>I have a few flea market chisels that need extensive lapping to remove rust pits. In the past I have tried SiC paper, drywall screens, Shapton stones, and diamond stones. SiC paper cuts fast but wears out quickly and you need to glue it down to avoid dubbing. I haven't had luck with drywall screens, they severely dub the edges. The 320 grit Shapton stone I bought was disappointing to me, it is very soft and needs flattening too often to be practical for me. I have had good luck with the coarse Norton diamond stone but after a few chisels it seems slow and worn out. I would like to try loose diamond sharpening for lapping the backs of plane blades and chisels. I thought I would use a cracked plane body for a lapping plate or try a piece of acrylic plastic. I have found suppliers for diamond powder, silicon base diamond compound, and water base diamond compound. Which one of these is the preferred one to use and how coarse can I go in microns? Are loose diamonds practical for removing pits or are they more for polishing and sharpening blades and irons in better shape? Maybe I should just invest in a surface grinder:)
>Paul, diamond seems misunderstood when it comes to lapping and honing tool steels. Diamond is as hard as it gets, and the grains are very sharp before you start lapping/grinding. But hardness and sharpness aren't everything. The larger grains of diamond, larger in relation to what's needed to remove tool steel, are especially prone to fracturing. This is not altogether bad, and is in fact desirable when diamond is properly charged onto a substrate which embeds the grains better than that which is being lapped. There are many variables: how deeply, how tightly, how long, at what angles, and how much these characteristics allow further fracturing and subsequent embedment. All which add up to what diamond does very well; diamond cuts other very hard substances easily while leaving a very consistant pattern.
When it comes to tools steels, coarse lapping with diamond is not always so appealing, which is why other compounds are chosen. For one thing, the steels used for hand woodworking tools can be quite 'gummy' at the Rockwell C-scale hardnesses we most encounter them in chisels and planes (52-62, say). This can mean more drag on very brittle, friable abrasives, which it turn means more dislodgments of individual grains from their substrate (often nickel-coated mild steel), and quicker attritional fragmentation.
Silicon carbide in it's best form for lapping is an extremely hard, highly orthogonal form with sharp edges both at the corners and along its blocky form that is superb for aggressively removing tool steel in a consistent pattern. No, it's not as hard as diamond, but it presents many sharp edges for steel removal opportunities in a very concentrated space. Used as a loose grit, when well graded, it's entirely adequate and highly appropriate to do the initial work on edge tools used for hand woodworking. While there are other abrasive choices such as the borons, silicon carbide is much cheaper and comes in a useful progression of grit sizes. So that coarse diamond stone, in my opinion, is better saved for regrinding damaged edges or changing the bevel angle on plane irons and chisels before progressing to the medium stones.
Also, even though your coarse stone seems 'dead' compared to it's early aggresiveness, that's about what I consider well broken-in for most handtool use. This is much more important when cutting much harder materials, such as sapphire (which I do), and using such an aggressive grit would take a long long time to work through subsequent grits, so coarse laps get a thorough break-in period on hard cheap lab-created material. But likewise for handtools, you don't really want deep scratches that take much more removal effort than is necessary before getting to actually to use the tool, which is the object anyway, unless you're someone more apt to benefit from manufacturing economies of diamond lapping. Then again, there are those who are completely with sharpening and abrasives fetishes, for whom cost is really no object when you're blithely indulging in... uh, nevermind.
>Paul, I found that 30 micron works very fast in terms of abrading the surface, though I have not used it on backs that are in rough shape. Though it might be heresy to some, if a back is in really bad shape, I use the LV powered sharpening sytem. The platters are extremely fast in removing material - though I admit is it tough presenting the back to the platter. It is like an airplane landing - you try to touch the platter with a part of the chisel back from the edge and then try to gently let the bevel end touch down like the pilot tries to let the nose wheel down. Good luck which ever way you go. Once you have them in fairly good shape, the diamond grit ( I use the oil paste - though I haven't tried the other forms) will hone it quickly
>I have done the same thing with a 8" stationary disc sander but it is a little dicey. It removes steel almost too fast and it is easy to make things worse.
>David, Thanks for your input. I am just looking for a better way to remove a lot of material. I am happy sharpening and maintaining edges with oil or water stones and have no gripes there. It just takes so long to flatten the backs.
>Please be advised, this is likely to make you cringe. On my most sad case flea market finds, I use a 4 1/2" angle grinder, to waste away unwanted metal. Use a thin(1/16")cutting wheel. Those are flexible enough, to bend in use, so that a large amount of wheel makes contact. This is important, because otherwise, gouging would occur.(not good) You must check progress often, as well as apply water, for cooling. This is not for the faint of heart, but when done carefully, will produce the desired result.
>I have had very poor results with the coarser grades of silicon carbide. The 120 grit Clovite compound has almost no life for your task. The grits crush readily and for this reason the abrasive manufacturers do not recommend silicon carbide for tool steel grinding.
If you got pits you will need some sort of power grinding to remove enough metal to be rid of them. I have used 60 micron diamond on a cast iron plate or acrylic for such a task and it works better than any other hand operation I have tried, but it doesn't compare to the minutes it will take on some sort of powered machine. I borrow my friends horizontal water stone Makita for these tasks and it works great.
If you are both good and lucky you might get by with using a belt sander and about 60 grit aluminium oxide or zirconia belt.
Do you use diamond powder or do you use a ready made paste? One of the suppliers I have found has both but the compound is a silicone base. I am concerned with using silicone in my shop as I understand it is very invasive and hard to get rid of.
>...Clovite compound is worthless for coarse lapping, but silicon carbide is indeed recommended for precisely this sort of work. However, not all silicon carbide is equal to the task. This company has had much acclaim and success with their lapping system and the SiC grit they sell does work and work well.
