if you are afraid, don't use gasoline. We washed down many,many parts when I was young. It should be done out doors,no smoking,and carefully. Use kerosene if you are terrified of gasoline. Everything mentioned so far will loosen the diamond's binding.

Any of the jet fuels as well as fuel kerosenes are highly unlikely to ignite when a match is tossed into a drum of the stuff - not a good test for safety of use. All are happy to ignite when atomized or dripped onto a flame or spark igniter. Douse yourself in Jet A while wearing your favorite polyester leisure suit, then undress to avoid smelling like the butt end of a jet...poly on cotton means static electricity...oops.

Exercise basic safety precautions...clean parts outside away from ignition sources (e.g., avoid using a welding torch to light your cigarette while cleaning parts in a gallon container of highly flammable solvent) and have a plan if the stuff gets a) on skin, b) clothing, or c) on Fido.

Other don'ts that no one should have to remind you of: don't spray lacquer, varnish, or other solvent finishes next to your fossil fuel burning furnace or water heater, don't cross busy highways at night in dark clothing with any expectation of pedestrian right-of-way, and definitely don't do Uncle Julio's double-bean chimichangas for lunch when a hot date is scheduled later that evening.