Where am IThis page discusses the abrasives I use for sharpening hand tools (plane blades, chisels, ...). There are much more detailed pages on honing and grinding using either a 1" belt sander or a bench stone.Why Microfinishing?There are lots manufacturers of sheet abrasives out there. I have tried many different makes and grades and have found that 3M Microfinishing Film is by far the best. This particular group of abrasives is not commonly found in Hardware stores, or even Autobody stores.Microfinishing abrasives are much different from standard abrasives used in woodworking. According to 3M advertising literature the abrasive used is much more finely graded (fewer large pieces of grit to cut deep scratches into your tools), with a bonding to the backing material strong enough to use for honing tool steels. You can spend a little less if you buy regular sheets rather than the PSA (pressure sensitive adhesive) backed papers. The problems attaching it to the glass eat up any saving quickly. Get the PSA backed papers.
I have spent some time looking for suppliers. Googling 3M Microfinishing Film produces lots of references within the 3M company sites, but the information there appears to be out of date. For example, the listing for 3M Product ID 468L(PSA), lists 30 and 40 micron grits. I have 15 and 40 micron grits with this designation. Dealing directly with 3M will require persistence - they may not know what they sell. It comes in rolls, sheets, discs and disc rolls. If buying in bulk, the rolls might be an economical alternative. Getting someone to give you a price on this may be difficult. Please let me know if you have any luck on this.
Where to get it?I buy these abrasives at Tools for Working Wood (search for 3M PSA) which has good prices and a bit more variety than other mail order companies.In Canada, Lee Valley has a somewhat smaller selection at somewhat higher prices. If any reader of this page outside North America finds finds a distributor local to them, I would certainly include a link on this page. How do I apply the PSA stuff to glass?The adhesive on the back of the 3M abrasives is very sticky - it grabs and holds. This makes it hard to get a piece smoothly adhered to a glass plate. The secret to applying the paper is a water spritzer. Just spritz the surface and slide the adhesive into place. I have put a video - well three actually - onto youtube showing me applying a half sheet of 15 micron 3M microfinishing abrasive to a sheet of glass.My Grit SelectionMy sharpening system has four steps:
Newer GritsThe dark green 0.5 micron Chromium Oxide based PSA sheets are no longer generally available.
Grinding the PrimaryFirst, you have to have a primary bevel.While most of the emphasis in these web pages is on honed microbevels, you cannot even start honing until you have a primary bevel and a reasonably good edge. In fact, if you grind the primary poorly, you probably cannot hone a good final edge. In my view, the best way to grind a primary bevel is by hand, using a coarse Silicon Carbide bench stone. Second best is a 1 inch belt sander. In either case, I grind almost to but do not actually grind the edge. That is, I leave the edge previously obtained by honing. (Except when I get a new blade with bad chips, or want to change the shape of the edge.) Pictures of primary bevels taken during belt sander grinding are on this page. Images of edges obtained using the Norton SiC bench stone are on this page. Be sure to double click on the images. It is possible to get so close to the edge while hand grinding, you might not be able to see the remnant of the old edge in the thumbnails. Grinding does not shorten the blade! Use and honing shorten the blade. Both operations remove metal on the order of a few thousands of an inch. A grinder can remove 10 times that in a split second. I have tried two types of sheet abrasive on glass: 40 grit 3M Microfinishing Film and 4 different grits of Norton 3X. Only the 60 grit Norton 3X removed metal fast enough to be useful, but it fractured the edge.
Grit ImagesThese images of various abrasives were taken using a standalone microscope and a digital camera. The large images are about 150 times magnification, so each 5" image represents about 0.05" of abrasive. (Digitally reduced 3 times from the originals.) The smaller images to the right are slices of 450 times originals.
Grit SizeThe importance of grit size is one of the first things any woodworker learns. Most first use abrasives for finishing their projects. In most cases a woodworker works through a series of abrasives in the 100 to 300 grit range. For sharpening though we need much finer abrasives.While there are many selections of abrasives recommended in various web sites, I have found that for the microbevels just 3 grits are required. I have tested about 40 different plane irons now, some many times. I use the same procedure for sharpening the blades each time, taking pictures with the QX3 often after each sharpening grit. An analysis of the geometry -- microbevel angles and widths -- has convinced me that each grit is taking off a depth of metal sufficient to remove all of the scratches left by the previous abrasive. There is no need for intermediate abrasive grits. The grits of the 3 abrasives are:
Abrasive HardnessWhile most woodworkers understand grit size, most don't worry about abrasive hardness. Fortunately, it is not a problem when working wood. It is possible to work wood with very soft abrasives. For example, Rottenstone is very soft. The standard abrasive for wood finishing, Garnet paper, is hard enough to work most woods.Stepping up to tool steel though, harder abrasives are needed. Natural stones may not be hard enough. Oil stones, based on Novaculite or Silicon Dioxide, and water stones, based on Alumina or Aluminum Oxide, are not hard enough for some of the harder tool steels. Both Silicon Carbide and Chromium Oxide are much harder than most AlO and SiO2, plenty hard enough for all tool steels used in plane blades. There is much more on abrasive hardness in the Jig FAQ. Home againBack to the Sharpening and Testing Plane Irons home page.Questions? Comments?You can email me here. |